Stereotactic Body Radiotherapy Plans Research Articles

Evaluation of a complementary metal oxide semiconductor detector as a tool for stereotactic body radiotherapy plan quality assurance.

A sub-mm resolution Complementary Metal Oxide Semiconductor sensor has been developed for stereotactic radiotherapy quality assurance. Herein we evaluate its basic dosimetric performance and its application for linac C-arm stereotactic body radiotherapy (SBRT) plan quality assurance. The detector was integrated into its accompanying phantom or in Water Equivalent Plastic (WEP). The measurement reproducibility, stability, dose linearity and dependence on angularity, dose rate and field size were investigated. Clinical plan measurements were compared to our radiotherapy treatment planning system and radiochromic film. Sensitivity to introduced Multi Leaf Collimator (MLC) offsets was evaluated by simulating single MLC offsets in SBRT plans and comparing measurements to expected doses. Signal reproducibility was within±0.1% and output calibration was stable over a 6month period. Detector showed good linearity with dose (r2=1). Signal decreased by 5% when dose rate was decreased from 1300 MU/min to 300 MU/min. Output factors agreed within 0.5% of chamber measurements for 1x1 cm field sizes or greater. Angularity measurements showed good agreement with reference. For measurement of planned clinical doses, gamma pass-rates were 98.5%±2.3% (treatment planning system reference, 2%/2mm) and 99.2%±1.0% (film reference, 2%,2mm). The detector also showed sensitivity to errors of 1mm offsets in MLC positioning. The detector performed well when used for pre-treatment SBRT plan quality assurance, offering a good alternative to radiochromic film.

Lung SBRT treatment planning: a study of VMAT arc selection guided by collision check software.

To evaluate the effects of arc geometry on lung stereotactic body radiation therapy (SBRT) plan quality, using collision check software to select safe beam angles. Thirty lung SBRT cases were replanned 10Gy x 5 using 4 volumetric modulated arc therapy (VMAT) geometries: coplanar lateral (cpLAT), coplanar oblique (cpOBL), noncoplanar lateral (ncpLAT) and noncoplanar oblique (ncpOBL). Lateral arcs spanned 180° on the affected side whereas the 180° oblique arcs crossed midline to spare healthy tissues. Couch angles were separated by 30° on noncoplanar plans. Clearance was verified with Radformation CollisionCheck software. Optimization objectives were the same across the four plans for each case. Planning target volume (PTV) coverage was set to 95% and then plans were evaluated for dose conformity, healthy tissue doses, and monitor units. Clinically treated plans were used to benchmark the results. The volumes of the 25%, 50% and 75% isodoses were smaller with noncoplanar than coplanar arcs. The volume of the 50% isodose line relative to the PTV (CI50%) was as follows: clinical 3.75±0.72, cpLAT 3.39 ± 0.37, cpOBL 3.36 ± 0.34, ncpLAT 3.02 ± 0.21 and ncpOBL 3.02 ± 0.22. The Wilcoxon signed rank test with Bonferroni correction showed p < 0.005 in all CI50% comparisons except between the cpLat and cpObl arcs and between the ncpLat and ncpObl arcs. The best lung sparing was achieved using ncpObl arcs, which was statistically significant (p < 0.001) compared with the other four plans at V12.5Gy, V13.5Gy and V20Gy. Chest wall V30Gy was significantly better using noncoplanar arcs in comparison to the other plan types (p < 0.001). The best heart sparing at V10Gy from the ncpOBL arcs was significant compared with the clinical and cpLat plans (p < 0.005). Arc geometry has a substantial effect on lung SBRT plan quality. Noncoplanar arcs were superior to coplanar arcs at compacting the dose distribution at the 25%, 50% and 75% isodose levels, thereby reducing the dose to healthy tissues. Further healthy tissue sparing was achieved using oblique arcs that minimize the pathlength through healthy tissues and avoid organs at risk. The dosimetric advantages of the noncoplanar and oblique arcs require careful beam angle selection during treatment planning to avoid collisions during treatment, which may be facilitated by commercial software.

Dosimetry implications of VMAT delivery angles and intra-fraction motion for SBRT prostate treatments.

For a cohort of retrospectively treated patients with prostate carcinoma using a Stereotactic body radiation therapy (SBRT) technique and with implanted localization fiducials, this study determined whether there were any correlations between theoretical fiducial visibility using intra-fraction megavoltage imaging and intra-fraction motion induced dosimetric effects. Treatment planning data for 20 retrospective patients that used a SBRT prostate technique were investigated in this study. An in-house script was created that split each of the 360-degree volumetric modulated arc therapy arcs into 12 sectors, with each sector being 30 degrees. The script created a total of 24 sectors for each SBRT plan ranging from 180 to 210 degrees to 180 to 150 degrees. Resulting data was assessed to determine whether there was dosimetric impact from intra-fractional prostate motion and if it correlated with the theoretical fiducial visibility. Forty percent of the 4 highest CTV D98% mean dose differences occurred within the angles of 240 and 270 degrees and 25% occurred between the angles of 90 and 120 degrees. The 4 highest average percentage differences in PTV D98% coverage during angular sectors of 270 to 240 degrees, 90 to 120 degrees, 240 to 270 degrees and 60 to 90 degrees were -1.19%, -1.14%, -1.10%, and 1.01% respectively. It was also the case for the PTV D95%in sectors 90 to 120 degrees, 240 to 270 degrees, 270 to 240 degrees and 270 to 300 degrees, which reduced by -0.97%, -0.93%, -0.92%, and -0.82% respectively. From the 4 highest rectal dose differences in V32Gy and V18Gy, it was found that 50% of the highest rectum V32Gyand V18Gy dose increases occurred between the angles of 90 and 120 degrees and 37.5% occurred between the angles of 240 and 270 degrees. The MU for each sector showed that 240_270, 240_210, 270_240, and 120_90 had the highest average MU with 150.8, 134.6, 129.2, and 124.3 respectively. This study demonstrated that there is a high correlation between dosimetric impact of intra-fractional motion and the theoretical fiducial visibility. As a result, modification to treatment plans to enable fiducial visibility at all angular sectors throughout a treatment may not be required. Further sector analysis tests are required to develop patient specific megavoltage imaging gantry angles for SBRT prostate patients.

Dose Prescription to Isodose Lines in Static Multi-Beam Stereotactic Body Radiotherapy for Lung Tumors: Which Line Is Optimal?

This study investigated the appropriate dose prescription method in static multi-beam stereotactic body radiotherapy for lung tumors. Static multi-beam stereotactic body radiotherapy is a mainstream treatment in Japan. Based on the hypothesis that dose prescription to lower isodose lines may improve planning target volume dose coverage and decrease doses to organs at risk, we investigated changes in dose-volume histograms with prescription to various isodose lines for planning target volume in static multi-beam stereotactic body radiotherapy. In all treatment plans, 45 Gy in 4 fractions were prescribed to 95% of the planning target volume. By adjusting the leaf margins of each beam, various prescription isodose lines encompassing 95% volume of the planning target volume were generated. The prescription isodose lines investigated were 40, 50, 60, 70, 80 and 90% lines relative to the maximum dose of each planning target volume. The conformity index, homogeneity index, mean lung dose, and V5-V40 of the lung were evaluated. The dose was calculated by the adaptive convolve algorithm. The conformity index was lowest in the 70% or 80% isodose plan. The mean lung doses and V10-V40 of the lung decreased steeply from the 90% to the 70% isodose plan, and was lowest in the 60% and 70% isodose plans. These indices increased in the 40% and 50% isodose plans. The optimal stereotactic body radiotherapy plans appeared to be dose prescription to the 60% or 70% isodose line. Further investigation is warranted to clarify the advantage of using this method clinically.

Early Detection of Leptomeningeal Metastases Among Patients Undergoing Spinal Stereotactic Radiosurgery

PurposeThe management of patients with advanced solid malignancies increasingly uses stereotactic body radiation therapy (SBRT). Advanced cancer patients are at risk for developing leptomeningeal metastasis (LM), a fatal complication of metastatic cancer. Cerebrospinal fluid (CSF) is routinely collected during computed tomography (CT) myelography for spinal SBRT planning, offering an opportunity for early LM detection by CSF cytology in the absence of radiographic LM or LM symptoms (subclinical LM). This study tested the hypothesis that early detection of tumor cells in CSF in patients undergoing spine SBRT portends a similarly poor prognosis compared with clinically overt LM. Methods and MaterialsWe retrospectively analyzed clinical records for 495 patients with metastatic solid tumors who underwent CT myelography for spinal SBRT planning at a single institution from 2014 to 2019. ResultsAmong patients planned for SBRT, 51 (10.3%) developed LM. Eight patients (1.6%) had subclinical LM. Median survival with LM was similar between patients with subclinical versus clinically evident LM (3.6 vs 3.0 months, P = .30). Patients harboring both parenchymal brain metastases and LM (29/51) demonstrated shorter survival than those with LM alone (2.4 vs 7.1 months, P = .02). ConclusionsLM remains a fatal complication of metastatic cancer. Subclinical LM detected by CSF cytology in spine SBRT patients has a similarly poor prognosis compared with standardly detected LM and warrants consideration of central nervous system-directed therapies. As aggressive local therapies are increasingly used for metastatic patients, more sensitive CSF evaluation may further identify patients with subclinical LM and should be evaluated prospectively.

A dosimetric comparison for SBRT plans of localized prostate cancer between Cyberknife and Varian Truebeam STX device

As the Stereotactic Body Radiotherapy (SBRT) approach began to increase in treating patients with localized prostate cancer, it became necessary to investigate which methods used in practice were better. The aim of this study is to perform a dosimetric comparison of the advantages and disadvantages of SBRT treatments for localized prostate cancer delivered by CyberKnife (CK) and Varian Truebeam STX (FF and FFF). Seventeen intermediate and high-risk patients with localized prostate cancer were included in the study. SBRT plans for the CK system and Varian Truebeam STX systems with and without Flattening Filters (Tru-FF and Tru-FFF) were prepared for each patient. Plans prepared for each patient were planned at a fraction dose of 6.7 Gy at 6 MV energy and a target dose of 33.5 Gy in 5 fractions. For all plans, cumulative dose-volume histograms (DVHs) were generated for target volumes and organs at risk (OAR). The maximum doses of PTV (41 Gy) in CK plans are higher than the maximum doses (35 Gy) in VMAT plans prepared with Tru-FF or Tru-FFF beams. The mean dose of the rectal wall (10.06 ± 2.40Gy for CK) is still relatively low compared to other plans (13.46 ± 2.16 Gy for Tru-FF and 13.61 ± 2.32 Gy for Tru-FFF). The bladder wall (14 Gy for CK, 26 Gy for Tru-FF and Tru-FFF) and femoral head (6.8 Gy for CK, 9 Gy for Tru-FF and 9.4 Gy Tru-FFF) doses were also lower for CK plans. The CK plans provide better tumour control due to low doses in critical organs and high target doses than the Tru-FF or Tru-FFF plans. It was observed that CK and VMAT plans for SBRT with 6 MV photon beams provided acceptable results in term of treatment planning criteria such as Conformity Index and Homogeneity Index. It is recommended to use a target tracking system to provide an accurate and reliable SBRT treatment with VMAT and CK techniques.

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.

Role of Functional MRI in Liver SBRT: Current Use and Future Directions.

Stereotactic body radiation therapy (SBRT) is an emerging treatment for liver cancers whereby large doses of radiation can be delivered precisely to target lesions in 3-5 fractions. The target dose is limited by the dose that can be safely delivered to the non-tumour liver, which depends on the baseline liver functional reserve. Current liver SBRT guidelines assume uniform liver function in the non-tumour liver. However, the assumption of uniform liver function is false in liver disease due to the presence of cirrhosis, damage due to previous chemo- or ablative therapies or irradiation, and fatty liver disease. Anatomical information from magnetic resonance imaging (MRI) is increasingly being used for SBRT planning. While its current use is limited to the identification of target location and size, functional MRI techniques also offer the ability to quantify and spatially map liver tissue microstructure and function. This review summarises and discusses the advantages offered by functional MRI methods for SBRT treatment planning and the potential for adaptive SBRT workflows.

Contour similarity and its implication on inverse prostate SBRT treatment planning.

Success of auto-segmentation is measured by the similarity between auto and manual contours that is often quantified by Dice coefficient (DC). The dosimetric impact of contour variability on inverse planning has been rarely reported. The main aim of this study is to investigate whether automatically generated organs-at-risk (OARs) could be used in inverse prostate stereotactic body radiation therapy (SBRT) planning and whether the dosimetric parameters are still clinically acceptable after radiation oncologists modify the OARs. Planning computed tomography images from 10 patients treated with SBRT for prostate cancer were selected and automatically segmented by commercially available atlas-based software. The automatically generated OAR contours were compared with the manually drawn contours. Two volumetric modulated arc therapy (VMAT) plans, autoRec-VMAT (where only automatically generated rectums were used in optimization) and autoAll-VMAT (where automatically generated OARs were used in inverse optimization) were generated. Dosimetric parameters based on the manually drawn PTV and OARs were compared with the clinically approved plans. The DCs for the rectum contours varied from 0.55 to 0.74 with a mean value of 0.665. Differences of D95 of the PTV between autoRec-VMAT and manu-VMAT plans varied from 0.03% to -2.85% with a mean value of -0.64%. Differences of D0.03cc of manual rectum between the two plans varied from -0.86% to 9.94% with a mean value of 2.71%. D95 of PTV between autoAll-VMAT and manu-VMAT plans varied from 0.28% to -2.9% with a mean value -0.83%. Differences of D0.03cc of manual rectum between the two plans varied from -0.76% to 6.72% with a mean value of 2.62%. Our study implies that it is possible to use unedited automatically generated OARs to perform initial inverse prostate SBRT planning. After radiation oncologists modify/approve the OARs, the plan qualities based on the manually drawn OARs are still clinically acceptable, and a re-optimization may not be needed.

Evaluation of the feasibility of cardiac gating for SBRT of ventricular tachycardia based on real-time ECG signal acquisition.

To investigate the feasibility of cardiac synchronized gating in stereotactic body radiation therapy (SBRT) of ventricular tachycardia (VT) using a real-time electrocardiogram (ECG) signal acquisition. Stability of beam characteristics during simulated ECG gating was examined by developing a microcontroller interface to a Varian Clinac iX linear accelerator allowing gating at frequencies and duty cycles relevant to cardiac rhythm. Delivery accuracy was evaluated by measuring dose linearity with an ionization chamber, and flatness and symmetry with a two-dimensional detector array, for different gating windows within typical human cardiac cycle periods. To establish a practical method of gating based on actual ECG signals, an AD8232 Heart Monitor board was used to acquire the ECG signal and synchronize the beam delivery. Real-time cardiac gated delivery measurements were performed for a single 10×10cm2 field and for a VT-SBRT plan using intensity-modulated radiation therapy (IMRT). Dose per monitor unit (MU) values were found to be linear within most gating windows investigated with maximum differences relative to non-gated delivery of <2% for gating windows ≥200ms and for >10MUs. Beam profiles for both gated and non-gated modes were also found to agree with maximum differences of 0.5% relative to central axis dose for all sets of beam-on/beam-off combinations. Comparison of dose distributions for intensity-modulated SBRT plans between non-gating and cardiac gating modes provided a gamma passing rate of 97.2% for a 2% 2mm tolerance. Beam output is stable with respect to linearity, flatness, and symmetry for gating windows within cardiac cycle periods. Agreement between dose distributions for VT-SBRT using IMRT in non-gated and cardiac cycle gated delivery modes shows that the proposed methodology is feasible. Technically, gating for delivery of SBRT for VT is possible with regard to beam stability.

Impact of Dosimetric Parameters on Local Control after SBRT for Pancreatic Cancer

<h3>Purpose/Objective(s)</h3> Stereotactic body radiotherapy (SBRT) for pancreatic cancer is an emerging modality to provide local control (LC) in patients deemed to have borderline resectable or locally advanced disease. Given the possibility for significant heterogeneity in dose distributions between SBRT plans, our objective was to determine if there were dosimetric variables associated with improved LC for pancreatic cancer patients. We hypothesized that higher gross tumor volume (GTV), internal target volume (ITV), and planning tumor volume (PTV) doses covering 90% of the volume (D90) as well as minimum (Dmin) and maximum (Dmax) doses would be associated with improved local control. <h3>Materials/Methods</h3> This was a retrospective cohort study including patients who received SBRT for pancreatic adenocarcinoma at a single institution, initially diagnosed as borderline resectable or locally advanced unresectable disease between 2015-2021. Baseline and treatment characteristics were collected including age, gender, performance status, tumor location, staging, CA 19-9 at diagnosis, and use of chemotherapy and/or surgery. Dosimetric parameters included target volume sizes as well as Dmin, Dmax, and D90 for the GTV, ITV, and PTV. These were then converted to BED using the linear/quadratic model based on an alpha/beta of 10Gy. The primary endpoint was LC. The secondary endpoint was overall survival (OS). The Cox proportional hazards model was used for univariate regression analyses (UVA) and the Kaplan-Meier method was utilized to estimate local control. <h3>Results</h3> A total of 55 patients were included in the study. Median follow-up was 9.5 months. All but two patients received 3300cGy in 5 fractions. 49 patients received chemotherapy non-concurrently. 8 patients underwent surgery following RT. On UVA, the Dmin was found to be significantly associated with LC for GTV (p=0.028), ITV (p=0.028), and PTV (p=0.037). D90 for ITV (p=0.010) and PTV (p=0.025) were also associated with LC. Dmax was not significantly associated for any target volume. Other variables significantly associated with LC were performance status (p=0.039), chemotherapy (p=0.008), GTV (p=0.019), and ITV (p=0.028). Patients receiving > 53Gy compared to those receiving ≤ 53Gy had a median time to local recurrence of 15.1 months vs 6.6 months, respectively (p=0.031). No variables on UVA were associated with OS. <h3>Conclusion</h3> This comprehensive dosimetric analysis demonstrates that local control after SBRT for pancreatic cancer is significantly associated with dosimetric factors despite our cohort's relative uniformity in prescription dose. Specifically, an ITV D90 with a BED_α/β10Gy> 53Gy was associated with better LC. This likely reflects the large impact that critical organs-at-risk may have on pancreatic SBRT plans and highlights the need for dosimetric parameter specifications in future studies. Further investigation is warranted for lesions amenable to dose escalation with SBRT to improve LC.

MRI-Guided Real-Time Online Adaptive Gated Stereotactic Body Radiation Therapy for Liver Tumors

<h3>Purpose/Objective(s)</h3> Liver tumors (primary or metastases) are commonly encountered in Oncology. Single breathing phase Magnetic resonance imaging (MRI)-guided gated and possible adaptive stereotactic body radiation therapy (SBRT) for liver tumors offer dosimetric advantages and facilitate safe dose escalation compared to non- MRI-guided SBRT (MRgSBRT). We aimed to evaluate the impact of MRgSBRT on treatment outcomes and report treatment related adverse events. <h3>Materials/Methods</h3> This IRB approved retrospective study included liver tumors patients treated consecutively with MRgSBRT between 2019 and 2021. All patients received Gadoxetate disodium contrast followed by single phase post-exhale CT and MRI simulations utilizing 0.35 T MRI-guided linear accelerator. MRI images (hybrid T1-T2 sequence) were used as the primary set of images for contouring and planning. Real-time online adaptive SBRT plans were generated to account for daily anatomical changes of nearby organs at risk (OARs) when indicated. Patients were followed up every 3 to 6 months with a brief history, physical examination and imaging as indicated. Patients' data were analyzed based on age, sex, histology, tumor volume and delivered biologically equivalent dose (BED <100 Gy vs BED > 100 Gy). Kaplan Meier curves were generated for freedom from local progression (FFLP) and overall survival (OS). <h3>Results</h3> The study included 35 eligible patients with 49 liver tumors (hepatocellular carcinoma 21 lesions, liver metastases 28 lesions). The median age was 69.5 years (range 25 - 95). The median planning target volume (PTV) was 43.5 cm³ (4.9 - 344.8 cm³). Most patients (80%) were prescribed 50 Gy in 5 fractions (median 50 Gy, range 35 - 50Gy) to PTV. A total of 255 fractions were delivered for patients who completed treatment, 22 of which were adapted (8.6%). The median follow up was 19.4 months (range 1-37 months). The median OS was 25.9 months with 95% confidence intervals (CI) (22.1 - 29.6). The 1-year OS rate was 77.7%, 3-year OS rate was 47.9%. Local control was a driver for OS with patients with durable local control having a median OS of 27.8 months [95%CI (23.8-31.6)] vs 13.5 months [95%CI (5.6-21.3), p=0.007] in patients with local progression. The median FFLP was 29.8 months [95%CI (27.2-32.3]. The estimated 1-year FFLP was 95.6 % and 3-year FFLP was 87.1%. The FFLP in patients with BED>100Gy was 30.9 months [95%CI (28.7-33.1)] vs 13.3 months [95%CI (5.3-21.3), p value=0.004] in patients who received <100Gy BED. Most patients tolerated treatment well, no acute grade 3 toxicity. Two patients developed late grade 3 toxicity (gastric ulcer and non-classic radiation induced liver disease). <h3>Conclusion</h3> In our study, Gated MRgSBRT with real-time online adaptive planning is feasible, provides durable local control and impacts overall survival in a heavily pretreated older group of liver tumor patients, with remarkably good safety profile. BED>100Gy improved local control. Improved local control seems to drive overall survival benefit in liver tumors.

CT-Based Online Adaptive Prostate SBRT Improves Target Coverage and Reduces Rectal Dose

<h3>Purpose/Objective(s)</h3> Stereotactic body radiation therapy (SBRT) is an emerging treatment modality for clinically localized prostate cancer (PCa). Online daily adaptive radiotherapy (ART) could potentially improve the therapeutic ratio of prostate SBRT by accounting for inter-fraction variation in target and OAR volumes. To-date no group has evaluated the clinical utility of a novel AI-augmented CT-based ART system for prostate SBRT. In this study we hypothesized that adaptive prostate SBRT plans would result in improved target coverage and lower dose to OARs versus the unadapted treatment plan. <h3>Materials/Methods</h3> Eight patients with unfavorable intermediate or high-risk PCa treated with 5-fx whole-pelvis prostate adaptive SBRT were reviewed. Patients were treated to the elective nodes (2500 cGy) with simultaneous boosts to the prostate/seminal vesicles (3625 cGy) and prostate (4000 cGy). 4 of 8 had rectal hydrogel spacer. For each Fx, CBCT was acquired and OARs (rectum, bladder, bowel, sigmoid, femurs) were segmented/deformed using AI. CTVs were rigidly registered. Volumes were adjusted manually and PTV expansions added. Adaptive treatment plans were developed based on the contoured targets and OARs and dose to these volumes for the adapted vs. initial plans were compared. The V100 (% PTV receiving prescription dose) and the D0.03 cc between scheduled and adapted treatment plans were compared using a Student's t-test, with significance threshold of <i>P<0.05</i>. <h3>Results</h3> Eight patients successfully completed 40 Fx's of adaptive RT as scheduled. Daily adaptation resulted in a statistically significant mean improvement in PTV V100 for all targets: [11.9% ± 2.1% for PTV 4000 (p <0.0001); 7.9% ± 1.5% for PTV 3625 (p <0.0001); and 6.1% ± 1.6% for PTV 2500 (p= 0.0007)]. Mean rectal D0.03 was significantly reduced by 45.4 cGy ± 10.8 cGy (p= 0.0002) per fraction (227 cGy/5 fractions) compared to the initial plans. There were no statistically significant differences between scheduled and adapted D0.03 to the bladder, sigmoid, or bowel (Table). No patients experienced acute CTCAE grade ≥3 GI/GU adverse events (median F/U 9.5 months). All statistically significant differences were maintained in the presence and absence of rectal hydrogel spacer. <h3>Conclusion</h3> CT-based online adaptive SBRT resulted in statistically significant and clinically meaningful improvements in PTV coverage and D0.03cc dose to the rectum. A trial evaluating CT adaptive prostate SBRT is underway.

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.

Use of single-energy proton pencil beam scanning Bragg peak for intensity-modulated proton therapy FLASH treatment planning in liver-hypofractionated radiation therapy.

The transmission proton FLASH technique delivers high doses to the normal tissue distal to the target, which is less conformal compared to the Bragg peak technique. To investigate FLASH radiotherapy (RT) planning using single-energy Bragg peak beams with a similar beam arrangement as clinical intensity-modulated proton therapy (IMPT) in a liver stereotactic body radiation therapy (SBRT) and to characterize the plan quality, dose sparing of organs-at-risk (OARs), and FLASH dose rate percentage. An in-house platform was developed to enable inverse IMPT-FLASH planning using single-energy Bragg peaks. A universal range shifter and range compensators were utilized to effectively align the Bragg peak to the distal edge of the target. Two different minimum MU settings of 400 and 800 MU/spot (Bragg-400 MU and Bragg-800 MU) plans were investigated on 10 consecutive hepatocellular carcinoma patients previously treated by IMPT-SBRT to evaluate the FLASH dose and dose rate coverage for OARs. The IMPT-FLASH using single-energy Bragg peaks delivered 50Gy in five fractions with similar or identical beam arrangement to the clinical IMPT-SBRT plans. NRG GI003 dose constraint metrics were used. Three dose rate calculation methods, including average dose rate (ADR), dose threshold dose rate (DTDR), and dose-ADR (DADR), were all studied. The novel spot map optimization can fulfill the inverse planning using single-energy Bragg peaks. All the Bragg peak FLASH plans achieved similar results for the liver-gross tumor volume (GTV) Dmean and heart , compared to SBRT-IMPT. The Bragg-800 MU plans resulted in 18.3% higher clinical target volume (CTV) compared with SBRT (p<0.05), and no significant difference was found between Bragg-400 MU and SBRT plans. For the CTV Dmax , SBRT plans resulted in 10.3% (p<0.01) less than Bragg-400 MU plans and 16.6% (p<0.01) less than Bragg-800 MU plans. The Bragg-800 MU plans generally achieved higher ADR, DADR, and DTDR dose rates than Bragg-400 MU plans, and DADR mostly led to the highest V40Gy/s compared to other dose rate calculation methods, whereas ADR led to the lowest. The lower dose rate portions in certain OARs are related to the lower dose deposited due to the farther distances from targets, especially in the penumbra of the beams. Single-energy Bragg peak IMPT-FLASH plans eliminate the exit dose in normal tissues, maintaining comparable dose metrics to the conventional IMPT-SBRT plans, while achieving a sufficient FLASH dose rate for liver cancers. This study demonstrates the feasibility of and sufficiently high dose rate when applying the Bragg peak FLASH treatment for a liver cancer hypofractionated FLASH therapy. The advancement of this novel method has the potential to optimize treatment for liver cancer patients.

Dose rate and dose robustness for proton transmission FLASH-RT treatment in lung cancer.

To evaluate the plan quality and robustness of both dose and dose rate of proton pencil beam scanning (PBS) transmission FLASH delivery in lung cancer treatment. An in-house FLASH planning platform was used to optimize 10 lung cancer patients previously consecutively treated with proton stereotactic body radiation therapy (SBRT) to receive 3 and 5 transmission beams (Trx-3fds and Trx-5fds, respectively) to 34 Gy in a single fraction. Perturbation scenarios (n=12) for setup and range uncertainties (5mm and 3.5%) were introduced, and dose-volume histogram and dose-rate-volume histogram bands were generated. Conventional proton SBRT clinical plans were used as a reference. RTOG 0915 dose metrics and 40 Gy/s dose rate coverage (V40Gy/s) were used to assess the dose and dose rate robustness. Trx-5fds yields a comparable iCTV D2% of 105.3%, whereas Trx-3fds resulted in inferior D2% of 111.9% to the clinical SBRT plans with D2% of 105.6% (p<0.05). Both Trx-5fds and Trx-3fds plans had slightly worse dose metrics to organs at risk than SBRT plans. Trx-5fds achieved superior dosimetry robustness for iCTV, esophagus, and spinal cord doses than both Trx-3fds and conventional SBRT plans. There was no significant difference in dose rate robustness for V40Gy/s coverage between Trx-3fds and Trx-5fds. Dose rate distribution has similar distributions to the dose when perturbation exists. Transmission plans yield overall modestly inferior plan quality compared to the conventional proton SBRT plans but provide improved robustness and the potential for a toxicity-sparing FLASH effect. By using more beams (5- versus 3-field), both dose and dose rate robustness for transmission plans can be achieved.

Imaging investigation of small motion target for lung stereotactic body radiotherapy (SBRT)

Purpose: Motion management is important in lung tumor SBRT. Small target four-dimensional computed tomography (4DCT) imaging could give significant target motion information. In this study, Quality of 4DCT for small target was investigated to provide information for treatment planning in SBRT of small target, which is in favor of magnifying the target margin characteristics during 4DCT scan. Method and Materials: A Real Time Position Management (RPM) phantom from Varian Medical System and 6 CT-spot pellets from Beekley Medical were employed for this study. Each pellet was equivalent to a 4mm diameter ball. The CT spot pellets were attached to the RPM phantom. Two pellet set planes formed 8cm distance and generated two speeds which were 0.2cm and 0.4mm per second given a period with 5.0 seconds. With a GE LightSpeedTM RT CT scanner, the phantom was executed with helical scan with 1.25mm slice thickness with and without motion. Then a 4D CT scan in axial cine mode was carried on at the same phantom. The image exams were exported the GE Advantage Workstation for process. The exams were reviewed and sorted into 10 phases retrospectively. The 0% phase image set, 50% phase image set, maximum intensity projection (MIP) image set, and average intensity projection (Ave-IP) image set were exported to the Eclipse treatment planning system for analysis. In image sets, multi-dimensional variables such as center coordinate of pellets, distance between pellets, and volume of the pellets were used for analysis. Results: At 350 HU window width and 40HU window level, image sets from helical scan for the static phantom is used as reference, comparing to helical scan from moving phantom, retrospective MIP, average intensity projection, 0% phase and 50% phase, for pellets moving at 0.4 cm per second, the average coordinate deviation are 0.49cm, 0.22cm, 0.19cm, 0.59cm and 0.06cm with standard deviation 0.83cm, 0.29cm, 0.32cm, 0.69cm, and 0.07cm, the percentage distance differences are 10.2%, 3.3%, 7.3%, 0.1%, and 0.1% with standard deviation of 13.2%, 4.4%, 9.6%, 0.1%, and 0.1%. and the percentage volumes changes are 17.4%, 171.5%, 18.4%, 14.2% and 7.8% with standard deviation at 25.4%, 201.8%, 23.7%, 19.85% and 10%. Conclusions: A simple and sensitive 4DCT quality assurance method was developed by combining Varian RPM phantom and CT-spot pellets from Beekley Medical and it shows that the 50% phase image set is better for SBRT planning. Further study will include dosimetric effect on the small target moving in different directions.

Dosimetric evaluation of respiratory gating on a 0.35-T magnetic resonance-guided radiotherapy linac.

The commercial 0.35-T magnetic resonance imaging (MRI)-guided radiotherapy vendor ViewRay recently introduced upgraded real-time imaging frame rates based on compressed sensing techniques. Furthermore, additional motion tracking algorithms were made available. Compressed sensing allows for increased image frame rates but may compromise image quality. To assess the impact of this upgrade on respiratory gating accuracy, we evaluated gated dose distributions pre- and post-upgrade using a motion phantom and radiochromic film. Seven motion waveforms (four artificial, two patient-derived free-breathing, and one breath-holding) were used to drive an MRI-compatible motion phantom. A treatment plan was developed to deliver a 3-cm diameter spherical dose distribution typical of a stereotactic body radiotherapy plan. Gating was performed using 4-frames per second (fps) imaging pre-upgrade on the "default" tracking algorithm and 8-fps post-upgrade using the "small mobile targets" (SMT) and "large deforming targets" (LDT) tracking algorithms. Radiochromic film was placed in a moving insert within the phantom to measure dose. The planned and delivered dose distributions were compared using the gamma index with 3%/3-mm criteria. Dose-area histograms were produced to calculate the dose to 95% (D95) of the sphere planning target volume (PTV) and two simulated gross tumor volumes formed by contracting the PTV by 3 and 5mm, respectively. Gamma pass rates ranged from 18% to 93% over the 21 combinations of breathing trace and gating conditions examined. D95 ranged from 206 to 514cGy. On average, the LDT algorithm yielded lower gamma and D95 values than the default and SMT algorithms. Respiratory gating at 8fps with the new tracking algorithms provides similar gating performance to the original algorithm with 4fps, although the LDT algorithm had lower accuracy for our non-deformable target. This indicates that the choice of deformable image registration algorithm should be chosen deliberately based on whether the target is rigid or deforming.

Stereotactic body radiation therapy for spine and non-spine bone metastases. GETUG (french society of urological radiation oncologists) recommendations using a national two-round modified Delphi survey

Backround and purposeThe relevance of metastasis-directed stereotactic body radiation therapy (SBRT) remains to be demonstrated through phase III trials. Multiple SBRT procedures have been published potentially resulting in a disparity of practices. Therefore, the french society of urological radiation oncolgists (GETUG) recognized the need for joint guidelines for metastasis-directed SBRT in order to standardize practice in trials carried out by the group. Materials and methodsAfter a comprehensive litterature review, 97 recommandation statements were created regarding planning and delivery of spine bone (SBM) and non-spine bone metastases (NSBM) SBRT. These statements were then submitted to a national online two-round modified Delphi survey among main GETUG investigators. Consensus was achieved if a statement received ≥ 75 % agreements, a trend to consensus being defined as 65–74 % agreements. Any statement without consensus at round one was re-submitted in round two. ResultsTweny-one out of 29 (72.4%) surveyed GETUG investigators responded to both rounds. Consensus was achieved for 91/97 statements (93.8%) allowing the edition of comprehensive guidelines encompassing all aspects of SBM and NSBM SBRT planning and delivery: patients selection (19 statements), treatment preparation (14 statements), target volume delineation (18 statements), dose and fractionation (11 statements), prescription and dose objectives (9 statements), organs at risk dose constraints (15 statements) and image guided radiation therapy (11 statements). ConclusionGETUG guidelines for SBM and NSBM SBRT were agreed upon using a validated modified Delphi approach. These guidelines will be used as per-protocol recommendations in ongoing and further GETUG clinical trials.

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.