Dosimetric Endpoints Research Articles

An Automated Treatment Plan Quality Control Tool for Intensity-Modulated Radiation Therapy Using a Voxel-Weighting Factor-Based Re-Optimization Algorithm.

Intensity-modulated radiation therapy (IMRT) currently plays an important role in radiotherapy, but its treatment plan quality can vary significantly among institutions and planners. Treatment plan quality control (QC) is a necessary component for individual clinics to ensure that patients receive treatments with high therapeutic gain ratios. The voxel-weighting factor-based plan re-optimization mechanism has been proved able to explore a larger Pareto surface (solution domain) and therefore increase the possibility of finding an optimal treatment plan. In this study, we incorporated additional modules into an in-house developed voxel weighting factor-based re-optimization algorithm, which was enhanced as a highly automated and accurate IMRT plan QC tool (TPS-QC tool). After importing an under-assessment plan, the TPS-QC tool was able to generate a QC report within 2 minutes. This QC report contains the plan quality determination as well as information supporting the determination. Finally, the IMRT plan quality can be controlled by approving quality-passed plans and replacing quality-failed plans using the TPS-QC tool. The feasibility and accuracy of the proposed TPS-QC tool were evaluated using 25 clinically approved cervical cancer patient IMRT plans and 5 manually created poor-quality IMRT plans. The results showed high consistency between the QC report quality determinations and the actual plan quality. In the 25 clinically approved cases that the TPS-QC tool identified as passed, a greater difference could be observed for dosimetric endpoints for organs at risk (OAR) than for planning target volume (PTV), implying that better dose sparing could be achieved in OAR than in PTV. In addition, the dose-volume histogram (DVH) curves of the TPS-QC tool re-optimized plans satisfied the dosimetric criteria more frequently than did the under-assessment plans. In addition, the criteria for unsatisfied dosimetric endpoints in the 5 poor-quality plans could typically be satisfied when the TPS-QC tool generated re-optimized plans without sacrificing other dosimetric endpoints. In addition to its feasibility and accuracy, the proposed TPS-QC tool is also user-friendly and easy to operate, both of which are necessary characteristics for clinical use.

A treatment planning comparison of volumetric modulated arc therapy and proton therapy for a sample of breast cancer patients treated with post-mastectomy radiotherapy

Post-mastectomy radiotherapy (PMRT) has been shown to improve disease-free survival and overall survival for locally advanced breast cancer. However, long term survivors may develop life threatening acute and chronic treatment-related toxicities after radiotherapy, like cardiac toxicity and second cancers. The more advanced techniques like volumetric arc therapy (VMAT), and proton therapy have the potential to improve treatment outcome by constraining doses to radiosensitive organs, but evidence from outcome study will not be available until years or decades later. Furthermore, the literature is largely incomplete regarding systematic comparison of potential benefits of advanced technologies for PMRT. The purpose of this study was to compare proton therapy, both passively scattered (PSPT) and intensity modulated (IMPT), to VMAT and develop an evidence-based rationale for selecting a treatment modality for left sided post-mastectomy radiotherapy (PMRT) patients. Eight left-sided PMRT patients previously treated with VMAT were included in this study. Planning target volumes (PTV) included the chest wall and regional lymph nodes. PSPT and IMPT plans were created using a commercial proton treatment planning system. The resulting plans were compared to the corresponding VMAT on the basis of dosimetric and radiobiological endpoints. The uncertainties in risk from proton range, set-up errors, and dose-response models were also evaluated. All modalities produced clinically acceptable treatment plans with nearly 100% tumor control probability. Both proton techniques provided significantly lower normal tissue complication probability values for the heart (p < 0.02) and lung (p < 0.001). Patient-averaged second cancer risk for the contralateral breast and lungs were also significantly lower (p < 0.001) with protons compared to VMAT. The findings of this study were upheld by the uncertainty analysis. All three techniques provided acceptable PMRT treatment plans. Proton therapy showed significant advantages in terms of predicted normal tissue sparing compared to VMAT, taking into account possible uncertainties.

Prospective Assessment of Deep Inspiration Breath Hold Using 3-Dimensional Surface Tracking for Irradiation of Left-Sided Breast Cancer

Deep inspiration breath hold (DIBH) is used to reduce dose and volume of cardiac irradiation during radiation therapy (RT) for breast cancer (BC). Patient subgroups most likely to benefit from DIBH and the effect of DIBH on treatment time remain largely unknown. We sought to identify predictors for the use of DIBH and to quantify differences in dosimetry and treatment time using a large, prospective registry of patients (pts) with BC. One hundred forty-eight pts (ages 26-81) with Stage I-IV left BC were enrolled on a prospective registry from 2011-2014. Pts with any type of breast surgery, systemic therapy and RT fields were included. All pts were deemed potential candidates for DIBH and were simulated with both free-breathing (FB) and DIBH. RT was delivered using either FB or DIBH scans based on physician assessment of tolerance, compliance, and anatomy, without formal dosimetric comparison. Alternate scans were also planned using deformable registration to include identical volumes within RT fields. DIBH pts were monitored using a real-time surface tracking system, AlignRT®. Baseline characteristics and treatment times were compared using Fisher’s exact test and Wilcoxon Rank Sum test. Coefficient of variation (CV) measured the dispersion of treatment times. Dosimetric endpoints were compared across paired DIBH and FB scans using Wilcoxon signed rank test. Linear regression was used to identify predictors for change in mean heart dose (ΔMHD) across paired scans. We treated 38 pts with FB and 110 with DIBH. FB pts were significantly older (median: 58 vs. 49.5 years; P=.002), more likely to have pre-existing heart (11% vs. 1%; P=.02) and lung disease (13% vs. 4%; P=.05), and less likely to receive chemotherapy (42% vs. 66%; P=.01) or immediate reconstruction (11% vs. 34%; P=.01) relative to DIBH pts. MHD (138 cGy vs. 256 cGy), heart V5 (3.8% vs. 8%), heart V30 (0% vs. 1.5%), mean lung dose (923 cGy vs. 992 cGy), and lung V20 (17.1% vs. 18.7%) were all significantly lower in DIBH than in paired FB plans. Of 146 evaluable pts, DIBH resulted in >20 cGy improvement in MHD in 107 (range: 22-881 cGy), neutral change in 25, and >20 cGy increase in MHD in 14 (range: 21-153 cGy). Treatment times were not significantly different between DIBH and FB pts (median 4.2 min vs. 3.8 min; P=.87), but DIBH pts had greater variability in times (P=.0002). On multivariate analysis, younger age (4.18 cGy/yr; P<.0001); higher BMI (6.06 cGy/kg/m2; P=.0018) and larger change in lung volumes (130 cGy/L; P = 0.003) were associated with greater ΔMHD. DIBH is an effective technique that improves cardiac dosimetry without significantly affecting treatment time in most pts receiving breast RT. As expected, greater inspiratory lung volumes augment the benefit in DIBH. As the benefit of DIBH was not seen in all pts, pts should be scanned with both FB and DIBH before choosing a treatment approach.

Automated Treatment Planning for Prostate Cancer Involving Seminal Vesicles and Lymph Nodes

It is labor-intensive and time-consuming to generate high quality radiation therapy treatment plans. Automated planning tools have the potential to improve both efficiency and quality of planning. This study compares automatically generated plans with clinical plans for prostate cancer involving seminal vesicles (SV) and pelvic lymph nodes (LN). We selected 8 patients with high risk prostate cancer, who were treated with various IMRT techniques (e.g. 7-beam step-and-shoot, 2-arc VMAT, and 3-arc VMAT). The treatment goal was to simultaneously deliver 70 Gy to ≥ 95% of the planning volume of prostate (PTV_Prostate), 60 or 56 Gy to ≥ 95% of PTV_SV, and 50.4 or 45 Gy to ≥ 95% of PTV_LN in 25 fractions. Using an auto-planning module (APM) in a commercial planning system, we developed a 4-arc VMAT planning technique and applied it to all patients. The four arcs included two full arcs and two partial arcs (from 200° to 270° and from 160° to 90°). The prescription doses to PTVs and the plan acceptance criteria to OARs were input to guide the APM to create optimization objectives automatically. The APM performed multiple optimization iterations automatically adjusting the objective parameters to meet the treatment goals and further improve OAR sparing. The auto-plans and clinical plans were compared using dosimetric endpoints, homogeneity index (HI), and conformity index (CI). Paired t-tests were performed with a significance level of 0.05. The averaged dosimetric endpoints, HI, and CI were shown in the table below. All auto-plans met the clinical plan acceptance criteria. The auto-plans had significantly better coverage of PTV_SV and similar coverage of PTV_prostate and PTV_LN compared with the clinical plans. The normal tissue sparing for the rectum and penile bulb was significantly improved by auto-planning. Compared with the clinical plans, the auto-plans had similar CI and slightly higher but still acceptable HI. A 4-arc VMAT auto-planning technique was developed using a commercial tool for challenging prostate cases originally treated with various techniques. Significant improvements in target coverage and normal tissue sparing using auto-planning were demonstrated. This auto-planning tool is promising in reducing clinical workload and improving plan quality and consistency.Poster Viewing Abstracts 3522; TablePlan comparison (n=8)OrganEndpointClinical plan (mean ± SD)Auto-plan (mean ± SD)p-valuePTV_prostateV70Gy (%)95.3 ± 0.195.4 ± 0.10.310PTV_SVV60/56Gy (%)96.3 ± 2.899.7 ± 0.40.012PTV_LNV50.4/45Gy (%)93.3 ± 3.496.4 ± 1.70.067BladderV63Gy (%)8.9 ± 4.09.2 ± 3.70.274RectumV63Gy (%)11.8 ± 2.79.1 ± 1.90.016RectumV45Gy (%)39.2 ± 8.625.2 ± 3.30.005Penile bulbDmean (Gy)38.2 ± 15.825.6 ± 13.60.001HI1.06 ± 0.011.08 ± 0.010.006CI1.00 ± 0.031.00 ± 0.020.888 Open table in a new tab

Novel Predictors of Esophageal Toxicity With Stereotactic Body Radiation Therapy (SBRT) for Central Lung Tumors

Purpose The aim of this study is to report on clinical and dosimetric endpoints for development of esophagus toxicity (ET) in patients treated with thoracic stereotactic body radiation therapy (SBRT).

Patient-specific dosimetric endpoints based treatment plan quality control in radiotherapy

In intensity modulated radiotherapy (IMRT), the optimal plan for each patient is specific due to unique patient anatomy. To achieve such a plan, patient-specific dosimetric goals reflecting each patient’s unique anatomy should be defined and adopted in the treatment planning procedure for plan quality control. This study is to develop such a personalized treatment plan quality control tool by predicting patient-specific dosimetric endpoints (DEs). The incorporation of patient specific DEs is realized by a multi-OAR geometry-dosimetry model, capable of predicting optimal DEs based on the individual patient’s geometry. The overall quality of a treatment plan is then judged with a numerical treatment plan quality indicator and characterized as optimal or suboptimal. Taking advantage of clinically available prostate volumetric modulated arc therapy (VMAT) treatment plans, we built and evaluated our proposed plan quality control tool. Using our developed tool, six of twenty evaluated plans were identified as sub-optimal plans. After plan re-optimization, these suboptimal plans achieved better OAR dose sparing without sacrificing the PTV coverage, and the dosimetric endpoints of the re-optimized plans agreed well with the model predicted values, which validate the predictability of the proposed tool. In conclusion, the developed tool is able to accurately predict optimally achievable DEs of multiple OARs, identify suboptimal plans, and guide plan optimization. It is a useful tool for achieving patient-specific treatment plan quality control.

Initial Report of a Prospective Dosimetric and Clinical Feasibility Trial Demonstrates the Potential of Protons to Increase the Therapeutic Ratio in Breast Cancer Compared With Photons

To compare dosimetric endpoints between proton therapy (PT) and conventional radiation and determine the feasibility of PT for regional nodal irradiation (RNI) in women with breast cancer. From 2012 to 2014, 18 women (stage IIA-IIIB) requiring RNI prospectively enrolled on a pilot study. Median age was 51.8 years (range, 42-73 years). The cohort included breast-conserving therapy (BCT) and mastectomy patients and right- and left-sided cancers. Treatment targets and organs at risk were delineated on computed tomography scans, and PT and conventional plans were developed. Toxicity was prospectively recorded using Common Terminology Criteria for Adverse Events version 4.0. A Wilcoxon signed-rank sum test compared the dose-volume parameters. The primary endpoint was a reduction in cardiac V5. Median follow-up was 20 months (range, 2-31 months). For all patients, the PT plan better met the dosimetric goals and was used for treatment. Proton therapy alone was used for 10 patients (9 postmastectomy, 1 after BCT) and combined proton-photon in 8 (6 BCT, 2 postmastectomy with immediate expander reconstruction). Proton therapy improved coverage of level 2 axilla (P=.0005). Adequate coverage of internal mammary nodes was consistently achieved with PT (median D95, 50.3 Gy; range, 46.6-52.1 Gy) but not with conventional radiation therapy (median D95, 48.2 Gy; range, 40.8-55 Gy; P=.0005). Median cardiac V5 was 0.6% with PT and 16.3% with conventional radiation (P<.0001). Median ipsilateral lung V5 and V20 were improved with PT (median V5 35.3% vs 60.5% [P<.0001]; and median V20, 21.6% vs 35.5% [P<.0001]). Grade 3 dermatitis developed in 4 patients (22%), which was the only grade 3 toxicity. No grade 4+ toxicities developed. Proton therapy for RNI after mastectomy or BCT significantly improves cardiac dose, especially for left-sided patients, and lung V5 and V20 in all patients without excessive acute toxicity. Proton therapy simultaneously improves target coverage for the internal mammary nodes and level 2 axilla, which may positively impact long-term survival in breast cancer patients.

Dosimetric impact of orthopedic metal artifact reduction (O-MAR) on Spine SBRT patients.

The dosimetric impact of orthopedic metal artifact reduction (O‐MAR) on spine SBRT patients has not been comprehensively studied, particularly with spinal prostheses in high‐dose gradient regions. Using both phantom and patient datasets, we investigated dosimetric effects of O‐MAR in combination of various metal locations and dose calculation algorithms. A physical phantom, with and without a titanium insert, was scanned. A clinical patient plan was applied to the artifact‐free reference, non‐O‐MAR, and O‐MAR phantom images with the titanium located either inside or outside of the tumor. Subsequently, five clinical patient plans were calculated with pencil beam and Monte Carlo (iPlan) on non‐O‐MAR and O‐MAR patient images using an extended CT‐density table. The dose differences for phantom plans and patient plans were analyzed using dose distributions, dose‐volume histograms (DVHs), gamma index, and selected dosimetric endpoints. From both phantom plans and patient plans, O‐MAR did not affect dose distributions and DVHs while minimizing metal artifacts. Among patient plans, we found that, when the same dose calculation method was used, the difference in the dosimetric endpoints between non‐O‐MAR and O‐MAR datasets were small. In conclusion, for spine SBRT patients with spinal prostheses, O‐MAR image reconstruction does not affect dose calculation accuracy while minimizing metal artifacts. Therefore, O‐MAR images can be safely used for clinical spine SBRT treatment planning.PACS numbers: 87.53.Bn, 87.55.K‐, 87.57.Q‐, 87.57.cp

Prospective assessment of deep inspiration breath-hold using 3-dimensional surface tracking for irradiation of left-sided breast cancer

PurposeDeep inspiration breath hold (DIBH) is used to decrease cardiac irradiation during radiation therapy (RT) for breast cancer. The patients most likely to benefit and the impact on treatment time remain largely unknown. We sought to identify predictors for the use of DIBH and to quantify differences in dosimetry and treatment time using a prospective registry. Methods and materialsA total of 150 patients with left breast cancer were enrolled. All patients were simulated with both free breathing (FB) and DIBH. RT was delivered by either modality. Alternate scans were planned with use of deformable registration to include identical RT volumes. DIBH patients were monitored by a real-time surface tracking system, AlignRT (Vision RT, Ltd, London, United Kingdom). Baseline characteristics and treatment times were compared by Fisher exact test and Wilcoxon rank sum test. Dosimetric endpoints were analyzed by Wilcoxon signed rank test, and linear regression identified predictors for change in mean heart dose (∆MHD). ResultsWe treated 38 patients with FB and 110 with DIBH. FB patients were older, more likely to have heart and lung disease, and less likely to receive chemotherapy or immediate reconstruction (all P < .05). Treatment times were not significantly different, but DIBH patients had greater variability in times (P = .0002). Of 146 evaluable patients, DIBH resulted in >20 cGy improvement in MHD in 107 patients but a >20 cGy increase in MHD in 14. Both MHD and lung V20 were significantly lower in DIBH than in paired FB plans. On multivariate analysis, younger age (4.18 cGy per year; P < .0001), higher body mass index (6.06 cGy/kg/m2; P = .0018), and greater change in lung volumes (130 cGy/L; P = .003) were associated with greater ∆MHD. ConclusionsDIBH improves cardiac dosimetry without significantly impacting treatment time in most patients. Greater inspiratory lung volumes augment this benefit. Because the improvement with DIBH was not uniform, patients should be scanned with both FB and DIBH.

SU‐E‐T‐01: (In)dependence of Plan Quality On Treatment Modalities and Target‐To‐Critical Structure Geometry for Brain Tumor

Purpose:To evaluate and test the hypothesis that plan quality may be systematically affected by treatment delivery techniques and target‐tocritical structure geometric relationship in radiotherapy for brain tumor.Methods:Thirty‐four consecutive brain tumor patients treated between 2011–2014 were analyzed. Among this cohort, 10 were planned with 3DCRT, 11 with RadipArc, and 13 with helical IMRT on TomoTherapy. The selected dosimetric endpoints (i.e., PTV V100, maximum brainstem/chiasm/ optic nerve doses) were considered as a vector in a highdimensional space. A Pareto analysis was performed to identify the subset of Pareto‐efficient plans.The geometric relationships, specifically the overlapping volume and centroid‐of‐mass distance between each critical structure to the PTV were extracted as potential geometric features. The classification‐tree analyses were repeated using these geometric features with and without the treatment modality as an additional categorical predictor. In both scenarios, the dominant features to prognosticate the Pareto membership were identified and the tree structures to provide optimal inference were recorded. The classification performance was further analyzed to determine the role of treatment modality in affecting plan quality.Results:Seven Pareto‐efficient plans were identified based on dosimetric endpoints (3 from 3DCRT, 3 from RapicArc, 1 from Tomo), which implies that the evaluated treatment modality may have a minor influence on plan quality. Classification trees with/without the treatment modality as a predictor both achieved accuracy of 88.2%: with 100% sensitivity and 87.1% specificity for the former, and 66.7% sensitivity and 96.0% specificity for the latter. The coincidence of accuracy from both analyses further indicates no‐to‐weak dependence of plan quality on treatment modality. Both analyses have identified the brainstem to PTV distance as the primary predictive feature for Pareto‐efficiency.Conclusion:Pareto evaluation and classification‐tree analyses have indicated that plan quality depends strongly on geometry for brain tumor, specifically PTV‐tobrain‐stem‐distance but minimally on treatment modality.

SU‐E‐T‐332: Dosimetric Impact of Photon Energy and Treatment Technique When Knowledge Based Auto‐Planning Is Implemented in Radiotherapy of Localized Prostate Cancer

Purpose:The aim of this study was to investigate the dosimetric impact of the combination of photon energy and treatment technique on radiotherapy of localized prostate cancer when knowledge based planning was used.Methods:A total of 16 patients with localized prostate cancer were retrospectively retrieved from database and used for this study. For each patient, four types of treatment plans with different combinations of photon energy (6X and 10X) and treatment techniques (7‐field IMRT and 2‐arc VMAT) were created using a prostate DVH estimation model in RapidPlan™ and Eclipse treatment planning system (Varian Medical System). For any beam arrangement, DVH objectives and weighting priorities were generated based on the geometric relationship between the OAR and PTV. Photon optimization algorithm was used for plan optimization and AAA algorithm was used for final dose calculation. Plans were evaluated in terms of the pre‐defined dosimetric endpoints for PTV, rectum, bladder, penile bulb, and femur heads. A Student's paired t‐test was used for statistical analysis and p &gt; 0.05 was considered statistically significant.Results:For PTV, V95 was statistically similar among all four types of plans, though the mean dose of 10X plans was higher than that of 6X plans. VMAT plans showed higher heterogeneity index than IMRT plans. No statistically significant difference in dosimetry metrics was observed for rectum, bladder, and penile bulb among plan types. For left and right femur, VMAT plans had a higher mean dose than IMRT plans regardless of photon energy, whereas the maximum dose was similar.Conclusion:Overall, the dosimetric endpoints were similar regardless of photon energy and treatment techniques when knowledge based auto planning was used. Given the similarity in dosimetry metrics of rectum, bladder, and penile bulb, the genitourinary and gastrointestinal toxicities should be comparable among the selections of photon energy and treatment techniques.

SU-D-BRB-02: Patient-Specific Rectal Toxicity Predictor Based Plan Quality Control for Prostate Stereotactic Body Radiation Therapy (SBRT)

Purpose: To develop a patient-specific rectal toxicity predictor guided plan quality control tool for prostate SBRT plans. Methods: For prostate SBRT cases, four segments of rectal walls including peri-prostatic anterior rectal wall, peri-prostatic lateral rectal walls, peri-prostatic posterior rectal wall and rectum superior to prostate are identified as organs at risk and the circumference of rectal wall receiving more than 39 Gy (CRW39) and 24 Gy (CRW24) are rectal toxicity predictors. In this new geometry-dosimetry model, a patient geometry descriptor, differential circumference of rectal wall (dCRW) is used as model input geometry parameters and plan dosimetric endpoints CRW39 and CRW24 are output dosimetric parameters. Linear models are built to correlate dCRW to both CRW39 and CRW24 and established with both a linear regression method and a modified bagging ensemble machine learning method. 27 SBRT prostate cases are retrospectively studied from a dose-escalated clinical trial research. 20 prescribed 50 Gy SBRT cases are recruited to train the model and the other rescaled 7 cases are used to evaluated model feasibility and accuracy. Results: Each solved linear coefficient sequence related to CRW39 or CRW24 is a one-dimensional decreasing function of the distance from the PTV boundary, indicating that the different locations of each rectal circumference have different contributions to each particular dosimetric endpoint. The fitting errors for those trained 20 prostate SBRT cases are small with mean values of 2.39%, 2.45% relative to the endpoint values for SBRT rectal toxicity predictor CRW39 and CRW24 respectively. 1 out of 7 evaluation plans is identified as poor quality plan. After re-planning, the CRW39 and CRW24 can be reduced by 3.34% and 3%, without sacrificing PTV coverage. Conclusion: The proposed patient geometry-plan toxicity predictor model for SBRT plans can be successfully applied to plan quality control for prostate SBRT cases.

SU‐E‐T‐628: Predicted Risk of Post‐Irradiation Cerebral Necrosis in Pediatric Brain Cancer Patients: A Treatment Planning Comparison of Proton Vs. Photon Therapy

Purpose:Post‐irradiation cerebral necrosis (PICN) is a severe late effect that can Result from brain cancers treatment using radiation therapy. The purpose of this study was to compare the treatment plans and predicted risk of PICN after volumetric modulated arc therapy (VMAT) to the risk after passively scattered proton therapy (PSPT) and intensity modulated proton therapy (IMPT) in a cohort of pediatric patients.Methods:Thirteen pediatric patients with varying age and sex were selected for this study. A clinical treatment volume (CTV) was constructed for 8 glioma patients and 5 ependymoma patients. Prescribed dose was 54 Gy over 30 fractions to the planning volume. Dosimetric endpoints were compared between VMAT and proton plans. The normal tissue complication probability (NTCP) following VMAT and proton therapy planning was also calculated using PICN as the biological endpoint. Sensitivity tests were performed to determine if predicted risk of PICN was sensitive to positional errors, proton range errors and selection of risk models.Results:Both PSPT and IMPT plans resulted in a significant increase in the maximum dose and reduction in the total brain volume irradiated to low doses compared with the VMAT plans. The average ratios of NTCP between PSPT and VMAT were 0.56 and 0.38 for glioma and ependymoma patients respectively and the average ratios of NTCP between IMPT and VMAT were 0.67 and 0.68 for glioma and ependymoma plans respectively. Sensitivity test revealed that predicted ratios of risk were insensitive to range and positional errors but varied with risk model selection.Conclusion:Both PSPT and IMPT plans resulted in a decrease in the predictive risk of necrosis for the pediatric plans studied in this work. Sensitivity analysis upheld the qualitative findings of the risk models used in this study, however more accurate models that take into account dose and volume are needed.

SU-E-J-67: Evaluation of Adaptive MLC Morphing for Online Correction of Prostate Cancer Radiotherapy

Purpose: Online adaptive MLC morphing is desirable over translational couch shifts to accommodate target position as well as anatomic changes. A reliable method of adaptive MLC segment to target during prostate cancer IMRT treatment is proposed and evaluated by comparison with daily online-image guidance (IGRT) correction and online-IMRT planning. Methods: The MLC adaptive algorithm involves following steps; move the MLC segments according to target translational shifts, and then morph the segment shape to maintain the spatial relationship between the planning-target contour and MLC segment. Efficacy of this method was evaluated retrospectively using daily-CBCT images on seven prostate patients treated with seven-beam IMRT treatment to deliver 64Gy in 20 fractions. Daily modification was simulated with three approaches; daily-IGRT correction based on implanted radio-markers, adaptive MLC morphing, and online-IMRT planning, with no-residual variation. The selected dosimetric endpoints and nEUD (normalized equivalent uniform dose to online-IMRT planning) of each organ of interest were determined for evaluation and comparison. Results: For target(prostate), bladder and rectal-wall, the mean±sd of nEUD were 97.6%+3.2%, 103.9%±4.9% and 97.4%±1.1% for daily-IGRT correction; and 100.2%+0.2%, 108.9%±5.1% and 99.8%±1.2% for adaptive MLC morphing, respectively. For daily-IGRT correction, adaptive MLC morphing and online-IMRT planning, target D99 was <95% of the prescription dosemore » in 30%, 0% and 0% of 140 fractions, respectively. For the rectal-wall, D5 exceeded 105% of the planned-D5 in 2.8%, 11.4% and 0% of 140 fractions, respectively. For the bladder, Dmax exceeded 105% of the planned-D5 in 2.8%, 5.6% and 0% of 140 fractions, respectively. D30 of bladder and rectal-wall were well within the planned-D30 for all three approaches. Conclusion: The proposed method of adaptive MLC morphing can be beneficial for the prostate patient population with large deformation and rotation. It is superior to the daily-IGRT correction, and comparable to the online-IMRT planning for dose to the target and rectal-wall.« less

SU‐E‐T‐334: Dosimetric Impacts of Organ at Risk (OAR) Contour Errors in Prostate Volumetric Modulated Arc Therapy (VMAT) Planning: A Sensitivity Study

Purpose:To investigate the dosimetric impacts of OAR contour errors in prostate VMAT planning with a sensitivity analysis method.Methods:Ten randomly selected prostate VMAT patients were used to simulate OAR contour error in rectum and bladder. For each OAR 12 even spaced contour control points were selected in every three slices. For each simulation, only one of the control points was simulated to move inwards or outwards by up to 6 voxels and coordinates of adjacent voxels to that moving control point were moved accordingly with a 3D‐spline smooth function. An in‐house software was used to predict OARs dosimetric endpoints based on geometric relationship between PTV and OAR. In this study, the V75, V70, V65, V60 of rectum and V80, V75, V70, V65 of bladder with and without perturbed contours were calculated and compared.Results:The percentage of OAR dose volume difference around the reference OAR contours were plotted as iso‐error lines overlaid on CT images. The significant difference was shown in OAR contours adjacent to the PTV where high dose gradient exists. When one of the 12 points adjacent to the PTV moved up to 6 voxels, maximum errors of 1.99%, 2.46%, 2.89%, and 3.19% were found in V75, V70, V65, and V60 of the rectum, and 1.59%, 1.67%, 1.87%, and 1.90% were found in V80, V75, V70, and V65 of the bladder, respectively.Conclusion:We can quantify the dosimetric impact of OAR contouring error by evaluating the percentage of OAR dose volume changes in prostate VMAT planning. For the adaptive radiation therapy, the iso‐error lines can provide the planner a guideline that which portion of contour needed to be checked carefully because dosimetric sensitivity of contour errors are different for different part of contours.

SU-C-210-01: Are Clinically Relevant Dosimetric Endpoints Significantly Better with Gating of Lung SBRT Vs. ITV-Based Treatment?: Results of a Large Cohort Investigation Analyzing Predictive Dosimetric Indicators as a Function of Tumor Volume and Motion A

Purpose: To quantitatively compare patient internal target volume (ITV)-based plans with retrospectively generated gated plans to evaluate potential dosimetric improvements in lung toxicity from gated radiotherapy. Methods: Evaluation was conducted for 150 stereotactic body radiation therapy (SBRT) treatment plans for 128 early-stage (T1–T3, <5cm) NSCLC patients. PTV margins were: ITV+5 mm (ITV_plan) and GTV+5 mm (Gated_plan). ITV-based and gated treatment plans were compared on the same free-breathing CT. ITV-based plan constraints were used to re-optimize and recalculate new gated plans. Plans were generated for 3 fractionation regimens: 3×18Gy, 4×12Gy (original), and 5×10Gy. Physical dose was converted to equivalent dose in 2Gy fractions (EQD2), which was used to determine mean lung dose (MLD) and percent volume of lung receiving ≥20Gy (V20). MLD and V20 differences between gating and ITV-based plans were analyzed as a function of both three-dimensional (3D) motion and tumor volume. The low dose region, V5, was also evaluated. Results: MLD and V20 differences between gated and ITV-based plans were larger for lower (1.48±1.32Gy and 1.44±1.29%) than for upper lobe tumors (0.89±0.74Gy and 0.92±0.71%) due to smaller tumor motion (2.9±3.4mm) compared to lower lobe tumors (8.1±6.1mm). Average differences of <1–2% were noted in V5 between ITV and gated plans. Dosimetric differences between gating and ITV-based methods increased with increasing tumor motion and decreasing tumor volume. Overall, average MLD (8.04±3.92Gy) and V20 (8.29±4.33%) values for ITV-based plans were already well below clinical guidelines, even for the 3×18Gy dose scheme, for which largest differences were noted relative to gated plans. Similar results were obtained for 5×10Gy and 4×12Gy regimens. Conclusion: Clinically relevant improvement in pulmonary toxicity, based on predictors of radiation pneumonitis (MLD and V20) was not generally observed, though improvement for tumors with 3D motion >15 mm, mainly concentrated in peripheral lower lobe tumors, may be considered clinically relevant. Work supported in part by a grant from Varian Medical systems, Palo Alto, CA.

SU‐E‐T‐205: Comparison of Dose‐Volume and Dose‐Mass 4D Inverse Optimization in NSCLC

Purpose:To compare dose‐volume (Dvh) and dose‐mass (Dmh) inverse treatment planning results, performed on time‐resolved (4D) CT scans for non‐small cell lung cancers (NSCLC), thereby accounting for the temporal variation in lung volume and mass.Methods:Ten NSCLC cases were retrospectively studied. The estimated range of tumor motion was in excess of 0.5 cm (0.7 cm to 1.8 cm) in at least one principal direction. For each case five phases, uniformly distributed over the breathing cycle, were chosen for planning. For each of the phases two IMRT plans, with Dvh and with Dmh quadratic objective cost function applied only for the organs at risk (OARs), were generated. Pure dose objective functions were used for the targets. The final dose in each case for each optimization scheme was achieved through a deformable dose accumulation over the five breathing phases. Plan quality of Dvh and Dmh optimization was evaluated by several dose indices (DIs), represented by the dose to certain volume of an OAR. Tallied DIs were cord1%, heart33%, lungs20%, lungs30%, and esophagus50%, as well as lung volumes encompassed by 20 Gy and 30 Gy isodose lines. Statistical significance in the differences among those indices was quantified with two‐tailed paired t‐tests with a significance level of 5%.Results:On average Dmh optimized plans outperformed the 4D Dvh optimized plans. The statistically significant dose reduction in cord DI1, heart DI33, esophagus DI50, lung DI20 and DI30 were 27.5%, 9.9%, 21.3%, 5.6%, and 14.7% respectively. Isovolumes of 20 Gy and 30 Gy differed by 9.6% and 6.7% respectively.Conclusions:The presented findings suggest that on average 4D Dmh optimized plans Result in better OAR sparing than 4D Dvh optimized plans. This is true for the majority of the tallied dosimetric endpoints, indicating that the potential sparing and its magnitude are patient specific.NIH

Abstract P1-15-20: Prospective study of proton radiotherapy for treatment of regional lymphatics in breast cancer

Abstract Purpose: To compare dosimetric endpoints between proton therapy (PT) and conventional radiation and to determine the feasibility of PT for regional nodal irradiation (RNI) in women with breast cancer. Methods: From May 2012 to February 2014, 18 women (stage IIA-IIIB) prospectively enrolled on a pilot study. Median age was 51.8 years (range, 42-73), with equal division between breast-conserving therapy (BCT) and mastectomy and right and left-sided cancers. Treatment targets (CTVs for breast/chest wall, supraclavicular, axillary, internal mammary nodes [IMNs]) and organs at risk were delineated on CT scans, and PT and conventional plans were developed. PT alone was used for 10 patients (9 post-mastectomy, 1 after BCT) and combined proton-photon in 8 (all BCT). Acute toxicity was prospectively recorded using CTCAE v4.0. A Wilcoxon signed-rank sum test compared the dose-volume parameters. Results: Median followup was 10.6 months (range, 1.1-19.1). For all patients, the PT plan better met the dosimetric goals and was used for treatment. Breast/chest wall coverage was adequate (V47.5=96.6% for both plans). PT improved coverage of level II axilla (median D95, 48.9Gy [range, 44.8-51.4Gy] with PT vs 45.4Gy [range, 39.8-51Gy] with conventional; p=0.0005). Adequate coverage of IMNs was consistently achieved with PT (median D95, 48.9Gy with a range of 44.8-51.4Gy) but not with conventional (median D95, 45.4Gy with range of 39.8-51Gy; p=0.0005); PT reduced heart dose in all patients. Median ipsilateral lung V5 measured 35.3% for PT compared to 60.5% for conventional (p&amp;lt;0.0001). Ipsilateral lung V20 improved with PT (median, 21.6% vs 35.5%; p&amp;lt;0.0001). Table 1: Cardiac dose in left-sided patients (comparison of medians of the cohort; range in parentheses) Heart V5Mean Heart DoseVentricle V5Mean Ventricle DoseMaximum LAD doseMean LAD doseProton Plan2.7% (0-12%)0.6Gy (0-3.2Gy)0.5% (0-15.6%)0.1Gy (0-4Gy)30.5 (13.4-42.6Gy)1.7Gy (0.5-28.5Gy)Conventional Plan34% (7-60%)5.9Gy (2-9.1Gy)31.4% (8.7-79.8%)5.4Gy (2.4-11Gy)44.6Gy (35.1-54.7Gy)27Gy (16.8-41.9Gy)p-value0.0040.0040.0040.0040.040.02 Table 2. Cardiac dose in right-sided patients (comparison of medians of the cohort; range in parentheses) Heart V5Mean Heart DoseVentricle V5Mean Ventricle DoseProton Plan0.3% (0-2.9%)0.5Gy (0-0.8Gy)3.3% (0-1%)0.2Gy (0-0.5Gy)Conventional Plan13.2% (0.4-24.7%)2.9Gy (1-5.1Gy)3.3% (0-7.1%)1.2Gy (0.8-1.7Gy)p-value0.0040.0040.0080.004 Grade 3 dermatitis developed in 4 patients (22%), which was the only grade 3 toxicity. All patients developed grade 2 dermatitis; other acute grade 2 toxicities included fatigue (n= 6) and esophagitis (n=5). No grade 4+ toxicities developed. Dermatitis resolved by 1 month after PT for all but 1 patient who developed cellulitis (grade 2). Conclusion: PT for RNI after mastectomy or BCT significantly improves cardiac dose especially for left-sided patients and lung V5 and V20 in all patients without excessive acute toxicity. PT simultaneously improves target coverage for the IMN and level II axilla, which may positively impact long-term survival in breast cancer patients. Citation Format: Julie A Bradley, Roi Dagan, Meng Wei Ho, Christopher G Morris, Zuofeng Li, Nancy P Mendenhall. Prospective study of proton radiotherapy for treatment of regional lymphatics in breast cancer [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P1-15-20.

Dose-mass inverse optimization for minimally moving thoracic lesions

In the past decade, several different radiotherapy treatment plan evaluation and optimization schemes have been proposed as viable approaches, aiming for dose escalation or an increase of healthy tissue sparing. In particular, it has been argued that dose-mass plan evaluation and treatment plan optimization might be viable alternatives to the standard of care, which is realized through dose-volume evaluation and optimization. The purpose of this investigation is to apply dose-mass optimization to a cohort of lung cancer patients and compare the achievable healthy tissue sparing to that one achievable through dose-volume optimization.Fourteen non-small cell lung cancer (NSCLC) patient plans were studied retrospectively. The range of tumor motion was less than 0.5 cm and motion management in the treatment planning process was not considered. For each case, dose-volume (DV)-based and dose-mass (DM)-based optimization was performed. Nine-field step-and-shoot IMRT was used, with all of the optimization parameters kept the same between DV and DM optimizations. Commonly used dosimetric indices (DIs) such as dose to 1% the spinal cord volume, dose to 50% of the esophageal volume, and doses to 20 and 30% of healthy lung volumes were used for cross-comparison. Similarly, mass-based indices (MIs), such as doses to 20 and 30% of healthy lung masses, 1% of spinal cord mass, and 33% of heart mass, were also tallied. Statistical equivalence tests were performed to quantify the findings for the entire patient cohort.Both DV and DM plans for each case were normalized such that 95% of the planning target volume received the prescribed dose. DM optimization resulted in more organs at risk (OAR) sparing than DV optimization. The average sparing of cord, heart, and esophagus was 23, 4, and 6%, respectively. For the majority of the DIs, DM optimization resulted in lower lung doses. On average, the doses to 20 and 30% of healthy lung were lower by approximately 3 and 4%, whereas lung volumes receiving 2000 and 3000 cGy were lower by 3 and 2%, respectively. The behavior of MIs was very similar. The statistical analyses of the results again indicated better healthy anatomical structure sparing with DM optimization.The presented findings indicate that dose-mass–based optimization results in statistically significant OAR sparing as compared to dose-volume–based optimization for NSCLC. However, the sparing is case-dependent and it is not observed for all tallied dosimetric endpoints.

Predictive Risk of Radiation Induced Cerebral Necrosis in Pediatric Brain Cancer Patients after VMAT Versus Proton Therapy.

Cancer of the brain and central nervous system (CNS) is the second most common of all pediatric cancers. Treatment of many of these cancers includes radiation therapy of which radiation induced cerebral necrosis (RICN) can be a severe and potentially devastating side effect. Risk factors for RICN include brain volume irradiated, the dose given per fraction and total dose. Thirteen pediatric patients were selected for this study to determine the difference in predicted risk of RICN when treating with volumetric modulated arc therapy (VMAT) compared to passively scattered proton therapy (PSPT) and intensity modulated proton therapy (IMPT). Plans were compared on the basis of dosimetric endpoints in the planned treatment volume (PTV) and brain and a radiobiological endpoint of RICN calculated using the Lyman-Kutcher-Burman probit model. Uncertainty tests were performed to determine if the predicted risk of necrosis was sensitive to positional errors, proton range errors and selection of risk models. Both PSPT and IMPT plans resulted in a significant increase in the maximum dose to the brain, a significant reduction in the total brain volume irradiated to low doses, and a significant lower predicted risk of necrosis compared with the VMAT plans. The findings of this study were upheld by the uncertainty analysis.