High Dose Spillage Research Articles

Radiotherapy dose escalation using pre-treatment diffusion-weighted imaging in locally advanced rectal cancer: a planning study.

Diffusion-weighted MRI (DWI) may provide biologically relevant target volumes for dose-escalated radiotherapy in locally advanced rectal cancer (LARC). This planning study assessed the dosimetric feasibility of delivering hypofractionated boost treatment to intra-tumoural regions of restricted diffusion prior to conventional long-course radiotherapy. Ten patients previously treated with curative-intent standard long-course radiotherapy (50 Gy/25#) were re-planned. Boost target volumes (BTVs) were delineated semi-automatically using 40th centile intra-tumoural apparent diffusion coefficient value with expansions (anteroposterior 11 mm, transverse 7 mm, craniocaudal 13 mm). Biased-dosed combined plans consisted of a single-fraction volumetric modulated arc therapy flattening-filter-free (VMAT-FFF) boost (phase 1) of 5, 7, or 10 Gy before long-course VMAT (phase 2). Phase 1 plans were assessed with reference to stereotactic conformality and deliverability measures. Combined plans were evaluated with reference to standard long-course therapy dose constraints. Phase 1 BTV dose targets at 5/7/10 Gy were met in all instances. Conformality constraints were met with only 1 minor violation at 5 and 7 Gy. All phase 1 and combined phase 1 + 2 plans passed patient-specific quality assurance. Combined phase 1 + 2 plans generally met organ-at-risk dose constraints. Exceptions included high-dose spillage to bladder and large bowel, predominantly in cases where previously administered, clinically acceptable non-boosted plans also could not meet constraints. Targeted upfront LARC radiotherapy dose escalation to DWI-defined is feasible with appropriate patient selection and preparation. This is the first study to evaluate the feasibility of DWI-targeted upfront radiotherapy boost in LARC. This work will inform an upcoming clinical feasibility study.

KV-CBCT-Guided Daily Adaptation Improves Target Coverage and Spares Normal Tissue for Accelerated Partial Breast Irradiation

Linear accelerator based accelerated partial breast irradiation (APBI) in early-stage breast cancer necessitates a reduction in non-target breast tissue to decrease long term toxicity and adverse cosmetic outcome. In particular, lumpectomy cavity seromas may decrease in size and deform in shape during the course of short course APBI. Online adaptive RT (oART) offers considerable prospect for treating mobile targets through pendulous breast anatomy. However, there are limited studies outlining the methods and outcomes of CBCT-based oART for APBI. Here we present a retrospective, single institutional study analyzing the treatment process for patients receiving stereotactic kV-CBCT guided oART APBI. Fourteen patients were treated to 30 Gy in 5 fractions for a total of 70 fractions. Time between simulation and treatment, change in gross tumor volume (GTV), and differences in DVH metrics with adaption were analyzed. The Wilcoxon paired, non-parametric test was utilized to test for DVH metric differences between the scheduled plans (initial plans recalculated on daily CBCT anatomy) and treated plans, which were either the scheduled or adapted plan (initial plans re-optimized using daily anatomy), depending on the preference of the treating physician or physicist. Median (inter-quartile range) time from simulation to first treatment was 28 days (21-33 days). During this same time, GTV volume reduced to 72.5% (57.8-87.3%) of the simulation lumpectomy cavity volume. Adaptive treatments required 31.1 min (27.2-37.1 min) from start of CBCT to treatment session end. Table 1 summarizes differences between scheduled and treated plan metrics for 70 fractions, 62 (89%) of which were treated adaptively and 8 (11%) of which were treated using the scheduled plan. Significant improvement in prescription planning target volume coverage (p = 0.003), significant reduction in 5/6 organ-at-risk metrics evaluated (p ≤ 0.003), and significant improvement in conformity index and high dose spillage (p ≤ 0.001) were realized with adaption. Table 1: Scheduled versus treated plan metrics. APBI using oART decreased most organs at risk DVH metrics, improved plan quality metrics, and increased target coverage, justifying the use of kV-CBCT-guided oART for APBI.

Cleaning the dose falloff with low modulation in SBRT lung plans

Purpose. This dosimetric study is intended to lower the modulation factor in lung SBRT plans generated in the Eclipse TPS that could replace highly modulated plans that are prone to the interplay effect. Materials and methods. Twenty clinical lung SBRT plans with high modulation factors (≥4) were replanned in Varian Eclipse TPS version 15.5 utilizing 2 mm craniocaudal and 1 mm axial block margins followed by light optimization in order to reduce modulation. A unique plan optimization methodology, which utilizes a novel shell structure (OptiForR50) for R50% optimization in addition to five consecutive concentric 5 mm shells, was utilized to control dose falloff according to RTOG 0813 and 0915 recommendations. The prescription varied from 34–54 Gy in 1–4 fractions, and the dose objectives were PTV D95% = Rx, PTV Dmax < 140% of Rx, and minimizing the modulation factor. Plan evaluation metrics included modulation factor, CIRTOG, homogeneity index (HI), R50%, D2cm, V105%, and lung V8–12.8Gy (Timmerman Constraint). A random-intercept linear mixed effects model was used with a p ≤ 0.05 threshold to test for statistical significance. Results. The retrospectively generated plans had significantly lower modulation factors (3.65 ± 0.35 versus 4.59 ± 0.54; p < 0.001), lower CIRTOG (0.97 ± 0.02 versus 1.02 ± 0.06; p = 0.001), higher HI (1.35 ± 0.06 versus 1.14 ± 0.04; p < 0.001), lower R50% (4.09 ± 0.45 versus 4.56 ± 0.56; p < 0.001), and lower lungs V8–12.8Gy (Timmerman) (4.61% ± 3.18% versus 4.92% ± 3.37%; p < 0.001). The high dose spillage V105% was borderline significantly lower (0.44% ± 0.49% versus 1.10% ± 1.64%; p = 0.051). The D2cm was not statistically different (46.06% ± 4.01% versus 46.19% ± 2.80%; p = 0.835). Conclusion. Lung SBRT plans with significantly lower modulation factors can be generated that meet the RTOG constraints, using our planning strategy.

Advancing Precision in Post-mastectomy Chest Wall Radiotherapy: A Comparative Dosimetric Analysis of Volumetric-Modulated Arc Therapy (VMAT) and Intensity-Modulated Radiotherapy (IMRT) Based on Institutional Experience.

Post-mastectomy radiation therapy (PMRT) is an important component in the management of breast cancer patients who have undergone mastectomy. Intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) are two popular methods of delivering PMRT. With IMRT, high radiation doses are directed at the tumor, while exposure to healthy tissue is kept to a minimum. VMAT, on the other hand, is a more advanced version of IMRT that allows for faster radiation dose delivery while maintaining precision. The complexity of the VMAT treatment planning and delivery process, on the other hand, may increase the risk of technical errors, which can reduce treatment effectiveness. Studies have compared VMAT and IMRT in PMRT for breast cancer patients, but most have found no significant differences in treatment outcomes between the two methods. Individual patient factors such as treatment goals, available resources, and other characteristics may influence the choice between the two techniques. This prospective observational study aimed to compare the dosimetry of two cutting-edge modern radiotherapy techniques for post-mastectomy breast cancer patients receiving hypofractionated doses. For 58 patients with breast cancer, 116 plans for radiotherapy treatment were generated by both VMAT and IMRT. To maintain the uniformity of contouring, every CT image was contoured by the same physician, and Radiotherapy Oncology Group (RTOG) contouring guidelines were strictly followed during contouring. Both techniques had comparable target volume coverage, but VMAT produced a significantly better conformity index than IMRT for both the left (0.71 vs. 0.65) and right (0.72 vs. 0.66) breasts (p-value < 0.05). VMAT plans had significantly higher low-dose spillage to the ipsilateral lung (V5Gy and V10Gy) but significantly lower high-dose spillage (V20Gy, V30Gy, and V40Gy) than IMRT plans (p-value < 0.05). Dmax and Dmean for the ipsilateral lung were comparable for both techniques. When compared to alternative treatment approaches, IMRT treatment plans were found to be more effective in minimizing radiation exposure to the heart for all patients with right-sided breast cancer, resulting in considerably lower levels of Dmean, V5Gy, V10Gy, V20Gy, and V35Gy. Plans for VMAT treatment were found to be significantly superior to left-side chest wall radiotherapy in terms of lower exposure to the heart for higher doses. IMRT plans, on the other hand, were successful in dramatically lowering the levels of Dmax that reached the spinal cord for both right- and left-sided breast cancers. Apart from similar planning target volume (PTV) coverage to IMRT plans, VMAT produced significantly better conformity. VMAT plans have more low-dose spillage to normal tissues, while IMRT plans spare various organs at risk significantly better at lower doses in both right and left-sided breast cancer. VMAT was found to be better at sparing the heart (in left-sided breast cancer only) and ipsilateral lung at a high dose range. The best radiotherapy approach for breast cancer should be established on an individual basis, taking into account tumor laterality and the risk-benefit ratio.

Dosimetric comparison of mDCAT and VMAT techniques according to 6MV-FFF and 10MV-FFF energies in patients with single adrenal metastasis.

To compare dosimetric and radiobiological terms of modified dynamic conformal arc therapy (mDCAT) and volumetric modulated arc therapy (VMAT) techniques using different flattening-filter free (FFF) energies in patients with single adrenal metastasis. In this study, plans were prepared for 10 patients drawing on the mDCAT and VMAT techniques with 6MV-FFF and 10MV-FFF energies. Target volume doses, biological effective doses (BED), quality indices, Monitor Unit (MU), number of segments, beam-on time and critical organ doses were compared in the plans. Plans with the significantly lower gradient index (GI) and conformity index (CI) values were obtained with 6MV-FFF energy VMAT planning (p < 0.05). The higher values were obtained for dose to 95% of internal target volume (ITVD95), ITVD95-BED10 with 10MV-FFF energy VMAT planning, whereas lower results were obtained for high dose spillage (HDS%) values (p < 0,05). With 10MV-FFF energy, HDS% values were 21.1% lower in VMAT plans and 5.6% lower in mDCAT plans compared to 6MV-FFF energy. Plans with approximately 50% fewer segments were obtained in mDCAT plans than VMAT plans (p < 0,05). Beam-on time values with mDCAT was 1.84 times lower when 6MV-FFF energies were analyzed, and 2.11 times lower when 10MV-FFF was analyzed (p < 0,05). Additionally, when 6MV-FFF and 10MV-FFF energies were examined, MU values with mDCAT were 2.1 and 2.5 times lower (p < 0,05). In general, the smaller the target volume size, the greater the differences between MU and beam-on time values mDCAT and VMAT. The study results implied that VMAT enabled to offer significantly more conformal SBRT plans with steeper dose fall-off beyond the target volume for single adrenal metastasis than the mDCAT, which attained at the cost of significantly higher MU and beam-on times. Especially with 10MV-FFF energy mDCAT plans, low-dose-bath zones can be reduced, and shorter-term treatments can be implemented with large segments. In adrenal gland SBRT, higher effective doses can be achieved with the right energy and technique, critical organ doses can be reduced, thus increasing the possibility of local control of the tumor with low toxicity.

Development and evaluation of a three-step automatic planning technique for lung SBRT based on performance examination of advanced settings in Pinnacle's auto-planning module

The benefits of Pinnacle's auto-planning module on clinical practice have been well documented. However, little is known regarding the efficiency of its Advanced Settings and the practicality of incorporating this module into Stereotactic Body Radiation Therapy (SBRT), which is why this research was conducted. To characterize the impact of Advanced Settings on plan quality, a total of 25 previously delivered postoperative cervical cases were re-planned and evaluated. Then a three-step automatic planning technique was developed and tested on ten lung SBRT cases based on the investigation. The differences between plans with fine-tuned Advanced Settings and the default were compared using a Wilcoxon signed-rank test with a significance threshold of 5%. The same statistical analysis was implemented to examine the quality variations in manual and automatic SBRT planning. When the Tuning Balance, Dose Fall-Off Margin, and Hot-Spot Maximum Goal were set to 100%, 1 cm, and 250%, respectively, better organ-at-risk (OAR) sparing was reached, but target quality was compromised. The OAR dose reduction and target homogeneity deterioration showed a strong correlation. The three-step methodology improved high dose spillage while saving time, with statistically significant reductions of 66.7% in V105% of non-PTV and 58.1% in planning time to the human-driven strategy. Except for urgent requirements for sparing OARs or processing SBRT plans, keeping the default is appropriate for Advanced Settings. The three-step methodology automatically searches for the available solution with purposeful Advanced Settings adjustments, demonstrating its ability to produce high-quality plans in less time. For the inexperienced or under-resourced clinics, our procedure can be introduced as a robust and handy strategy in SBRT, notably for expedited quality planning.

Plan quality and consistency in spine radiosurgery treatment planning: comparison between automatic treatment planning with Elements Spine SRS and manual inverse planning with Varian Eclipse.

Spine radiosurgery treatment planning can be a challenging task since a high radiation dose is delivered to target volumes close to the spinal cord, therefore a steep dose gradient is required. Plan quality is greatly influenced by the planner skills, so automatic treatment planning has been proposed to overcome this issue and assure high-quality plans. The Brainlab Elements Spine SRS treatment planning system is specially designed for spine radiosurgery treatments. It is an automatic treatment planning system that works through predefined protocols, with minimal planner interaction required. In this work, we evaluated the plan quality and consistency among the planners within the same institution when using the Elements Spine SRS compared to manual inverse planning with the Varian Eclipse system. Six planners produced a plan for 3 sample target volumes representing different spine metastases in the thoracic region using both treatment planning systems. Dose prescription was 16 Gy in a single fraction, at more than 80% of the target volume. The most important organ at risk was the spinal canal. The dose constraint was V10 Gy < 0.35 cm3. High dose spillage outside the target volume, the homogeneity index, the Paddick conformity index and the number of monitor units were also evaluated. The mean dose to the target volumes in the Elements Spine SRS plans were consistently higher by 0.8 Gy to 1.5 Gy and the maximum dose to the target volumes were consistently higher by 1.8 Gy to 3.1 Gy. Spinal cord sparing was comparable to the Eclipse plans. However, the number of monitor units was greatly reduced, up to 2270 monitor units less. No difference was found in plan quality variability among the planners.

Dosimetric comparison of robotic- and LINAC-based treatment of spine stereotactic body radiotherapy.

To determine which treatment technique and modality would offer better dosimetric results and be preferable for spinal stereotactic body therapy (SBRT) depending on the three different regions of the vertebrae. Linear accelerator (LINAC)- and CyberKnife (CK)-based treatment techniques were compared in terms of their dosimetric quality, treatment efficiency, and delivery accuracy. Thirty previously treated patients were included in this study. Intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) techniques were used for LINAC-based treatment, whereas CK-based treatment plans were generated for two different collimator systems: fixed and multileaf collimator (MLC). The plans were compared based on spinal cord sparing, dose homogeneity, conformity index (CI), gradient index (GI), monitor unit (MU), and beam-on time. The percentage volumes of V2Gy, V5Gy (representing volume low of the dose spillage region), V10Gy, and V20Gy (representing the volume of the high-dose spillage region) of the healthy tissue were analyzed. The CI and GI of the VMAT plans were better than those of the IMRT plans. For spinal cord sparing, the VMAT and MLC-based CK (CK-MLC) techniques were superior. The percentage of low-dose spillage regions was the lowest for IMRT and fixed cone-based CK (CK-FIX) plans. The percentage of the high-dose spillage region was the lowest for the VMAT and CK-MLC plans. In terms of treatment efficiency, the VMAT and CK-MLC plans were superior to the IMRT and CK-FIX plans. The VMAT technique lowered the MU and beam-on time values. The plan delivery accuracy of the VMAT and CK-FIX plans was better than that of the IMRT plans. VMAT is the best option for LINAC-based spinal SBRT. For CK-based spinal SBRT, MLC-based plans are preferred. If the clinic has both treatment modalities and the patient can tolerate long treatment times, CK-MLC-based treatment should be chosen because of its superiority in sparing the spinal cord and sharp dose fall-off.

The new SRS/FSRT technique HyperArc for benign brain lesions: a dosimetric analysis

To evaluate the potential benefit of HyperArc (HA) fractionated stereotactic radiotherapy (FSRT) for the benign brain lesion. Sixteen patients with a single deep-seated, centrally located benign brain lesion treated by CyberKnife (CK, G4 cone-based model) were enrolled. Treatment plans for HA with two different optimization algorithms (SRS NTO and ALDO) and coplanar RapidArc (RA) were generated for each patient to meet the corresponding treatment plan criteria. These four FSRT treatment plans were divided into two groups—the homogeneous delivery group (HA-SRS NTO and coplanar RA) and the inhomogeneous delivery group (HA-ALDO and cone-based CK)—to compare for dosimetric outcomes. For homogeneous delivery, the brain V5, V12, and V24 and the mean brainstem dose were significantly lower with the HA-SRS NTO plans than with the coplanar RA plans. The conformity index, high and intermediate dose spillage, and gradient radius were significantly better with the HA-SRS NTO plans than with the coplanar RA plans. For inhomogeneous delivery, the HA-ALDO exhibited superior PTV coverage levels to the cone-based CK plans. Almost all the doses delivered to organs at risk and dose distribution metrics were significantly better with the HA-ALDO plans than with the cone-based CK plans. Good dosimetric distribution makes HA an attractive FSRT technique for the treatment of benign brain lesions.

Automated treatment planning of prostate stereotactic body radiotherapy with focal boosting on a fast-rotating O-ring linac: Plan quality comparison with C-arm linacs.

PurposeThe integration of auto‐segmentation and automated treatment planning methods on a fast‐rotating O‐ring linac may improve the time efficiency of online adaptive radiotherapy workflows. This study investigates whether automated treatment planning of prostate SBRT with focal boosting on the O‐ring linac could generate plans that are of similar quality as those obtained through manual planning on clinical C‐arm linacs.MethodsFor 20 men with prostate cancer, reference treatment plans were generated on a TrueBeam STx C‐arm linac with HD120 MLC and a TrueBeam C‐arm linac with Millennium 120 MLC using 6 MV flattened dual arc VMAT. Manual planning on the Halcyon fast‐rotating O‐ring linac was performed using 6 MV FFF dual arc VMAT (HA2‐DL10) and triple arc VMAT (HA3‐DL10) to investigate the performance of the dual‐layer MLC system. Automated planning was performed for triple arc VMAT on the Halcyon linac (ET3‐DL10) using the automated planning algorithms of Ethos Treatment Planning. The prescribed dose was 35 Gy to the prostate and 30 Gy to the seminal vesicles in five fractions. The iso‐toxic focal boost to the intraprostatic tumor nodule(s) was aimed to receive up to 50 Gy. Plan deliverability was verified using portal image dosimetry measurements.ResultsCompared to the C‐arm linacs, ET3‐DL10 shows increased seminal vesicles PTV coverage (D99%) and reduced high‐dose spillage to the bladder (V37Gy) and urethra (D0.035cc) but this came at the cost of increased high‐dose spillage to the rectum (V38Gy) and a higher intermediate dose spillage (D2cm). No statistically significant differences were found when benchmarking HA2‐DL10 and HA3‐DL10 with the C‐arm linacs. All plans passed the patient‐specific QA tolerance limit.ConclusionsAutomated planning of prostate SBRT with focal boosting on the fast‐rotating O‐ring linac is feasible and achieves similar plan quality as those obtained on clinical C‐arm linacs using manual planning.

The feasibility and efficacy of new SBRT technique HyperArc for recurrent nasopharyngeal carcinoma: noncoplanar cone-based robotic system vs. noncoplanar high-definition MLC based Linac system

The purpose of this study was to evaluate the feasibility and efficacy of HyperArc (HA) for recurrent nasopharyngeal cancer (NPC) by comparing it with the CyberKnife system (CK). Fifteen patients with recurrent nasopharyngeal cancer who were treated using the noncoplanar cone-based robotic CK system were enrolled. CK was delivered with a median dose of 37.5 Gy in 5 fractions. The delivered CK treatment plans were the sources for the corresponding homogeneous HA (HA-H) and inhomogeneous HA (HA-IH) plans. The HA-H plans were generated to meet the corresponding treatment plan criteria for the CK plans. The HA-IH plans were designed to emulate the corresponding inhomogeneous CK isodose distributions. These three SBRT treatment plans were compared with target coverage, sparing of organs at risk (OARs), and dose distribution metrics. The HA-H and HA-IH plans consistently exhibited CTV and PTV coverage levels similar or better to those of the CK plans but significantly reduced the dose to OARs. Using the HA techniques (both HA-H and HA-IH plans), the mean maximal doses to the spinal cord, brainstem, optic nerves, and optic chiasm were reduced by approximately 60%, compared to the CK plans. The high dose spillage, conformity, and homogeneity indices of the HA-H plans were significantly better than those of the CK plans. The HA-IH plans showed faster dose falloff and similar conformity of the HA-H plans and dose heterogeneity of the CK plans. Here we demonstrated the HA treatment plan system for recurrent NPC is feasible, either homogeneous or inhomogeneous delivery. Excellent sparing of OARs and dosimetric distribution and very efficient delivery make HA an attractive SBRT technique for recurrent NPC treatment.

Dosimetric and planning efficiency comparison for lung SBRT: CyberKnife vs VMAT vs knowledge-based VMAT

This is the first study that compared treatment plan quality and planning efficiency for lung stereotactic body radiation therapy (SBRT) using CyberKnife (CK) Multiplan vs Varian Eclipse treatment planning systems, including volumetric modulated arc therapy (VMAT) and knowledge-based VMAT (KBP-VMAT). Thirteen lung SBRT patients treated with 50 to 55 Gy in 3 or 5 fractions were retrospectively included in this study. CK plans created with Multiplan V. 4.6.1 using 2 fixed circular cones were previously approved used for treatment. For the comparison, the computed tomography (CT) data sets and contours from the CK plans were used to generate VMAT and KBP-VMAT plans (University of California San Diego publicly-shared RapidPlan model) using Eclipse V. 13.7. Metrics used for the comparison of CK, VMAT, and KBP-VMAT plans included monitor units (MUs), conformity indices, dose heterogeneity, high-dose spillage, low-dose spillage, adjacent organs at risk (OAR) doses, and treatment planning time. One-way analysis of variance with post-hoc Tukey tests and paired t-tests were used to analyze the difference of these metrics corresponding to the different planning techniques. All of the 3 planning techniques achieved our clinical goals. With similar planning target volume (PTV) coverage, CK plans yielded the most MU (p< 0.001), the least dose homogeneity (p < 0.002), and the least D2cm dose (p < 0.001), while KBP-VMAT plans resulted in the most OAR sparing. No significant difference was found among other dosimetric metrics such as high-dose spillage, lung V20 and volume receiving 50% of the prescription dose. Compared to VMAT, KBP-VMAT improved OAR sparing (p < 0.05), but required significantly more MU (p < 0.001). KBP-VMAT was associated with the shortest planning time. Eclipse-based VMAT can achieve comparable plan quality for lung SBRT as CK, in a more efficient manner. RapidPlan can facilitate the planning process of KBP-VMAT, with potentially better OAR sparing but higher MU requirements. Further improvement for KBP-VMAT is likely achievable by developing site-specific patient models.

Robust Optimization of SBRT Planning for Patients With Early Stage Non-Small Cell Lung Cancer.

Purpose:Setup uncertainty is a known challenge for stereotactic body radiotherapy planning. Using the internal target volume-based robust optimization was proposed as a more accurate way than the conventional planning target volume-based optimization when considering the robustness criteria. In this study, we aim to investigate the feasibility of internal target volume-based robust optimization in stereotactic body radiotherapy planning using 4-dimensional computed tomography and develop a novel dose–volume histogram band width metric to quantitatively evaluate robustness.Method and Materials:A total of 50 patients with early stage non-small cell lung cancer, who underwent stereotactic body radiotherapy, were retrospectively selected. Each of the 50 patients had 2 stereotactic body radiotherapy plans: one with the conventional planning target volume-based optimization and the other with patient-specific robustly optimized internal target volume and with a uniform 5 mm setup error. These were compared with the planning target volume-based optimization method based on both plan quality and robustness. The quality was evaluated using dosimetric parameters and radiobiology parameters, such as high-dose spillage (V 90%RX, conformity index), intermediate-dose spillage (dose falloff products), low-dose spillage (normal tissue: V 50%RX), and lung tissue complication probability. The robustness was evaluated under a uniform 3 to 5 mm setup errors with a novel proposed metric: dose–volume histogram band width.Results:When compared with planning target volume-based optimization plans, the internal target volume-based robust optimization plans have better conformity of internal target volume coverage (conformity index: 1.17 vs 1.27, P < .001), intermediate-dose spillage (dose falloff product: 129 vs 167, P < .001), low-dose spillage in normal tissue (V 50%RX: 0.8% vs 1.5%, P < .05), and lower risk of radiation pneumonitis (lung tissue complication probability: 4.2% vs 5.5%, P < .001). For the robustness, dose–volume histogram band width analysis shows that the average values in internal target volume, D 95%, D 98%, and D 99%, of internal target volume-based robust optimization are smaller than that of planning target volume-based optimization (unit cGy) under 3-, 4-, and 5-mm setup uncertainties (3-mm setup uncertainty: 42 vs 73 cGy; 4-mm setup uncertainty: 88 vs 176 cGy; 5-mm setup uncertainty: 229 vs 490 cGy), which might indicate that internal target volume-based robust optimization harbored a greater robustness regardless of the setup errors.Conclusions:Internal target volume-based robust optimization may have clinical potential in offering better plan quality in both target and organs at risk and lower risk of radiation pneumonitis. In addition, the proposed internal target volume-based robust optimization may demonstrate robustness regardless of different setup uncertainties in the stereotactic body radiotherapy planning.Registration:Retrospective study with local ethics committee approval.

Optimization of motion management parameters in a synchrotron-based spot scanning system.

PurposeTo quantify the effects of combining layer‐based repainting and respiratory gating as a strategy to mitigate the dosimetric degradation caused by the interplay effect between a moving target and dynamic spot‐scanning proton delivery.MethodsAn analytic routine modeled three‐dimensional dose distributions of pencil‐beam proton plans delivered to a moving target. Spot positions and weights were established for a single field to deliver 100 cGy to a static, 15‐cm deep, 3‐cm radius spherical clinical target volume with a 1‐cm isotropic internal target volume expansion. The interplay effect was studied by modeling proton delivery from a clinical synchrotron‐based spot scanning system and respiratory target motion, patterned from surrogate patient breathing traces. Motion both parallel and orthogonal to the beam scanning direction was investigated. Repainting was modeled using a layer‐based technique. For each of 13 patient breathing traces, the dose from 20 distinct delivery schemes (combinations of four gate window amplitudes and five repainting techniques) was computed. Delivery strategies were inter‐compared based on target coverage, dose homogeneity, high dose spillage, and delivery time.ResultsNotable degradation and variability in plan quality were observed for ungated delivery. Decreasing the gate window reduced this variability and improved plan quality at the expense of longer delivery times. Dose deviations were substantially greater for motion orthogonal to the scan direction when compared with parallel motion. Repainting coupled with gating was effective at partially restoring dosimetric coverage at only a fraction of the delivery time increase associated with very small gate windows alone. Trends for orthogonal motion were similar, but more complicated, due to the increased severity of the interplay.ConclusionsLayer‐based repainting helps suppress the interplay effect from intra‐gate motion, with only a modest penalty in delivery time. The magnitude of the improvement in target coverage is strongly influenced by individual patient breathing patterns and the tumor motion trajectory.

Improving Quality and Consistency in NRG Oncology Radiation Therapy Oncology Group 0631 for Spine Radiosurgery via Knowledge-Based Planning

Touse knowledge-based planning (KBP) as a method of producing high-quality, consistent, protocol-compliant treatment plans in a complex setting of spine stereotactic body radiation therapy on NRG Oncology Radiation Therapy Oncology Group (RTOG)0631. An internally developed KBP model was applied to an external validation cohort of 22 anonymized cases submitted under NRG Oncology RTOG 0631. The original and KBP plans were compared via their protocol compliance, target conformity and gradient index, dose to critical structures, and dose to surrounding normal tissues. The KBP model generated plans meeting all protocol objectives in a single optimization when tested on both internal and protocol-submitted NRG Oncology RTOG 0631 cases. Two submitted plans that were considered to have a protocol-unacceptable deviation were made protocol compliant through the use of the model. There were no statistically significant differences in protocol spinal cord metrics (D10% and D0.03cc) between the manually optimized plans and the KBP plans. The volume of planning target volume receiving prescription dose increased from 93.3%±3.2% to 98.3%±1.4% (P=.01) when using KBP. High-dose spillage to surrounding normal tissues (V105%) showed no significant differences (2.1±7.3cm3 for manual plans to 1.8±0.6cm3 with KBP), and dosimetric outliers with large amounts of spillage were eliminated through the use of KBP. Knowledge-based planning plans were also found to be significantly more consistent in several metrics, including target coverage and high dose outside of the target. Incorporation of KBP models into the clinical trial setting may have a profound impact on the quality of trial results, owing to the increase in consistency and standardization of planning, especially for treatment sites or techniques that are nonstandard.

Abstract ID: 177 A robust Monte Carlo treatment planning optimization algorithm for dose painting clinical implementation

Purpose In order to start accurate clinical trials based on treatments more aggressive than traditional margin approaches, a robust optimization algorithm has been developed for dose calculation full Monte Carlo-based. Specifically, the algorithm is presented here to manage uncertainties on dose painting from PET/CT image data. Material and Methods CARMEN platform [1] was updated to allow heterogeneous dose prescription by means the recurrent both approaches. For the approach considered as true dose painting by number (tDPBN) where the restriction of dose to volumes makes no sense, it was necessary to develop a novel algorithm including an optimization method at the voxel level under Lineal Programming (LP) formulation [2] . For the approach based on the discretization of functional information into several clusterings (DPBN), instead of a recurrent equidistant isolevel, we implemented several algorithms able to reflect the diffuse and multifocal nature of the uptake regions. For this study, the affinity propagation proposed by Foster [3] in order to reduce random errors due to the PET images registration process. Full Monte Carlo simulations were performed for pre-optimization and final dose calculation for taking into account the interactions of particles by means an explicit transport along the beam modifiers in the linac head. Axesse/Synergy linacs of Elekta were modellized with the EGSnrc/BEAMnrc code. The dose calculation in patient was carried out with the BEAMDOSE code, a modified version of DOSXYZnr for calculate the specific beamlet dose contribution on each voxel. A grid calculation consisting on 256 × 256 voxels per slice was used from the interpolation of PET/CT images reconstructed by keeping a compromise with EARL (ResEARch4Life®) accreditation requirements. Linear Programming formulation at voxel level allowed stablishing a tractable robustness of the uncertainties related to the heterogeneous dose prescription, imposing lower and upper-bound constraints to each voxel in accordance to the clustering volume to which they belong. For tDPBN, an inverse planning schema was previously developed [4] . For DPBN by clusterings approach, a specific direct aperture optimization (BIOMAP) based on the sequencing of biophysical maps [2] has been modified to generate apertures with Boolean combinations of the clustering projections. Results tDPBN by means inverse planning showed excellent QVHs, although they were achieved by means of solutions with high MUs (around 2000 MU/fraction with more than 350 segments). Secondary contribution from MLC meant a high dose spillage to the body, so cannot be directly assumed for clinical implementation. The robust solutions for DPBN by means BIOMAP allow the accurate clinical implementation (around 750 MU/fraction with less than 200 segments) with Q index (planned_dose_matrix/true_prescribed_dose_matrix) over 95%.

Improving Quality and Consistency in Clinical Trials via Knowledge-Based Planning, NRG Oncology RTOG 0631

Consistency and standardization of radiotherapy planning quality for conducting a multi-institutional clinical trial are always challenging, and non-protocol compliant radiotherapy plans have been shown to impact patient outcomes. This study was aimed to utilize knowledge-based planning (KBP) as a method of producing high quality, consistent, protocol compliant treatment plans, in a complex setting of spine SBRT on RTOG 0631. An internally developed KBP model was applied to an external validation cohort of 22 anonymized cases submitted to NRG for RTOG 0631. The original and KBP plans were compared via their protocol scores, target conformity and gradient index, dose to critical structures, and dose to surrounding normal tissues. The KBP model generated plans meeting all protocol objectives in a single optimization when tested on both internal and external RTOG 0631 cases. Two external plans that were considered to have a protocol-unacceptable deviation were made protocol compliant through the use of the model. There were no statistically significant differences in conformity or gradient indices between the manually optimized plans and the KBP plans. The volume of PTV receiving prescription dose increased from 93.3 ± 3.2% to 99.5 ± 0.7% (P<0.001) when using KBP. High-dose spillage to surrounding normal tissues (V105%) showed no significant differences (2.1 ± 7.3 cc for manual plans to 1.2 ± 0.4 cc with KBP), and dosimetric outliers with large amounts of spillage were eliminated through the use of KBP. Incorporation of KBP models into the clinical trial setting may have a profound impact on the quality of trial results due to the increase in consistency and standardization of planning, especially for treatment sites or techniques that are non-standard. The KBP model will be made available on the Center for Innovation in Radiation Oncology website.

Deterministic direct aperture optimization using multiphase piecewise constant segmentation.

Direct aperture optimization (DAO) attempts to incorporate machine constraints in the inverse optimization to eliminate the post-processing steps in fluence map optimization (FMO) that degrade plan quality. Current commercial DAO methods utilize a stochastic or greedy approach to search a small aperture solution space. In this study, we propose a novel deterministic direct aperture optimization that integrates the segmentation of fluence map in the optimization problem using the multiphase piecewise constant Mumford-Shah formulation. The Mumford-Shah based direct aperture optimization problem was formulated to include an L2-norm dose fidelity term to penalize differences between the projected dose and the prescribed dose, an anisotropic total variation term to promote piecewise continuity in the fluence maps, and the multiphase piecewise constant Mumford-Shah function to partition the fluence into pairwise discrete segments. A proximal-class, first-order primal-dual solver was implemented to solve the large scale optimization problem, and an alternating module strategy was implemented to update fluence and delivery segments. Three patients of varying complexity-one glioblastoma multiforme (GBM) patient, one lung (LNG) patient, and one bilateral head and neck (H&N) patient with 3 PTVs-were selected to test the new DAO method. For each patient, 20 non-coplanar beams were first selected using column generation, followed by the Mumford-Shah based DAO (DAOMS ). For comparison, a popular and successful approach to DAO known as simulated annealing-a stochastic approach-was replicated. The simulated annealing DAO (DAOSA ) plans were then created using the same beam angles and maximum number of segments per beam. PTV coverage, PTV homogeneity D95D5, and OAR sparing were assessed for each plan. In addition, high dose spillage, defined as the 50% isodose volume divided by the tumor volume, as well as conformity, defined as the van't Riet conformation number, were evaluated. DAOMS achieved essentially the same OAR doses compared with the DAOSA plans for the GBM case. The average difference of OAR Dmax and Dmean between the two plans were within 0.05% of the plan prescription dose. The lung case showed slightly improved critical structure sparing using the DAOMS approach, where the average OAR Dmax and Dmean were reduced by 3.67% and 1.08%, respectively, of the prescription dose. The DAOMS plan substantially improved OAR dose sparing for the H&N patient, where the average OAR Dmax and Dmean were reduced by over 10% of the prescription dose. The DAOMS and DAOSA plans were comparable for the GBM and LNG PTV coverage, while the DAOMS plan substantially improved the H&N PTV coverage, increasing D99 by 6.98% of the prescription dose. For the GBM and LNG patients, the DAOMS and DAOSA plans had comparable high dose spillage but slightly worse conformity with the DAOMS approach. For the H&N plan, DAOMS was considerably superior in high dose spillage and conformity to the DAOSA . The deterministic approach is able to solve the DAO problem substantially faster than the simulated annealing approach, with a 9.5- to 40-fold decrease in total solve time, depending on the patient case. A novel deterministic direct aperture optimization formulation was developed and evaluated. It combines fluence map optimization and the multiphase piecewise constant Mumford-Shah segmentation into a unified framework, and the resulting optimization problem can be solved efficiently. Compared to the widely and commercially used simulated annealing DAO approach, it showed comparable dosimetry behavior for simple plans, and substantially improved OAR sparing, PTV coverage, PTV homogeneity, high dose spillage, and conformity for the more complex head and neck plan.

Dosimetric trade-offs in breast treatment with VMAT technique.

Breast planning with volumetric modulated arc therapy (VMAT) has been explored, especially for left-sided breast treatments, with the primary intent of lowering the heart dose and improving target dose homogeneity. As a trade-off, larger healthy tissue volumes would receive low dose levels, with the potential risk of increasing late toxicities and secondary cancer induction, although no clinical data are available today to confirm the risk level. The scope of this work is to explore the dosimetric trade-offs using two different VMAT plans. Two planning strategies for dual-partial-arc VMAT, RA_avoid and RA_full, with and without avoidance sectors, were explored in a cohort of 20 patients, for whole left breast irradiation for 40.05 Gy to the mean target dose in 15 fractions. Common dose objectives included a stringent dose homogeneity, mean dose to the heart <5 Gy, ipsilateral lung (Ilung) <8 Gy, contralateral lung (Clung) <2 Gy and contralateral breast (Cbreast) <3 Gy. RA_full showed a better dose conformity, lower high-dose spillage in the healthy tissue and lower skin dose. RA_avoid presented a reduction of the mean doses for all critical structures: 51% to the heart, 12% to the Ilung, 81% to the Clung and 73% to the Cbreast. All differences were significant with p < 0.0001. The adaptation of VMAT options to planning objectives reduced significantly the healthy tissue dose levels at the price of some high-dose spillage. Evaluation of the trade-offs for application to the different critical structures should drive in improving the usage of the VMAT technique for breast cancer treatment. Advances in knowledge: Different planning strategies in the same VMAT technique could give significant variations in dose distributions. The choice of the trade-offs would affect the possible future late toxicity and secondary cancer induction risk.

SU‐F‐T‐635: Lung SBRT: Dosimetric and Treatment Time Comparison of Volumetric‐Modulated Arc Therapy and Three‐Dimensional Conformal Radiotherapy in Clinically Treated Cases

Purpose:To compare three‐dimensional conformal radiotherapy (3D CRT) and volumetric‐modulated arc therapy (VMAT) in lung stereotactic body radiation therapy (SBRT)Methods:A retrospective study of clinically treated lung SBRT cases treated between 2010 and 2015 at our hospital was performed. All treatment modalities were included in this evaluation (VMAT, 3D CRT, static IMRT, and dynamic conformal arc therapy). However, the majority of treatment modalities were either VMAT or 3D CRT. Treatment times of patients and dosimetric plan quality metrics were compared. Treatment times were calculated based on the time the therapist opened and closed the patient's treatment plan. This treatment time closely approximates the utilization time of the treatment room. The dosimetric plan quality metrics evaluated include ICRU conformity index, the volume of 105% prescribed dose outside PTV, the ratio of volume of 50% prescribed dose to the volume of PTV, the percentage of maximum dose at 2 cm away from PTV to the prescribed dose, and the V20 (percentage of lung volume receiving 20 Gy or more).Results:Treatment time comparisons show that on average VMAT has shorter treatment times than 3D CRT. Dose conformity, defined by the ICRU conformity index, and high dose spillage, defined by the volume of 105% dose outside the PTV, is reduced when using VMAT compared to 3D CRT. V20 and intermediate dose spillage/fall‐off metrics of VMAT and 3D are not significantly different.Conclusion:Clinically treated lung SBRT cases indicate VMAT is superior to 3D with regard to shorter treatment times, plan dose conformity, and plan high dose spillage.