Lung Stereotactic Body Radiotherapy Plans Research Articles

Feasibility of a Single-Fraction Stereotactic Dose of 30 Gy to Solitary Lung Lesions on Halcyon.

Purpose We sought to explore the feasibility of using the current co-planar Halcyon ring delivery system (RDS) with a novel multileaf collimator (MLC) aperture shape controller in delivering a single high dose of 30 Gy to solitary lung lesions via stereotactic body radiotherapy (SBRT). Materials and methods Thirteen non-small-cell lung cancer (NSCLC) patients previously treated with a single dose of 30 Gy to lung lesions via SBRT on the TrueBeam (6MV-FFF) using non-coplanar volumetric modulated arc therapy (VMAT) arcs were anonymized and replanned onto the Halcyon RDS (6MV-FFF) following RTOG-0915 single-fraction criteria. The Halcyon plans utilized a novel dynamic conformal arc (DCA)-based MLC-fitting approach before VMAT optimization with a user-defined aperture shape controller option. The clinical TrueBeam and Halcyon plans were compared via their protocol compliance, target conformity, gradient index, and dose to organs-at-risk (OAR). Treatment delivery efficacy and accuracy were assessed through end-to-end quality assurance (QA) tests on Halcyon and independent dose verification via in-house Monte Carlo (MC) second-check validation. Results All Halcyon lung SBRT plans met RTOG-0915 protocol's requirements for target coverage, conformity, and gradient indices, and maximum dose 2 cm away from the target (D2cm) while being statistically insignificant (p > 0.05) when compared to clinical TrueBeam plans. Additionally, Halcyon provided a similar dose to OAR except for the ribs, where Halcyon demonstrated a lower maximum dose (15.22 Gy vs 17.01 Gy, p < 0.001). However, Halcyon plans required a higher total monitor unit (8892 MU vs 7413 MU, p < 0.001), resulting in a higher beam modulation factor (2.96 MU/cGy vs 2.47 MU/cGy, p < 0.001) and an increase in beam-on time by a factor of 2.1 (11.11 min vs 5.3 min, p < 0.005). End-to-end QA measurements demonstrate that Halcyon plans were clinically acceptable with an average gamma passing rate of 99.8% for 2%/2mm criteria and independent MC 2nd checks within ±2.86%. Conclusion Our end-to-end testing and validation study demonstrates that by utilizing a DCA-based MLC aperture shape controller before VMAT optimization, Halcyon can be used for delivering a single dose of lung SBRT treatment. However, future improvements of Halcyon RDS are recommended to allow higher output rates, rotational couch corrections, and an integrated intrafraction motion management system that will further enhance Halcyon's capability for site-specific single dosage of SBRT.

Clinical Outcomes of Synchronous Lung Stereotactic Body Radiation Therapy Lesions via Single-Isocenter/Multi-Lesion Treatments

Lung cancer patients with synchronous primary or oligometastatic (< 5 lesions) with associated co-morbidities may not retain their treatment position for the extended stereotactic body radiotherapy (SBRT) treatment times with individual isocenter plans for each lesion due to discomfort or shortness of breath. SBRT using a single-isocenter/multi-lesion (SIML) volumetric arc therapy (VMAT) plan with flattening filter free (FFF) beam could significantly shorten overall treatment time, improve patient comfort and compliance and clinic efficiency. We report clinical results of treating multiple lung metastases or synchronous primary lung cancers with SIML lung SBRT. Eighty patients with synchronous primary lung cancers or oligometastatic lung tumors (two, n = 61; three, n = 10; four, n = 4; five n = 5; total treated lesions, n = 193) were simulated with abdominal compression and/or 4D-CT MIP images and treated with a highly-conformal SIML VMAT lung SBRT plans via 3-4 non-coplanar arcs. Common prescriptions were 50-55 Gy/5 fractions and 54 Gy/3 fractions prescribed 70-80% isodose line to the each PTV. Acuros dose calculation for 6MV-FFF beam was used for tissue heterogeneity corrections. RTOG-0618/0813 criteria were used to dose constraints to organs at risk (OAR) and target conformality. Treatment was delivered every other day or twice weekly with CBCT-guidance, adjustments made with 6DOF couch corrections on a medical linear accelerator, and treatment delivery time within 15 minutes. Local control rates and toxicity profile was evaluated using CTCAE v. 5.0 grading for pneumonitis, rib fracture and chest wall pain. All plans met RTOG-0618/0813 requirements for each tumor coverage, dose to OAR including normal lung and ribs. Mean follow up after last fraction of treatment was 16.9 months (range, 1.0-54.2 months). PTV volume ranged from 2.17 to 167.8 cc with mean volume of 16.1 cc. Of the 80 patients treated, 71 had adequate post-treatment thoracic CT imaging to assess local control. Local control was achieved in 167/175 (95.4%) of treated and followed lesions. CTCAE grade 1 asymptomatic pneumonitis was noted on thoracic CT scans in 42/71 (59.2%) of patients and occurred, on average 4.8 months after SBRT. Symptomatic pneumonitis and rib fracture did not occur in any patient. CTCAE grade 2 chest wall pain occurred in 4/80 (5.0%) treated patients and was managed conservatively with over-the-counter NSAIDS or acetaminophen. SIML lung SBRT for synchronous primary lung cancers or multiple lung metastases can be used as a variant to traditional multiple isocenter SBRT or chronologically separate treatment courses, and has excellent local control rates and low toxicity profile in our patient population. It can help improve comfort and compliance of the patients who have difficulty lying still for an extended treatment course, and significantly reduces treatment time via isocenter shifts/repeated CBCTs for image guidance, thus improving clinic efficiency.

Uncertainties in the dosimetric heterogeneity correction and its potential effect on local control in lung SBRT

Objective. Dose calculation in lung stereotactic body radiation therapy (SBRT) is challenging due to the low density of the lungs and small volumes. Here we assess uncertainties associated with tissue heterogeneities using different dose calculation algorithms and quantify potential associations with local failure for lung SBRT. Approach. 164 lung SBRT plans were used. The original plans were prepared using Pencil Beam Convolution (PBC, n = 8) or Anisotropic Analytical Algorithm (AAA, n = 156). Each plan was recalculated with AcurosXB (AXB) leaving all plan parameters unchanged. A subset (n = 89) was calculated with Monte Carlo to verify accuracy. Differences were calculated for the planning target volume (PTV) and internal target volume (ITV) Dmean[Gy], D99%[Gy], D95%[Gy], D1%[Gy], and V100%[%]. Dose metrics were converted to biologically effective doses (BED) using α/β = 10Gy. Regression analysis was performed for AAA plans investigating the effects of various parameters on the extent of the dosimetric differences. Associations between the magnitude of the differences for all plans and outcome were investigated using sub-distribution hazards analysis. Main results. For AAA cases, higher energies increased the magnitude of the difference (ΔDmean of −3.6%, −5.9%, and −9.1% for 6X, 10X, and 15X, respectively), as did lung volume (ΔD99% of −1.6% per 500cc). Regarding outcome, significant hazard ratios (HR) were observed for the change in the PTV and ITV D1% BEDs upon univariate analysis (p = 0.042, 0.023, respectively). When adjusting for PTV volume and prescription, the HRs for the change in the ITV D1% BED remained significant (p = 0.039, 0.037, respectively). Significance. Large differences in dosimetric indices for lung SBRT can occur when transitioning to advanced algorithms. The majority of the differences were not associated with local failure, although differences in PTV and ITV D1% BEDs were associated upon univariate analysis. This shows uncertainty in near maximal tumor dose to potentially be predictive of treatment outcome.

Predicted Inferior Outcomes for Lung SBRT With Treatment Planning Systems That Fail Independent Phantom-Based Audits

More than 15% of radiotherapy clinics fail external audits with anthropomorphic phantoms conducted by Imaging and Radiation Oncology Core-Houston (IROC-H) while passing other industry-standard quality assurance (QA) tests. We seek to evaluate the predicted impact of such failed plans on outcomes for patients treated with stereotactic body radiation therapy (SBRT) for lung tumors. We conducted a retrospective study of 55 patients treated with SBRT for lung tumors with prescription BED10 ≥100Gy using a treatment planning system (TPS) that passed IROC-H phantom audits. Sample linac beam models with introduced errors were commissioned by varying the multi-leaf collimator leaf tip offset parameter (i.e., 'dosimetric leaf gap') over the range +/-1.0 mm relative to the validated model. These models mimic TPS that pass internal QA measures but fail IROC-H tests. Patient plans were recalculated on sample beam models. Predicted tumor control probability (TCP) and normal tissue complication probability (NTCP) were calculated based on published dose-response models. A leaf tip offset value of -1.0 mm decreased the fraction of plans receiving a PTV BED10 ≥100 Gy from 95% to 27%. This translated to a significant decrease in 2-yr TCP of 4.8% (95% CI: 2.0-5.5%) with a decrease in TCP up to 21%. Conversely, leaf tip offset of +1.0 mm resulted in 36% of patients exceeding previously met OAR constraints, including two instances of spinal cord and brachial plexus overdoses and a small increase in chest wall NTCP by 0.7%, (95% CI 0.5%-0.8%). Simulated treatment plans with modest MLC leaf offsets result in lung SBRT plans that significantly under-dose tumor or exceed OAR constraints. These dosimetric endpoints translate to significant detriments in TCP. These simulated plans mimic planning systems that pass internal QA measures but fail independent phantom-based tests, underscoring the need for enhanced quality assurance including external audits of TPS.

Treatment plan comparison of volumetric-modulated arc therapy to intensity-modulated radiotherapy in lung stereotactic body radiotherapy using either 6- or 10-MV photon energies.

PurposeThe aim of this study was to dosimetrically compare volumetric‐modulated arc therapy (VMAT) with intensity‐modulated radiotherapy (IMRT) techniques using either 6‐ or 10‐MV photon beam energies in lung stereotactic body radiation therapy (SBRT) plans.MethodsThirty patients with primary or metastatic lung tumors eligible for SBRT were randomly selected. VMAT and IMRT treatment plans using either 6‐ or 10‐MV photon energies were generated through automatic SBRT planning software in the RayStation treatment planning system.ResultsFor planning target volume, there was no difference in D 95% for all plans, whereas D 2% and D 50% were significantly increased by 5.22%–5.98% and 2.47%–2.59%, respectively, using VMAT6/10‐MV plans compared to IMRT6/10‐MV plans. When comparing the D max of organs at risk (OARs), VMAT6/10‐MV was 18.32%–47.95% lower than IMRT6/10‐MV for almost all OARs. VMAT6/10‐MV obviously decreased D mean, V 5Gy, V 10Gy, and V 20Gy of whole lung by 9.68%–20.92% than IMRT6/10‐MV. Similar results were found when comparing VMAT6‐MV with IMRT10‐MV or VMAT10‐MV with IMRT6‐MV. The differences in the D 2%, heterogeneity index, and conformity index between 6‐ and 10‐MV plans are not statistically significant. Plans using 6‐MV performed 4.68%–8.91% lower levels of D max of spinal cord, esophagus, great vessels, and trachea and proximal bronchial tree than those using 10‐MV plans. Similarly, D mean, V 5Gy, V 10Gy, and V 20Gy of whole lung were also reduced by 2.79%–5.25% using 6‐MV. For dose fall‐off analysis, the D 2cm and R 50% of VMAT6/10‐MV were lower than those of IMRT6/10‐MV. Dose fall‐off curve based on 10 rings was steeper for VMAT plans than IMRT plans regardless of the energy used.ConclusionsFor lung SBRT plans, VMAT‐based plans significantly reduced OARs dose and steepened dose fall‐off curves compared to IMRT‐based plans. A 6‐MV energy level was a better choice than 10‐MV for lung SBRT. In addition, the dose differences between different techniques were more obvious than those between different energy levels.

Feasibility of using ring-mounted Halcyon Linac for single-isocenter/two-lesion lung stereotactic body radiation therapy.

PurposeTo demonstrate the plan quality and delivery efficiency of volumetric‐modulated arc therapy (VMAT) with the Halcyon Linac ring delivery system (RDS) in the treatment of single‐isocenter/two‐lesion lung stereotactic body radiation therapy (SBRT).Materials/methodsSixteen previously treated non‐coplanar VMAT single‐isocenter/two‐lesion lung SBRT plans delivered with SBRT‐dedicated C‐arm TrueBeam Linac were selected. Prescribed dose was 50 Gy to each lesion over five fractions with treatment delivery every other day and AcurosXB algorithm as the final dose calculation algorithm. TrueBeam single‐isocenter plans were reoptimized for Halcyon Linac with coplanar geometry. Both TrueBeam and Halcyon plans were normalized for identical combined target coverage and evaluated. Conformity indices (CIs), heterogeneity index (HI), gradient index (GI), gradient distance (GD), and D 2cm were compared. The normal lung V5Gy, V10Gy, V20Gy, mean lung dose (MLD), and dose to organs at risk (OAR) were evaluated. Treatment delivery parameters, including beam‐on time, were recorded.ResultsHalcyon plans were statistically similar to clinically delivered TrueBeam plans. No statistical differences in target conformity, dose heterogeneity, or intermediate‐dose spillage were observed (all, p > 0.05). Halcyon plans, on average, demonstrated statistically insignificant reduced maximum dose to most adjacent OAR and normal lung. However, Halcyon yielded statistically significant lower maximal dose to the ribs (p = 0.041) and heart (p = 0.026), dose to 1 cc of ribs (p = 0.035) and dose to 5 cc of esophagus (p = 0.043). Plan complexity slightly increased as seen in the average increase of total monitor units, modulation factor, and beam‐on time by 480, 0.48, and 2.78 min, respectively. However, the estimated overall treatment time was reduced by 2.22 min, on average. Mean dose delivery accuracy of clinical TrueBeam plans and the corresponding Halcyon plans was 98.9 ± 0.85% (range: 98.1%–100%) and 98.45 ± 0.99% (range: 97.9%–100%), respectively, demonstrating similar treatment delivery accuracy.ConclusionSBRT treatment of synchronous lung lesions via single‐isocenter VMAT on Halcyon RDS is feasible and dosimetrically equivalent to clinically delivered TrueBeam plans. Halcyon provides excellent plan quality and shorter overall treatment time that may improve patient compliance, reduce intrafraction movement, improve clinic efficiency, and potentially offering lung SBRT treatments for underserved patients on a Halcyon only clinic.

Dosimetric Comparison of Single-Isocenter and Multiple-Isocenter Techniques for Two-Lesion Lung SBRT Using the RefleXion High-Speed Ring-Gantry System

Due to the technical challenges that accompany stereotactic body radiotherapy (SBRT) to multiple lesions, a single-isocenter treatment plan technique for multi-target scenarios may optimize the final, aggregate dose distribution. The RefleXion™ X1 is a new ring-gantry based system with several features that may optimize dosimetry in a multi-target SBRT scenario, including high speed rotation (60 revolutions per minute), a high-speed multi-leaf collimator (100 Hz), and a variable pitch couch. A comprehensive dosimetric comparison for multiple targets that contrasts multiple isocenters versus a single-isocenter technique has not been performed. Therefore, this investigation performed such a comparison in two-lesion lung SBRT cases.Three patients with two synchronous lung lesions with maximum dimension of 5 cm underwent single-isocenter and two-isocenter planning with the RefleXion treatment planning system using a Collapsed Cone Convolution Superposition algorithm. Treatment doses were in accordance with AAPM TG101 criteria (50Gy in 5 fractions) and planned such that 95% of the planning target volume (PTV) would receive at least 95% of the prescription dose. In the single-isocenter plans, the isocenter was placed between the two-lesions, whereas the two-isocenter plans localized the isocenters on each target with a limitation on the IEC-X direction to avoid couch-bore collisions. Conformity index (CI), homogenous index (HI), R50%, and D2cm were calculated. The volume of the normal lung receiving 5 Gy (V5), V10, V15, V20, mean lung dose, and dose to other OARs were evaluated. In addition, the treatment time to deliver each plan was recorded.The mean isocenter to the tumor distance in XYZ-space was 8.53 cm (range: 7.66-10.62 cm). The mean combined PTV was 42.47cc (range: 9.7- 97.8 cc). There was no significant difference in target coverage, maximum target dose, CI, HI, R50%, V20, V15, and V10 between single-isocenter and two-isocenters for lung SBRT plans. On average, single-isocenter plans contributed an increase in OAR dose, especially to the carina and esophagus, by 5.17% (range: 2.91-6.52%) and 15.62% (range: 6.44-22.97%), respectively. With the RefleXion X1 set at 850MU/min, the average difference in total beam-on time was approximately 33 seconds between the two approaches. However, set-up time for the second isocenter in the two-isocenter plan would likely prolong the total treatment delivery time.This study shows that a single-isocenter technique for the scenario of two lung lesions is dosimetrically feasible and clinically efficient with the RefleXion X1 in SBRT mode. Single-isocenter treatment delivery techniques may be a desirable option for efficient radio-ablative therapies directed at multiple metastatic targets.

Fast generation of lung SBRT plans with a knowledge-based planning model on ring-mounted Halcyon Linac.

PurposeTo demonstrate fast treatment planning feasibility of stereotactic body radiation therapy (SBRT) for centrally located lung tumors on Halcyon Linac via a previously validated knowledge‐based planning (KBP) model to support offline adaptive radiotherapy.Materials/methodsTwenty previously treated non‐coplanar volumetric‐modulated arc therapy (VMAT) lung SBRT plans (c‐Truebeam) on SBRT‐dedicated C‐arm Truebeam Linac were selected. Patients received 50 Gy in five fractions. c‐Truebeam plans were re‐optimized for Halcyon manually (m‐Halcyon) and with KBP model (k‐Halcyon). Both m‐Halcyon and k‐Halcyon plans were normalized for identical or better target coverage than clinical c‐Truebeam plans and compared for target conformity, dose heterogeneity, dose fall‐off, and dose tolerances to the organs‐at‐risk (OAR). Treatment delivery parameters and planning times were evaluated.Results k‐Halcyon plans were dosimetrically similar or better than m‐Halcyon and c‐Truebeam plans. k‐Halcyon and m‐Halcyon plan comparisons are presented with respect to c‐Truebeam. Differences in conformity index were statistically insignificant in k‐Halcyon and on average 0.02 higher (p = 0.04) in m‐Halcyon plans. Gradient index was on average 0.43 (p = 0.006) lower and 0.27 (p = 0.02) higher for k‐Halcyon and m‐Halcyon, respectively. Maximal dose 2 cm away in any direction from target was statistically insignificant. k‐Halcyon increased maximal target dose on average by 2.9 Gy (p < 0.001). Mean lung dose was on average reduced by 0.10 Gy (p = 0.004) in k‐Halcyon and increased by 0.14 Gy (p < 0.001) in m‐Halcyon plans. k‐Halcyon plans lowered bronchial tree dose on average by 1.2 Gy. Beam‐on‐time (BOT) was increased by 2.85 and 1.67 min, on average for k‐Halcyon and m‐Halcyon, respectively. k‐Halcyon plans were generated in under 30 min compared to estimated dedicated 180 ± 30 min for m‐Halcyon or c‐Truebeam plan.Conclusion k‐Halcyon plans were generated in under 30 min with excellent plan quality. This adaptable KBP model supports high‐volume clinics in the expansion or transfer of lung SBRT patients to Halcyon.

Automation and integration of a novel restricted single-isocenter stereotactic body radiotherapy (a-RESIST) method for synchronous two lung lesions.

Synchronous treatment of two lung lesions using a single‐isocenter volumetric modulated arc therapy (VMAT) stereotactic body radiation therapy (SBRT) plan can decrease treatment time and reduce the impact of intrafraction motion. However, alignment of both lesions on a single cone beam CT (CBCT) can prove difficult and may lead to setup errors and unacceptable target coverage loss. A Restricted Single‐Isocenter Stereotactic Body Radiotherapy (RESIST) method was created to minimize setup uncertainties and provide treatment delivery flexibility. RESIST utilizes a single‐isocenter placed at patient’s midline and allows both lesions to be planned separately but treated in the same session. Herein is described a process of automation of this novel RESIST method. Automation of RESIST significantly reduced treatment planning time while maintaining the benefits of RESIST. To demonstrate feasibility, ten patients with two lung lesions previously treated with a single‐isocenter clinical VMAT plan were replanned manually with RESIST (m‐RESIST) and with automated RESIST (a‐RESIST). a‐RESIST method automatically sets isocenter, creates beam geometry, chooses appropriate dose calculation algorithms, and performs VMAT optimization using an in‐house trained knowledge‐based planning model for lung SBRT. Both m‐RESIST and a‐RESIST showed lower dose to normal tissues compared to manually planned clinical VMAT although a‐RESIST provided slightly inferior, but still clinically acceptable, dose conformity and gradient indices. However, a‐RESIST significantly reduced the treatment planning time to less than 20 min and provided a higher dose to the lung tumors. The a‐RESIST method provides guidance for inexperienced planners by standardizing beam geometry and plan optimization using DVH estimates. It produces clinically acceptable two lesions VMAT lung SBRT plans efficiently. We have further validated a‐RESIST on phantom measurement and independent pretreatment dose verification of another four selected 2‐lesions lung SBRT patients and implemented clinically. Further development of a‐RESIST for more than two lung lesions and refining this approach for extracranial oligometastastic abdominal/pelvic SBRT, including development of automated simulated collision detection algorithm, merits future investigation.

Dosimetric evaluation of MRI-guided multi-leaf collimator tracking and trailing for lung stereotactic body radiation therapy.

PurposeThe treatment margins for lung stereotactic body radiotherapy (SBRT) are often large to cover the tumor excursions resulting from respiration, such that underdosage of the tumor can be avoided. Magnetic resonance imaging (MRI)‐guided multi‐leaf collimator (MLC) tracking can potentially reduce the influence of respiration to allow for smaller treatment margins. However, tracking is accompanied by system latency that may induce residual tracking errors. Alternatively, a simpler mid‐position delivery combined with trailing can be used. Trailing reduces influences of respiration by compensating for baseline motion, to potentially improve target coverage. In this study, we aim to show the feasibility of MRI‐guided tracking and trailing to reduce influences of respiration during lung SBRT.MethodsWe implemented MRI‐guided tracking on the MR‐linac using an Elekta research tracking interface to track tumor motion during intensity modulated radiotherapy (IMRT). A Quasar MRI4D phantom was used to generate Lujan motion (cos4, 4 s period, 20 mm peak‐to‐peak amplitude) with and without 1.0 mm/min cranial drift. Phantom tumor positions were estimated from sagittal 2D cine‐MRI (4 or 8 Hz) using cross‐correlation‐based template matching. To compensate the anticipated system latency, a linear ridge regression predictor was optimized for online MRI by comparing two predictor training approaches: training on multiple traces and training on a single trace. We created 15‐beam clinical‐grade lung SBRT plans for central targets (8 × 7.5 Gy) and peripheral targets (3 × 18 Gy) with different PTV margins for mid‐position based motion management (3–5 mm) and for MLC tracking (3 mm). We used a film insert with a 3 cm spherical target to measure the spatial distribution and quantity of the delivered dose. A 1%/1 mm local gamma‐analysis quantified dose differences between motion management strategies and reference cases. Additionally, a dose area histogram (DAH) revealed the target coverage relative to the reference scenario.ResultsThe prediction filter gain was on average 25% when trained on multiple traces and 44% when trained on a single trace. The filter reduced system latency from 313 ± 2 ms to 0 ± 5 ms for 4 Hz imaging and from 215 ± 3 ms to 3 ± 3 ms for 8 Hz. The local gamma analysis for the central delivery showed that tracking improved the gamma pass‐rate from 23% to 96% for periodic motion and from 14% to 93% when baseline drift was applied. For the peripheral delivery during periodic motion, delivery pass‐rates improved from 22% to 93%. Comparing mid‐position delivery to trailing for periodic+drift motion increased the local gamma pass rate from 15% to 98% for a central delivery and from 8% to 98% for a peripheral delivery. Furthermore, the DAHs revealed a relative D98% GTV coverage of 101% and 97% compared to the reference scenario for, respectively, central and peripheral tracking of periodic+drift motion. For trailing, a relative D98% of 99% for central and 98% for peripheral trailing was found.ConclusionsWe provided a first experimental demonstration of the technical feasibility of MRI‐guided MLC tracking and trailing for central and peripheral lung SBRT. Tracking maximizes the sparing of healthy tissue, while trailing is highly effective in mitigating baseline motion.

Development and clinical validation of a robust knowledge-based planning model for stereotactic body radiotherapy treatment of centrally located lung tumors.

PurposeTo develop a robust and adaptable knowledge‐based planning (KBP) model with commercially available RapidPlanTM for early stage, centrally located non‐small‐cell lung tumors (NSCLC) treated with stereotactic body radiotherapy (SBRT) and improve a patient's“simulation to treatment” time.MethodsThe KBP model was trained using 86 clinically treated high‐quality non‐coplanar volumetric modulated arc therapy (n‐VMAT) lung SBRT plans with delivered prescriptions of 50 or 55 Gy in 5 fractions. Another 20 independent clinical n‐VMAT plans were used for validation of the model. KBP and n‐VMAT plans were compared via Radiation Therapy Oncology Group (RTOG)–0813 protocol compliance criteria for conformity (CI), gradient index (GI), maximal dose 2 cm away from the target in any direction (D2cm), dose to organs‐at‐risk (OAR), treatment delivery efficiency, and accuracy. KBP plans were re‐optimized with larger calculation grid size (CGS) of 2.5 mm to assess feasibility of rapid adaptive re‐planning.ResultsKnowledge‐based plans were similar or better than n‐VMAT plans based on a range of target coverage and OAR metrics. Planning target volume (PTV) for validation cases was 30.5 ± 19.1 cc (range 7.0–71.7 cc). KBPs provided an average CI of 1.04 ± 0.04 (0.97–1.11) vs. n‐VMAT plan'saverage CI of 1.01 ± 0.04 (0.97–1.17) (P < 0.05) with slightly improved GI with KBPs (P < 0.05). D2cm was similar between the KBPs and n‐VMAT plans. KBPs provided lower lung V10Gy (P = 0.003), V20Gy (P = 0.007), and mean lung dose (P < 0.001). KBPs had overall better sparing of OAR at the minimal increased of average total monitor units and beam‐on time by 460 (P < 0.05) and 19.2 s, respectively. Quality assurance phantom measurement showed similar treatment delivery accuracy. Utilizing a CGS of 2.5 mm in the final optimization improved planning time (mean, 5 min) with minimal or no cost to the plan quality.ConclusionThe RTOG‐compliant adaptable RapidPlan model for early stage SBRT treatment of centrally located lung tumors was developed. All plans met RTOG dosimetric requirements in less than 30 min of planning time, potentially offering shorter “simulation to treatment” times. OAR sparing via KBPs may permit tumorcidal dose escalation with minimal penalties. Same day adaptive re‐planning is plausible with a 2.5‐mm CGS optimizer setting.

An Automated knowledge-based planning routine for stereotactic body radiotherapy of peripheral lung tumors via DCA-based volumetric modulated arc therapy.

PurposeTo develop a knowledge‐based planning (KBP) routine for stereotactic body radiotherapy (SBRT) of peripherally located early‐stage non‐small‐cell lung cancer (NSCLC) tumors via dynamic conformal arc (DCA)‐based volumetric modulated arc therapy (VMAT) using the commercially available RapidPlanTM software. This proposed technique potentially improves plan quality, reduces complexity, and minimizes interplay effect and small‐field dosimetry errors associated with treatment delivery.MethodsKBP model was developed and validated using 70 clinically treated high quality non‐coplanar VMAT lung SBRT plans for training and 20 independent plans for validation. All patients were treated with 54 Gy in three treatments. Additionally, a novel k‐DCA planning routine was deployed to create plans incorporating historical three‐dimensional‐conformal SBRT planning practices via DCA‐based approach prior to VMAT optimization in an automated planning engine. Conventional KBPs and k‐DCA plans were compared with clinically treated plans per RTOG‐0618 requirements for target conformity, tumor dose heterogeneity, intermediate dose fall‐off and organs‐at‐risk (OAR) sparing. Treatment planning time, treatment delivery efficiency, and accuracy were recorded.ResultsKBPs and k‐DCA plans were similar or better than clinical plans. Average planning target volume for validation was 22.4 ± 14.1 cc (7.1–62.3 cc). KBPs and k‐DCA plans provided similar conformity to clinical plans with average absolute differences of 0.01 and 0.01, respectively. Maximal doses to OAR were lowered in both KBPs and k‐DCA plans. KBPs increased monitor units (MU) on average 1316 (P < 0.001) while k‐DCA reduced total MU on average by 1114 (P < 0.001). This routine can create k‐DCA plan in less than 30 min. Independent Monte Carlo calculation demonstrated that k‐DCA plans showed better agreement with planned dose distribution.ConclusionA k‐DCA planning routine was developed in concurrence with a knowledge‐based approach for the treatment of peripherally located lung tumors. This method minimizes plan complexity associated with model‐based KBP techniques and improve plan quality and treatment planning efficiency.

Assessment of the Air-Electron Streaming Effect for MRI Guided Lung SBRT: A Treatment Planning Study

Electron streams outside the body are inevitably generated in the presence of magnetic field during MRI guided radiotherapy. This study aims to investigate the air-electron streaming effects for lung Stereotactic Body Radiotherapy (SBRT) treated on the 1.5T MRI Linac. Three lung SBRT cases with prescription dose ranging from 30Gy to 60Gy in four to five fractions were investigated in this study. The original curative treatment plan was designed by using step-and-shoot intensity modulated radiotherapy (IMRT) technique and then calculated with Monaco (v5.40.01) Monte Carlo dose engine under the 1.5 T magnetic field. To investigate the effect of air-electron streaming, another comparable treatment plan without 1.5T magnetic field was generated using the same planning parameters. In order to display the in-air dose distribution outside the body in Monaco system, an auxiliary structure was created by extending the external with margin of 8cm in anterior. The two plans with or without magnetic field were then compared in terms of in air dose distribution outside the body and dose to the chin. Electron stream effects outside the body were obviously observed for all 3 lung SBRT cases when in the presence of magnetic field, while which were not observed in the absence of magnetic field. From sagittal view, the electron stream can travel a long distance in air along the magnetic field direction, even can reach to the patient’s chin. But only two of the three cases have CT scan including the chin region. For that two cases, the chin surface skin doses were 1.47Gy (2.45% of 60Gy) and 2.24Gy (7.47% of 30Gy) respectively. While for the plans without magnetic field, the corresponding chin surface skin dose were 0.18Gy (0.3% of 60Gy) and 0.32Gy (1.07% of 30Gy) respectively. For the last case, the streaming electrons were expected to reach at the chin area, but due to which was not scanned in CT image, it’s hard to evaluate quantitatively. Electron streaming effects outside the body were also observed in MRI guided lung SBRT plans and which could lead to a higher chin dose compared to conventional scenario without magnetic field. It is therefore recommended for the MRI guided lung SBRT patients that the chin region should be scanned and included in planning CT image. Other suggestions for example shielding with bolus or keeping the patient’s head tilted to the opposite side should also be considered.

Comprehensive Dosimetric Evaluation of a Biology-Guided Radiotherapy Machine in Treatment Plans for Brain, Lung, Head and Neck, Esophagus, and Prostate Malignancies

A new biology-guided radiotherapy (BgRT) delivery machine is currently under development. It combines a compact 6 MV linear accelerator and a binary multileaf collimator with PET, kVCT, and MV imaging systems on the same ring gantry that rotates continuously at 60 rotations per minute while the patient is translated through the bore with small increments. We aimed to perform inter-comparison of treatment plans for typical clinical disease sites between the new BgRT machine and existing IMRT delivery techniques. To ensure objective comparison, biology guidance was not assumed in planning for the new system and the same planning margins were used for each case with all the delivery techniques. We retrospectively retrieved one clinical external-beam radiotherapy treatment plan for each of the five disease sites: intracranial lesion for stereotactic radiosurgery (SRS), early-stage lung lesion for stereotactic body radiotherapy (SBRT), head and neck, esophagus, and prostate with seminal vesicle and pelvic lymph node treatments. Both a helical tomotherapy (HT) plan (using a 1-cm or 2-cm jaw size) and a volumetric modulated arc therapy (VMAT) plan (using 0.5-cm MLC or 0.25-cm MLC leaves) were generated for each clinical case. A treatment plan (the prototype plan) for the new system was generated for each case using either a 1-cm jaw size for the intracranial SRS and lung SBRT plans, and a 2-cm jaw size for the other sites. Dosimetric constraints in national radiotherapy treatment protocols (RTOG-0813, NRG-HN001, RTOG-1010, and RTOG-0534) were used to facilitate dosimetric comparison of treatments plans. The prototype plans met all the dosimetric constraints in all disease sites. The relative strengths with each delivery technique differ with respect to the disease site. With the head & neck case, while the VMAT and HT plans shows similar normal organ dose, the prototype plan achieved comparable PTV dose coverage and lower dose to most critical organs including a 40% reduction in parotid mean dose compared to the HT and the VMAT plans. With the intracranial SRS case, the VMAT plan achieved slightly better dose conformity and faster dose drop-off compared to the HT and prototype plans, while all the plans achieved comparable dose to the normal brain and other organs-at-risk. With the lung SBRT case, both the VMAT and the prototype plans achieve similar dose to the normal lung volume and other organs. In both the prostate and esophagus cases, mixed results were observed with one technique achieving better sparing to certain organs. While all the delivery technique met given dosimetric constraints in most disease sites, the prototype system was shown to achieve better normal organ sparing with the head & neck site, while the VMAT technique with smaller MLC leaves was more suitable for intracranial SRS. Actual dosimetric measurements will follow to verify dose delivery accuracy with the new BgRT machine.

Similar-cases-based planning approaches with beam angle optimizations using water equivalent path length for lung stereotactic body radiation therapy.

This study aimed to propose automated treatment planning approaches based on similar cases with beam angle optimizations using water equivalent path length (WEPL) to avoid lung and rib doses for lung stereotactic body radiation therapy (SBRT). Similar cases to an objective case were defined as cases, which were close to the objective case with respect to the Euclidean distances based on geometrical features. Initial similar-case-based (ISC) plans were generated by applying lung SBRT plans of similar cases to objective cases. Similar cases were selected using the Euclidean distances based on lung shape and geometrical features from a radiation treatment planning database with 174 cases. Beam angles of the ISC plans were optimized using a greedy algorithm based on a cost function to include absorbed doses in the lung and ribs in the WEPL. The 12 dose evaluation indices for the planning target volume, lung, spinal cord, and ribs were evaluated in the original plans, ISC plans, and optimized similar-case-based (OSC) plans with and without WEPL for 20 test cases to investigate its dosimetric impact. These findings revealed that V10 and the mean dose for the lung and V20, V30, and V40 for the ribs in the OSC plan with WEPL improved more significantly than those in the original and ISC plans. This study indicates a potential of similar cases, whose beam angle configurations were optimized with WEPL to avoid lung and rib doses in lung SBRT plans.

Evaluation of the target dose coverage of stereotactic body radiotherapy for lung cancer using helical tomotherapy: A dynamic phantom study.

To evaluate the target dose coverage for lung stereotactic body radiotherapy (SBRT) using helical tomotherapy (HT) with the internal tumor volume (ITV) margin settings adjusted according to the degree of tumor motion. Lung SBRT with HT may cause a dosimetric error when the target motion is large. Two lung SBRT plans were created using a tomotherapy planning station. Using these original plans, five plans with different ITV margins (4.0-20.0mm for superior-inferior [SI] dimension) were generated. To evaluate the effects of respiratory motion on HT, an original dynamic motion phantom was developed. The respiratory wave of a healthy volunteer was used for dynamic motion as the typical tumor respiratory motion. Five patterns of motion amplitude that corresponded to five ITV margin sizes and three breathing cycles of 7, 14, and 28 breaths per minute were used. We evaluated the target dose change between a static delivery and a dynamic delivery with each motion pattern. The target dose difference increased as the tumor size decreased and as the tumor motion increased. Although a target dose difference of <5 % was observed at ≤10mm of tumor motion for each condition, a maximum difference of -9.94 % ± 7.10 % was observed in cases of small tumors with 20mm of tumor motion under slow respiration. Minimizing respiratory movement is recommended as much as possible for lung SBRT with HT, especially for cases involving small tumors.

Clinical evaluation of photon optimizer (PO) MLC algorithm for stereotactic, single-dose of VMAT lung SBRT

Recently implemented photon optimizer (PO) MLC optimization algorithm is mandatory for RapidPlan modeling in Eclipse. This report quantifies and compares the dosimetry and treatment delivery parameters of PO vs its predecessor progressive resolution optimizer (PRO) algorithm for a single-dose of volumetric modulated arc therapy (VMAT) lung stereotactic body radiation therapy (SBRT). Clinical SBRT treatment plans for 12 early-stage non–small-cell lung cancer patients receiving 30 Gy in 1 fraction using PRO-VMAT were re-optimized using the PO-VMAT MLC algorithm with identical planning parameters and objectives. Average planning target volume derived from the 4D CT scans was 13.6 ± 12.0 cc (range: 4.3 to 41.1 cc) Patients were treated with 6 MV flattening filter free beam using Acuros-based calculations and 2.5 mm calculation grid-size (CGS). Both treatment plans were normalized to receive same target coverage and identical CGS to isolate effects of MLC positioning optimizers. Original PRO and re-optimized PO plans were compared via RTOG–0915 protocol compliance criteria for target conformity, gradient indices, dose to organs at risks and delivery efficiency. Additionally, PO-VMAT plans with a 1.25 mm CGS were evaluated. Both plans met RTOG protocol requirements. Conformity indices showed no statistical difference between PO 2.5 mm CGS and PRO 2.5 mm CGS plans. Gradient index (p = 0.03), maximum dose to 2 cm away from planning target volume in any direction (D2cm) (p < 0.05), and gradient distance (p < 0.05) presented statistically significant differences for both plans with 2.5 mm CGS. Some organs at risks showed statistically significant differences for both plans calculated with 2.5 mm CGS; however, no clinically significant dose differences were observed between the plans. Beam modulation factor was statistically significant for both PO 1.25 mm CGS (p = 0.001) and PO 2.5 mm CGS (p < 0.001) compared to clinical PRO 2.5 mm CGS plans. PO-VMAT plans provided decreased beam-on time by an average of 0.2 ± 0.1 minutes (up to 1.0 minutes) with PO 2.5 mm and 1.2 ± 0.39 minutes (maximum up to 3.22 minutes) with PO 1.25 mm plans compared to PRO 2.5 mm plans. PO-VMAT single-dose of VMAT lung SBRT plans showed slightly increased intermediate-dose spillage but boasted overall similar plan quality with less beam modulation and hence shorter beam-on time. However, PO 1.25 mm CGS had less intermediate-dose spillage and analogous plan quality compared to clinical PRO-VMAT plans with no additional cost of plan optimization. Further investigation into peripheral targets with PO-MLC algorithm is warranted. This study indicates that PO 1.25 mm CGS plans can be used for RapidPlan modeling for a single dose of lung SBRT patients. PO-MLC 1.25 mm algorithm is recommended for future clinical single-dose lung SBRT plan optimization.

Comparing Maximum Dose to Central Airways in Mid-Ventilation Versus Average Intensity Projection-Based 4DCT Planning for Lung Stereotactic Body Radiation Therapy (SBRT)

Four-Dimensional Computed Tomography (4DCT) is typically used in radiation treatment planning for improved targeting of intrathoracic tumors. While tumor definition uses all in- and expiration image sets to account for the full range of respiratory motion, the definition of organs at risk (OARs) is less well defined. We investigated OAR dose differences for two commonly used image sets for SBRT dose calculation: average intensity projection (AIP) and mid-point in ventilation (midV). Determining accurate OAR doses is particularly important for proximal airways, as RTOG 0813 results have shown that increased dose to these structures can result in unacceptable, at times fatal toxicity. Fifteen 4DCT-based lung SBRT plans originally generated on 30% MidV image sets were transferred to associated AIP images. Fractionation schedules were 50 Gy in 5 fractions in 5, 48 Gy in 4 fractions in 9, and 40 Gy in 5 fractions in 1 plan. Internal gross tumor volumes (iGTV) were delineated on maximum intensity projection (MIP) images. OARs were contoured per RTOG 0813 protocol on both MidV and AIP datasets. Maximum doses were reported for each OAR (trachea, proximal bronchial tree (PBT), heart, and great vessels (GV)) on original MidV plans and after plan transfer to AIP image sets with field arrangements and monitor unit (MU) values unchanged. AIP volumes were, on average, greater by 4.26 ± 5.44 cc for trachea and 13.11 ± 12.21 for PBT, and reduced by 21.6 ± 70.42 for heart and 6.06 ± 41.14 cc for GV. Average superior-inferior movement of tumor and carina were 7.1 ± 4.1 and 7.6 ± 4.7 mm, respectively. The average absolute difference between AIP and MidV in maximum doses to trachea, PBT, heart, and GV were 1.01 ± 1.37, 1.68 ± 3.10, 1.45 ± 2.05, and 1.07 ± 1.21 Gy, respectively. Maximum dose to PBT was increased in 3 of 15 (20%) plans after transfer to AIP image sets, with an average increase of 3.03 ± 2.97 Gy (0.09 to 7.10). Of the remaining 12 plans (80%), this was decreased by an average of 1.35 ± 3.04 Gy (0.01 to 11.31). Fourteen (93%) plans exhibited an absolute difference in maximum dose of > 1 Gy to at least one OAR. Absolute difference in maximum dose to heart showed a positive correlation with carina motion (R2 = 0.61), and GV a weak positive correlation with tumor motion (R2 = 0.40). Our results show a difference in maximum dose to OARs between AIP and MidV image sets. These differences are confounded by tumor and carina motion and associated differences in contoured volumes. Although this study does not allow to identify the better image set, AIP images are more likely to account for a true positional average, and AIP-based plans are expected to be more representative of the true maximum OAR dose. To calculate correct OAR doses, both tumor and OARs require contouring and planning on all respiratory phases which might be necessary for critical central structures such as PBT, if they are in the high dose area.

Evaluation of plan quality and treatment efficiency for single-isocenter/two-lesion lung stereotactic body radiationtherapy.

Purpose/objectivesTo evaluate the plan quality and treatment delivery efficiency of single‐isocenter/two‐lesions volumetric modulated arc therapy (VMAT) lung stereotactic body radiation therapy (SBRT).Materials/methodsEight consecutive patients with two peripherally located early stage nonsmall‐cell‐lung cancer (NSCLC) lung lesions underwent single‐isocenter highly conformal noncoplanar VMAT SBRT treatment in our institution. A single‐isocenter was placed between the two lesions. Doses were 54 or 50 Gy in 3 and 5 fractions respectively. Patients were treated every other day. Plans were calculated in Eclipse with AcurosXB algorithm and normalized to at least 95% of the planning target volume (PTV) receiving 100% of the prescribed dose. For comparison, two‐isocenter plans (isocenter placed centrally in each target) were retrospectively created. Conformity indices (CIs), heterogeneity index (HI), gradient index (GI), gradient distance (GD), and D2cm were calculated. The normal lung V5, V10, V20, mean lung dose (MLD) and other organs at risk (OARs) doses were evaluated. Total number of monitor units (MUs), beam‐on time, and patient‐specific quality assurance (QA) results were recorded.ResultsThe mean isocenter to tumor distance was 6.7 ± 2.3 cm. The mean combined PTV was 44.0 ± 23.4 cc. There was no clinically significant difference in CI, HI, GD, GI, D2cm, and V20 including most of the OARs between single‐isocenter and two‐isocenter lung SBRT plans, evaluated per RTOG guidelines. However, for single‐isocenter plans as the distance between the lesions increased, the V5, V10, and MLD increased, marginally. The total number of MUs and beam‐on time was reduced by a factor of 1.5 for a single‐isocenter plan compared to a two‐isocenter plan. The single‐isocenter/two‐lesions VMAT lung SBRT QA plans demonstrated an accurate dose delivery of 98.1 ± 3.2% for clinical gamma passing rate of 3%/3 mm.ConclusionThe SBRT treatment of two peripherally located lung lesions with a centrally placed single‐isocenter was dosimetrically equivalent to two‐isocenter plans. Faster treatment delivery for single‐isocenter treatment can improve patient compliance and reduce the amount of intrafraction motion errors for well‐suited patients.

The dosimetric differences in calculating lung SBRT plans on different image data sets: Comparison of the free breathing scan to both the average intensity projection scan and to the sum of calculations on each respiratory phase of the 4DCT scan

The purpose of this study is to evaluate the dosimetric differences to the lung internal target volume (ITV) in stereotactic body radiation therapy (SBRT) when calculated on the free breathing (FB) scan in comparison to calculations on the average intensity projection (AIP) scan as well as the sum of dose calculated on each of the treated respiratory phases. The clinical treatment plan data for 16 SBRT lung patients were retrospectively chosen for this study, 5 of which were 30% to 70% respiratory phase gated. All patients had ITV contours and fixed monitor units from the treatment approved plan copied over to each scan on which calculations were to be made. The results of this study yielded 6 out of 16 patients with greater than or equal to 2% difference in ITV maximum dose (D0) and 2 of 16 patients with greater than or equal to 2% difference in ITV minimum dose (D100). The range of ITV dose differences for these 8 patients was 2% to 4.7% with no patients exceeding a 5% difference in D0 or D100. None of the patients had greater than or equal to 2% difference in ITV mean dose (Dmean). Sixty-three percent of patients with greater than 2% ITV dose difference in any of the categories reviewed were those patients with greater than 1 cm gross tumor motion. This study concluded that in order to reduce uncertainty in dose to the ITV, tumor motion should be assessed at simulation and limited to below 1 cm in any direction if possible.