PTV Size Research Articles

190. Correlation between modulation degree and dosimetric accuracy of prostate VMAT treatment planning

Purpose To evaluate the impact of modulation degree (MD) on patient specific pre-treatment quality assurance for prostate VMAT treatment planning. Methods Ten patients with similar PTV and rectum size were included in the study. The prescribed fractional dose to PTV was 2,5 Gy daily. VMAT plans were calculated using a MonteCarlo-based TPS. For each patient we optimized 4 different plans using the same constraint cost-functions for PTV and organ-at-risk (OARs), the same plan parameters (arc number, maximum number of control points for arc) but changing the minimum distance between opposing MLC leafs (MDW = 0.5, 0.8, 1, 1.5 cm). Such parameter influences the MD and the plan quality. MD was calculated from the DICOM-RT files using the mean segment area value and total Monitor Unit. To evaluate the plan quality we took into account the PTV coverage and rectum dose. Gamma index analysis was performed to compare measured and calculated dose. Plans were accepted if 95% of the points satisfied the 2% dose-difference and distance-to-agreement 2 mm (γ2%,2 mm). All plans were delivered by a 6 MV Linac and the patient specific pre-treatment quality assurance verification was performed using a cylindrical diode array-phantom. Descriptive statistic was performed to evaluate the correlation between MD and gamma passing rate measured. Results We observed a negative linear correlation (Fig. 1) between MD values and γ2%,2 mm (correlation coefficient r = 0.7, p = 0.0004). The plans with MD 4 yielded a γ2%,2 mm inferior than 95% in 100% of cases (Fisher-exact-test p = 0.002). 93% of plans with MDW > 0.8 cm had a MD Conclusions The gamma index value was significantly related to MD 4. Rival plans with a MD 0.8 cm allows to obtain a desirable MD value. The calculation of MD for prostate VMAT treatment planning is important to predict the patient-QA in term of gamma index analysis.

Evaluation of Dynamic Tumor-tracking Intensity-modulated Radiotherapy for Locally Advanced Pancreatic Cancer

Intensity-modulated radiotherapy (IMRT) is now regarded as an important treatment option for patients with locally advanced pancreatic cancer (LAPC). To reduce the underlying tumor motions and dosimetric errors during IMRT as well as the burden of respiratory management for patients, we started to apply a new treatment platform of the dynamic tumor dynamic tumor-tracking intensity-modulated radiotherapy (DTT-IMRT) using the gimbaled linac, which can swing IMRT toward the real-time tumor position under patients’ voluntary breathing. Between June 2013 and March 2015, ten patients were treated, and the tumor-tracking accuracy and the practical benefits were evaluated. The mean PTV size in DTT-IMRT was 18% smaller than a conventional ITV-based PTV. The root-mean-squared errors between the predicted and the detected tumor positions were 1.3, 1.2, and 1.5 mm in left-right, anterior-posterior, and cranio-caudal directions, respectively. The mean in-room time was 24.5 min. This high-accuracy of tumor-tracking with reasonable treatment time are promising and beneficial to patients with LAPC.

Outcomes following Stereotactic Body Radiation Therapy for Patients with Child-Pugh B or C Hepatocellular Carcinoma

SBRT is a treatment option for patients (pts) with hepatocellular carcinoma (HCC). Due to concern for causing worsening liver function, caution has been recommended when using SBRT in pts with Child-Pugh (CP) B or C disease. Here we report our institutional outcomes. An institutional review board-approved registry was used to identify CP B and C patients with non-metastatic HCC treated with SBRT between 2011 and 2017. Baseline patient and disease factors, laboratory values, and treatment information were recorded. Patients were followed with radiographic imaging as well as clinical and laboratory follow-up. CP score after treatment was calculated at 2 or 3 months as available. Overall survival (OS) was evaluated using the Kaplan-Meier method and logistic regression analysis was used to identify factors predictive for CP change ≥2 and grade III hepatotoxicity. Thirty-seven pts treated with SBRT to 44 lesions met study criteria. About half of pts (54%) had received prior therapies including embolization, radiofrequency ablation, or y90 radioembolization. Twenty pts were treated with definitive intent, 8 as a bridge to transplant, and 9 were treated for downstaging for transplant. At the time of treatment, 27 pts had CP score of B (7-9) and 10 pts had CP score of C (10-12). The median overall survival (OS) was 14.8 months. Thirteen pts proceeded on to liver transplant after SBRT (7 of 8 bridging, 6 of 9 downstaging), and transplantation was associated with a significant improvement in median OS (32.2 months vs 7.3 months, p=0.018). Among non-transplant patients, CP score of B7 predicted for improved OS relative to CP B8-C scores (1 yr OS 56% vs 18.8% p=0.04). The most commonly used regimen was 40 Gy in 5 fractions, median PTV was 103.7 cc, and median liver mean dose was 10.3 Gy. The median radiographic follow up was 8.3 months, during which only one patient experienced local progression. Among the 30 pts undergoing follow-up with laboratory studies, 8 pts (26.7%) had a rise in CP score exceeding 2 points at 2 or 3 month follow-up and only a single patient died of liver toxicity. Among pts treated with definitive intent, only 10% experienced a change in CP score of ≥2. Neither liver mean dose nor PTV size predicted for CP score change ≥2. Following SBRT, 2 pts developed new grade III hypoalbuminemia, 11 pts developed new grade III hyperbilirubinemia, and 6 pts developed new grade III transaminitis. Liver mean dose, PTV size, and prescription dose did not predict for development of new grade III hepatotoxicity. However, in patients who did not have disease progression in the liver, RT dose ≥40 Gy predicted for grade III hepatotoxicity (p=0.003). SBRT can be successfully employed as local therapy for well-selected patients with CP B and C HCC with low rates of clinically significant hepatotoxicity. SBRT is also feasible in such patients as a part of bridging or downstaging prior to liver transplant.

Quality of Life: A Prospective Randomized Trial of Palliative Volumetric Arc Therapy (VMAT) Versus Conformal Radiation Therapy (CRT)

VMAT spares normal tissues from high/intermediate radiation (RT) doses but increases the volume of tissues receiving low doses of RT as compared to CRT. We hypothesized that palliative VMAT would induce less acute quality of life (QOL) detriment than CRT. This phase 2 trial randomized patients to palliative RT using VMAT or CRT to 1 painful site of metastatic disease in the trunk. Eligible patients were able to complete questionnaires (EORTC QLQ C30 and SF-BPI) and were expected to live >3 months. Patients were ineligible if their KPS<50, they had prior radiation to the same site, there were planned changes in analgesics or cancer therapy within 7 days of the RT. Treating physicians could choose 8Gy in 1 fraction or 20Gy in 5 fractions, a choice used to stratify randomization. The primary endpoint was EORTC QLQ C30 global QOL subscales at 1-week post-RT (QOL at 4 weeks was a secondary endpoint). Paired t-test was used to compare the effect of RT to all patients. Wilcoxon signed-rank test (2-tailed) was used to compare changes in QOL. Only clinically important (≥10 point) QOL differences are reported. From July 2014-November 2017, 72 patients were accrued. Three patients did not receive RT, 12 did not return 1-week post-RT. There were 55 and 46 evaluable patients at 1 and 4 weeks post-RT, respectively. The most common diagnoses were NSCLC, breast, and prostate cancer. Median overall survival was 9 months. Median pain level was 7/10 at baseline and 5/10 at 1 and 4 weeks post-RT (p<0.0001 and p=0.0013). Median time from consultation to RT was 6 days. Baseline characteristics (KPS, pain, QOL subscales and time to 1st RT fraction) were balanced between the groups. At 1 week post-RT, global QOL subscale was not significantly (p=0.46) different between VMAT vs. CRT, but VMAT induced significantly (p= 0.027) less nausea and vomiting than CRT. At 4 weeks post-RT, VMAT induced significantly (p=0.048) less global QOL deterioration. At 4 weeks, patients who received VMAT maintained better role (p=0.022) and social (p=0.018) functions, but reported more diarrhea symptoms (p=0.033) than patients treated with CRT. Patient age, gender, RT dose, PTV size and target location (abdomino-pelvic vs. thoracic) were not associated with global QOL changes at 4 weeks. Palliative VMAT appears to induce less QOL detriments than CRT at 4 weeks post-RT. VMAT reduced iatrogenic nausea and vomiting, which was partially offset by an increase in diarrhea.

Treatment-Related Lymphopenia is an Independent Predictor of Survival in Patients with Lung Cancer Treated via Stereotactic Body Radiation Therapy

Treatment-related lymphopenia (TRL) is frequently associated with radiation therapy. Multiple prior studies have shown associations between TRL and inferior outcomes in various solid tumors, including patients treated with conventionally fractionated radiation therapy for lung cancer. TRL was recently shown to occur in patients with lung cancer treated with stereotactic body radiation therapy (SBRT), but the survival impact of TRL in this setting remains unknown. We seek to evaluate the impact of severe TRL in the setting of lung cancer treated with SBRT on overall survival and evaluate potential strategies to minimize TRL. We retrospectively identified 346 patients treated with thoracic SBRT between 2008 and 2016. Of these patients, 137 had complete blood counts with differentials within 12 months preceding and following SBRT. This was further refined to exclude patients who received prior radiation therapy to the head, neck or thorax; patients who received chemotherapy; and patients with pre-treatment absolute lymphocyte counts less than 500x106 cells/L. The remaining 91 patients were included in the analyses. All patients were treated with 5 fraction regimens, following RTOG 0813 or 0915 as appropriate. We defined severe TRL as an absolute lymphocyte count less than 500x106 cells/L. Kaplan-Meier curves, log-rank analysis and cox regression were performed to assess for survival differences associated with severe TRL using statistical software. Of the 91 included patients, median age was 71.9 years, 55% were female, and 39% had central lung lesions. Planning target volumes ranged from 4.3-183 cm3. Patients with severe TRL had worse overall survival (log rank, 2=7.62, p=0.006). On univariate cox regression, minimum absolute lymphocyte count was directly related to survival, with a hazard ratio of 0.353 (p=0.002). Flattening filter free-arcs were associated with less lymphocyte depletion compared to step and shoot IMRT, with a mean difference in depletion of 211x106 cells/L between modalities (p=0.04). PTV size also had a statistically significant positive correlation with lymphopenia. Severe TRL after lung cancer SBRT is associated with worse overall survival. The use of flattening filter free-arcs reduced lymphocyte depletion compared to step and shoot IMRT.

Diabetes mellitus and radiation induced lung injury after thoracic stereotactic body radiotherapy

BackgroundRadiographic radiation induced lung injury (RILI) is frequently observed after stereotactic body radiotherapy (SBRT). Models of radiographic change can identify patient risk factors that predict clinical toxicity. We examined the association between radiographic lung changes and lung tissue dose-density response over time with clinical risk factors for RILI, such as diabetes. Methods424 baseline and follow up CT scans at 3, 6, and 12 months post-treatment were analyzed in 116 patients (27 with diabetes) undergoing thoracic SBRT. Volumes of dense/hazy regions and lung parenchyma dose-density response curves were evaluated with respect to follow up time, diabetes, and other factors. ResultsDense and hazy tissue regions were larger in the diabetic population, with the effect most pronounced at 3 months. Similarly, dose-density response curves showed greater density change versus dose in the diabetic group (all p < 0.05). Diabetes, time, the interaction of diabetes and time, smoking status, African American race, baseline lung density, and tumor location were significantly associated with radiographic changes on mixed effect analyses. PTV size, pulmonary function, and medication exposure did not significantly impact RILI. Clinical grade 1–2 pneumonitis was more prevalent in diabetic patients (p = 0.02). However, radiographic change did not correlate with clinical pneumonitis. ConclusionsThe presence of diabetes and other clinical factors is associated with increased volume and density of radiographic RILI after lung SBRT, most prominently early after treatment. This is the first report demonstrating the increased severity of RILI after SBRT in diabetic patients. Increased caution treating diabetic patients may be warranted.

23. FFF 6 MV beam commissioning tests on agility

Introduction This work reports some aspects of the FFF 6 MV commissioning on an Elekta Agility accelerator, associated with the TPS Philips Pinnacle 9.8. Documentation manuals and other “instructions for use” give informations on the technical/metrical parameters to be entered and the measurement conditions for the modeling. But there is little information describing a similar installation with clinical conditions at SSD 90 cm. Recently in the literature some data were published [1] , [2] , [3] . Such published informations bring an undeniable support. Modeling could be intrinsically estimated via the fitting but an end-to-end test is necessary to validate the model [4] . Generation of typical clinical plans with different PTV size and their delivery was particularly focused on. Methods FFF beam is matched with 6 FF beam on our both Agility machines. Measurements for the modeling are done with MP3-M PTW water phantom. Data describing linac and relative OF are entered in Pinnacle. Automatic modeling sequences are essentially used to generate a definitive model up to good fitting quality. The agreement between the planned and measured doses was determined using different plastic water phantoms for point dose measurements and with an array or a film for planar dose measurements. Static phantom plans mimic clinical situations: different tissues, field shapes and depth prescriptions. Dynamic plans calculated on patients CT images for 5 cases per localization (breast (DIBH), prostate, lung, and SBRT) are measured with the Octavius 4D on both matched linacs. Results For all the tested square fields, the dose measurement at 10 cm and SSD 90 cm with the Farmer chamber is similar to the prescribed dose (within ±0.13%). For the other configurations, the difference between calculation and measurement is smaller than 1.3%. The QA for VMAT plans of patients are performed with the Octavius 4D. The averaged volume analysis passing the 3 mm-3% criteria for all the localizations and both matched machines is 98 ± 1.5%. Conclusions Verification based on phantom studies offers the possibility to directly compare dose measurement and prescribed dose. Verification based on patient’s treatment plans is not considered to be complete (the QA phantom dose is not really the prescribed patient dose) but mimic all real clinical situations. Moreover, it confirms a beam time 25% to 35% (SBRT) shorter with FFF.

20. Pre-treatment quality assurance for flattening filter free dynamic arcs: A detector comparison

Introduction Pre-treatment quality assurance (QA) with flattening filter free (FFF) beams is challenging because of the high dose rate and the required spatial resolution as small field are usually used. Our current clinical practice is to perform pre-treatment QA with a diode array in a rotational phantom. This study investigates the replacement of this process by an electronic portal imager (EPI) measurement without a phantom in place in order to shorten the QA time on the machine. Methods Pre-treatment QA was performed with PTW Octavius 4D phantom equipped with SRS1000 diode array and with Varian aS1000 EPI. Diode array measurements were compared with the dose calculated in Octavius phantom with Varian Eclipse 13.7. EPI measurements was converted to water measurement and compared with water calculation performed in Dosisoft EPIBEAM (ThinkQA 1.0.2.2ALPHA). QA results were compared for Varian Truebeam 10 FFF dynamic arcs of 10 liver stereotactic radiotherapy plans and for plans with dose and MLC errors voluntarily introduced during beam delivery. Results Clinical plans covered a wide range of PTV size (average = 129.5 cm3, minimum = 9.8 cm3, maximum = 327.5 cm3) and dose prescription (average = 11.8 Gy, minimum = 6 Gy, maximum = 18 Gy) resulting in 1866 MU on average (minimum = 806 MU, maximum = 3072). Diode array and portal imager QA process gave both gamma values higher than 97% for 2%-2 mm criteria (10% threshold). Analysis of plans with errors is pending. Conclusions EPI QA process for FFF beams is achievable. It is much faster than diode array process as there is no need for a phantom setup and it provides similar results.

Sequential proton boost after standard chemoradiation for high-grade glioma

PurposeTo retrospectively assess the feasibility and safety of a sequential proton boost following conventional chemoradiation in high-grade glioma (HGG). Method and materialsSixty-six consecutive patients with HGG were treated with 50.0 Gy photons (50.0–50.4 Gy) in 2.0 Gy (1.8–2.0 Gy) fractions, followed by a proton boost with 10 Gy equivalent (Gy(RBE)) in 2.0 Gy(RBE) fractions. Patients were matched one to one with 66 patients with HGG undergoing conventional radiation therapy (RT) with 60.0 Gy photons (59.4–60.0 Gy) in 2.0 Gy fractions (1.8–2.0 Gy). Matching criteria were age, WHO grade, Karnofsky’s performance status, PTV size, temozolomide therapy (each p > 0.1). The study assessed progression-free survival (PFS), overall survival (OS), acute treatment-related toxicity (CTCAE v.4.03) and pseudoprogression (RANO criteria). ResultsMedian PFS and OS were similar in both treatment groups (bimodality RT, PFS: 8.8 months [2–32 months], OS 19.1 months [4–41 months]; photon-only RT, PFS: 7.2 months [2–39 months], 20.9 months [3–53 months]; p = 0.430 and p = 0.125). The median PTV of the proton boost was significantly smaller than the photon plan PTVs (each p < 0.001). Acute toxicity was mild. Toxicity ≥grade 2 was observed in 6 patients (9%) receiving bimodality RT and 9 patients (14%) receiving photon-only RT. Two types of severe adverse events (CTCAE grade 3) occurred solely in the photon-only group: severe increase in intracranial pressure (5%); and generalized seizures (3%). Pseudoprogression was rare, occurring on average 6 weeks after radiotherapy, and was balanced in both treatment groups (n = 4 each; 8%). ConclusionDelivering a proton boost to significantly smaller target volumes when compared to photon-only plans, yielded comparable progression and survival rates at lower CTCAE grade 3 acute toxicity rates. Pseudoprogression occurred rarely and evenly distributed in both treatment groups. Thus, bimodality RT was at least equivalent regarding outcome and potentially superior with respect to toxicity in patients with HGG. SummaryTreating patients with HGG with 50.0 Gy photons in 2.0 Gy fractions, followed by a proton boost with 10 Gy(RBE) in 2.0 Gy(RBE) fractions, is safe and feasible. Severe radiation-induced acute toxicity and pseudoprogression were rare in both treatment groups. Therefore, in this clinical setting, combined proton radiotherapy might be beneficial in terms of further risk reduction for treatment-related side effects. Interestingly, treatment volume reduction using a proton boost led to comparable survival and progression rates with decreased severe treatment-related toxicity compared to conventional photon radiotherapy.

Do We Need to Dose-Escalate Fractionated Stereotactic Radiation Therapy for Resected Brain Metastases?

Fractionated stereotactic radiotherapy (FSRT) represents an attractive therapeutic option for brain metastases or intracranial resection cavities not amenable to stereotactic radiosurgery. Multiple institutions utilizing various dose-fractionation schedules have demonstrated good local control. Our purpose is to report our FSRT experience in patients with 1-3 intact or resected brain metastases. Patients treated between 2014-16 were retrospectively reviewed. Planning target volume was defined as gross lesion or resection cavity plus 2-3 mm. FSRT schedules consisted of 25 Gy in 5-fractions (BED10 = 37.5 Gy, EQD2 = 31.25 Gy) and 30 Gy in 5-fractions (BED10= 48 Gy, EQD2 = 40 Gy), typically prescribed to the 95% isodose line. One-year local control (LC), local progression-free survival (PFS), and overall survival (OS) are reported using the Kaplan-Meier method. Cox regression analysis was performed to identify factors predicting for local failure. Forty-two lesions from 32 patients, including intact (N=12) and resection cavities (N=30, 71%) were identified. Two of the cavities were treated post-subtotal resection. Twenty-three (72%) patients had one target treated with FSRT. Median age for the cohort was 69 years (IQR 60, 73.5). The majority (59.5%) of metastases originated from primary lung malignancy. Twenty patients received 25 Gy and 22 patients received 30 Gy in 5-fractions. Median PTV volume was 13.3 cc (IQR 21.6,39.7). Dosimetric evaluation revealed that most (98%) of the PTVs received 95% of the prescribed dose. At a median follow-up of 6 months, local failure occurred in 3 intact lesions and 7 resection cavities. One-year actuarial LC, local PFS, and OS were: 70.8% (95% CI 51.9-83.4), 48% (95% CI 30.5-63.5), and 61.3% (95% CI 42-75.9), respectively. There was a trend toward lower 1-year LC with 25 Gy compared to 30 Gy (65% vs 87%, p=0.076). When resection cavities were evaluated separately, 1-year local failure rate was 23.3% (N=7). Six of these were in-field failures, one was a marginal failure. All of the resection cavity failures initially underwent gross total resection followed by 25 Gy in 5-fractions. There was a trend towards inferior LC with cavities treated with 25 Gy compared to 30 Gy (59% vs 100%, p=0.06). No other clinical or dosimetric parameters including histology or PTV size predicted for local failure. The observed 1-year LC rate for intact and resected metastases was comparable to that seen in other institutional experiences. Of the resection cavities that failed, almost all were in-field recurrences despite excellent target dose coverage. Resection cavities treated with a dose of 25 Gy had a higher failure rate compared to those treated with 30 Gy. These data suggest that 25 Gy in 5-fractions may not be sufficient for long-term control of resected brain metastases and higher doses should be considered.

Comparative Evaluation of Treatment Planning Systems Used in Lung SBRT

The effectiveness of lung SBRT depends on the choice of treatment planning system (TPS) used for dose calculations. Whereas Monte-Carlo (MC) based TPS calculate the most accurate dose, conventional planning systems cannot adequately account for dose variation caused by heterogeneous lung tissue. We aim to evaluate the performance of several commercial TPS used in lung SBRT. We hypothesize that the use of MC-based TPS will result in more accurate and consistent delivered doses in lung SBRT. SBRT plans for 30 early-stage lung cancer patients (31 targets) were retrospectively evaluated in this multi-institutional study. Patient data with structure sets were anonymized and sent to four participating institutions to plan on five different TPS: one pencil-beam (PB), two convolution-superposition (CS1 & CS2), and two Monte Carlo (MC1 & MC2) systems. A test case with homogenous slab phantom was also included to allow benchmarking of each TPS. Patients were grouped according to PTV size: small (<25cc), medium (25 – 60cc), large (>60cc); and location: island, adjacent to chest-wall/mediastinum, and mixed. All patients were initially planned with MC1TPS to deliver 50Gy PTV dose in 5 fractions using 7-12 non-coplanar, conformal, 6 MV fields. Treatment plans were optimized for > 95% PTV dose coverage as per the Radiation Therapy Oncology Group guidelines. Using the same beam parameters, calculation geometry, and MUs, all patient plans were re-computed on other TPS. Dose-volume-histograms (DVH) for PTV and organs-at-risk (OARs) including lung, esophagus, trachea, heart, and spinal cord were evaluated. Two-tailed student t-test was used for statistical analysis. Average PTV was 53.5cc (range: 8.6 – 149.4cc). The largest dose difference was found between MC1 and PB for small island targets. Compared to MC1, average values of PTV Dmean for all patients were higher by 12.5%, 3%, 0.5% and 2% respectively in PB, CS1, CS2 and MC2 TPS (p < 0.01). Larger dose differences were seen for PTV Dmin: 21.8%, 4.6%, 10.6% and 12.8% (p < 0.01), indicating greater sensitivity of dose calculation algorithms in the target periphery due to density changes. Whereas the prescription dose conformality (CI) was similar in CS and MC TPS, dose spillage (D2cm) was 6.2%, 4.8%, 1.5% and 0.9% higher respectively for PB, CS1, CS2 and MC2 (p< 0.01). Compared to MC1, average lung V20 showed an 11.4% increase in PB, but was 2.4%, 2.4% and 4.4% lower in CS1, CS2 and MC2 TPS respectively (p < 0.01). Other OARs showed similar trend with MC1 having closest agreement with MC2, followed by CS2, CS1 and PB. Conventional TPS for lung SBRT result in significant dose variation depending on the calculation algorithm. MC based TPS provide the greatest accuracy and consistency in delivered doses, thereby allowing easier plan comparison among different institutions. In light of observed dose differences, current RTOG dose guidelines may require further evaluation or revision.

Evaluation of Brain Edema Formation Defined By MRI After LINAC-based Stereotactic Radiosurgery.

BackgroundPeri-lesional edema is a serious and well-known complication of stereotactic radiosurgery (SRS). Here we evaluated edema risk after SRS and assessed its formation and resolution dynamics.Patients and methods107 patients underwent SRS for heterogeneous diagnoses: 34 (29%) with arteriovenous malformations, 38 (35%) with meningiomas, 16 (15%) with metastatic tumors, 16 (15%) with acoustic neuromas, 3 with (3%) cavernomas, and 2 (2%) each with anaplastic astrocytomas and anaplastic oligoastrocytomas. Edema area was delineated in MRI T2-FLAIR sequences 0, 6, 12, 18, 24, 30, and 38 months after treatment. Lesion location was defined as either above (n = 80) or below (n = 32) the “Frankfurt modified line” (FML).Results17% of patients developed or had worsening post-treatment edema. Edema volume was maximal at 6 months (mean 7.2, SD 1.2) post radiosurgery. Post-SRS edema was 5.1 (1.06 – 24.53) times more likely in patients with lesions above the FML. There was no association between edema development and age, PTV size, number of beams, and diagnosis (p = 0.07).ConclusionsRadiosurgery-associated edema develops within 6 months of treatment and decreases over time. Edema occurrence is strongly related to lesion location, and its presence is much more likely when the treated lesions are situated above the Frankfurt line.

P21. Treatment planning for liver stereotactic body radiation therapy using FFF photon beams

Introduction Since April 2015, patients are treated in our department with liver stereotactic body radiation therapy on a linear accelerator equipped with flattening filter free (FFF) photon beams. The energy and the use of respiratory gating are adapted for each patient. This study presents the results for the first 14 patients. Material and methods. Three gold fiducials are implanted inside or in the vicinity of the lesion 7–10 days prior planning imaging. The minimum 4 h fasting patient undergoes first a 4D CT scan with systematic audio coaching and a thoracic thermoplastic mask (Orfit Industries, Wijnegem, Belgium). An MRI scan in treatment position is acquired and registered to the CT images based on the fiducials. The radiation oncologist outlines the GTV on each of the six respiratory phases. A 6 phases ITV (entire breathing cycle) and a 3 phases ITV (expiratory phases) are then created. The corresponding PTVs are created by adding a 5 mm margin. When the ratio PTV 6/ PTV 3 is higher than 1.5, the treatment is achieved with respiratory gating. The beam used is a conformal dynamic arc of 180 to 200° amplitude, conformed to the PTV. 10 MV FFF beams were preferably used as the dose rate is higher than for 6 MV FFF beams (2400 and 1200 MU/min respectively). For organs at risk, constraints from Timmerman et al. [1] were used. Results For the past year, 14 patients have been treated (8 with 3 fractions of 18 Gy, 3 with 5 fractions of 11 Gy and 3 with 5 fractions of 10 Gy), without any restriction on PTV size (from 11 to 273 cm 3 ). The prescription isodose is 80%. 5 patients were treated in free breathing (PTV 6) and 9 with respiratory gating (PTV 3). Sometimes it has been necessary to treat with respiratory gating in order to fulfill the constraints to the organs at risk. The systematic comparison for the first 10 patients between the two FFF energies showed that the dosimetric objectives were fulfilled similarly for both energy except when the PTV was close to an organ at risk. In this case, better results were obtained with 6MV FFF beam. Conclusion Despite the fact that the liver is a very mobile organ, it has been possible to treat some patients in free breathing, without compromising the dosimetric objectives, thus allowing a better patient comfort (treatment time 3 times faster). The 10 MV FFF with its 2400 MU/min dose rate was most often used, allowing treatment times lower than 10 min even with respiratory gating and the highest prescribed dose.

Dosimetric comparison of flattened and unflattened beams for stereotactic body radiation therapy: Impact of the size of the PTV on dynamic conformal arc and volumetric modulated arc therapy

PurposeFlattening filter free (FFF) beams are frequently used for stereotactic body radiation therapy with various treatment modalities: conventional static fields, dynamic conformal arc (DCA) or Rapid Arc (RA). The goal of this study was to obtain some criteria to enable a conscious choice of the employment of FFF beams and of the DCA or RA technique, depending on the PTV size. Methods and materials24 PTVs from 1.52cm3 to 445.24cm3 were studied in various sites: virtual phantom, lung and liver. For each PTV, DCA and RA plans were prepared using two flattened (FF) and two unflattened photon beams. Parameters such as conformity index, gradient index, healthy-tissue and organs at risk mean doses, number of monitor units (MU), beam on time (BOT) were used to quantify obtained dose distributions. Friedman tests and Spearman’s rank correlation coefficients were also performed. ResultsNo significant differences were found between FF and FFF beams for RA regarding conformity and gradient indices. For DCA, 10FFF is less suitable and forward planning becomes more challenging as PTV volume increases. FFF beams provided a better sparing of healthy-tissues except for 10FFF used with DCA. 6FFF was slightly better than 10FFF in terms of healthy-tissue mean doses. FFF beams generated significantly reduced BOTs and increased MUs. These effects were more pronounced for larger volumes and especially for RA plans. ConclusionsFFF showed better results than FF beams for the considered plans. 10FFF used with DCA should be used with caution for medium and large volumes.

Hippocampal sparing radiotherapy for glioblastoma patients: a planning study using volumetric modulated arc therapy.

BackgroundThe purpose of this study is to investigate the potential to reduce exposure of the contralateral hippocampus in radiotherapy for glioblastoma using volumetric modulated arc therapy (VMAT).MethodsDatasets of 27 patients who had received 3D conformal radiotherapy (3D-CRT) for glioblastoma with a prescribed dose of 60Gy in fractions of 2Gy were included in this planning study. VMAT plans were optimized with the aim to reduce the dose to the contralateral hippocampus as much as possible without compromising other parameters. Hippocampal dose and treatment parameters were compared to the 3D-CRT plans using the Wilcoxon signed-rank test. The influence of tumour location and PTV size on the hippocampal dose was investigated with the Mann–Whitney-U-test and Spearman’s rank correlation coefficient.ResultsThe median reduction of the contralateral hippocampus generalized equivalent uniform dose (gEUD) with VMAT was 36 % compared to the original 3D-CRT plans (p < 0.05). Other dose parameters were maintained or improved. The median V30Gy brain could be reduced by 17.9 % (p < 0.05). For VMAT, a parietal and a non-temporal tumour localisation as well as a larger PTV size were predictors for a higher hippocampal dose (p < 0.05).ConclusionsUsing VMAT, a substantial reduction of the radiotherapy dose to the contralateral hippocampus for patients with glioblastoma is feasible without compromising other treatment parameters. For larger PTV sizes, less sparing can be achieved. Whether this approach is able to preserve the neurocognitive status without compromising the oncological outcome needs to be investigated in the setting of prospective clinical trials.

Sci‐Fri PM: Radiation Therapy, Planning, Imaging, and Special Techniques ‐ 06: Patient‐specific QA Procedure for Gated VMAT SABR Treatments using 10x Beam in Flattening‐Filter Free Mode

Purpose:To develop a highly sensitive patient specific QA procedure for gated VMAT stereotactic ablative radiotherapy (SABR) treatments.Methods:A platform was constructed to attach the translational stage of a Quasar respiratory motion phantom to a pinpoint ion chamber insert and move the ion chamber inside the ArcCheck. The Quasar phantom controller uses a patient‐specific breathing pattern to translate the ion chamber in a superior‐inferior direction inside the ArcCheck. With this system the ion chamber is used to QA the correct phase of the gated delivery and the ArcCheck diodes are used to QA the overall dose distribution. This novel approach requires a single plan delivery for a complete QA of a gated plan. The sensitivity of the gating QA procedure was investigated with respect to the following parameters: PTV size, exhale duration, baseline drift, gating window size.Results:The difference between the measured dose to a point in the penumbra and the Eclipse calculated dose was under 2% for small residual motions. The QA procedure was independent of PTV size and duration of exhale. Baseline drift and gating window size, however, significantly affected the penumbral dose measurement, with differences of up to 30% compared to Eclipse.Conclusion:This study described a highly sensitive QA procedure for gated VMAT SABR treatments. The QA outcome was dependent on the gating window size and baseline drift. Analysis of additional patient breathing patterns is currently undergoing to determine a clinically relevant gating window size and an appropriate tolerance level for this procedure.

SU‐D‐BRB‐01: A Comparison of Learning Methods for Knowledge Based Dose Prediction for Coplanar and Non‐Coplanar Liver Radiotherapy

Purpose:The predictive power of knowledge based planning (KBP) has considerable potential in the development of automated treatment planning. Here, we examine the predictive capabilities and accuracy of previously reported KBP methods, as well as an artificial neural networks (ANN) method. Furthermore, we compare the predictive accuracy of these methods on coplanar volumetric‐modulated arc therapy (VMAT) and non‐coplanar 4π radiotherapy.Methods:30 liver SBRT patients previously treated using coplanar VMAT were selected for this study. The patients were re‐planned using 4π radiotherapy, which involves 20 optimally selected non‐coplanar IMRT fields. ANNs were used to incorporate enhanced geometric information including liver and PTV size, prescription dose, patient girth, and proximity to beams. The performance of ANN was compared to three methods from statistical voxel dose learning (SVDL), wherein the doses of voxels sharing the same distance to the PTV are approximated by either taking the median of the distribution, non‐parametric fitting, or skew‐normal fitting. These three methods were shown to be capable of predicting DVH, but only median approximation can predict 3D dose. Prediction methods were tested using leave‐one‐out cross‐validation tests and evaluated using residual sum of squares (RSS) for DVH and 3D dose predictions.Results:DVH prediction using non‐parametric fitting had the lowest average RSS with 0.1176(4π) and 0.1633(VMAT), compared to 0.4879(4π) and 1.8744(VMAT) RSS for ANN. 3D dose prediction with median approximation had lower RSS with 12.02(4π) and 29.22(VMAT), compared to 27.95(4π) and 130.9(VMAT) for ANN.Conclusion:Paradoxically, although the ANNs included geometric features in addition to the distances to the PTV, it did not perform better in predicting DVH or 3D dose compared to simpler, faster methods based on the distances alone. The study further confirms that the prediction of 4π non‐coplanar plans were more accurate than VMAT.NIH R43CA183390 and R01CA188300

SU‐F‐T‐641: Comparative Study On Planning and Delivery Efficiency of Volumetric Modulated Arc Therapy Between Varian and Elekta Platforms for SBRT Lung Treatment

Purpose:To compare the plan quality between Eclipse (Varian) and Monaco (Elekta) TPS. To ascertain, if SBRT lung treatment could be delivered in a single coplanar arc (360 degrees) with both Elekta and Varian platforms. To assess if the smaller leaf width in Varian Millennium and Elekta Agility MLC heads have a dosimetric advantage over Elekta MLCi2 headMethods:Ten SBRT lung patients (PTV volumes ranging from 11 cc to 103cc) who were previously treated on Varian Linac with non‐coplanar arcs and received 50Gy in 5 fractions were chosen for this study. The patients were replanned in Eclipse TPS (AAA algorithm) using a 360 degree coplanar single arc (SA) delivery technique and 2 partial complimentary 180 degree arcs (PA). Treatment planning using single coplanar arc (360 degree arc) was also done on Monaco TPS (Montecarlo) for both Agility (160 leaf) and MLCi2 (80 leaf) Elekta MLC headsResults:The average monitor units to deliver 10 Gy across all delivery methods were 3000 ± 474 MU and did not vary with PTV size. Coplanar single arc and partial arc techniques did not compromise either the RTOG 0813 or 0915 low dose spillage criteria for R50% or the maximum dose to any point 2cm away from the PTV. OAR doses to spinal cord, heart, great vessels, esophagus, rib and lung were comparable on both Eclipse (Varian) and Monaco (Elekta) platforms regardless of the delivery method.Conclusion:SBRT lung tumors can be treated with a single coplanar 360 degree arc in both Varian and Elekta platforms. Non coplanar arcs and increasing arc degrees more than 360 degrees had no benefit in this study regardless of the volume of PTV. 0.5 cm leaf width used in Millennium and Agility MLC heads had no significant dosimetric improvement over 1 cm leaves in the MLCi2 head.

SU‐F‐T‐528: Relationship Between Tumor Size and Plan Quality Using FFF and Non‐FFF Modes in Rapidarc

Purpose:For a give PTV dose, beam‐on time is shorter in the FFF than the non‐FFF mode because of higher MU/min. Larger tumors usually require more complex intensity modulation, which might affect plan quality and total MU. We investigated the relationship between PTV size and plan quality using FFF and non‐FFF modes.Methods:Two different PTV volumes (PTV and PTV+1 cm margin) were drawn in brain, lung and liver. 3‐full to 7‐partial arc (Rapidarc) of 6 MV, 1400 MU/min were studied. Plan quality was evaluated by: (a) DVH for PTV and normal tissues, (b) total MU and beam‐on time, and (c) passing rate for IMRT plan QA.Results:For the same PTV coverage, DVH for normal tissue was the same or slightly lower in the FFF compared with non‐FFF. Total MU was 13% higher in FFF than non‐FFF in the 3‐arc, 7 Gy treatment, but the difference became smaller when arc number increased to 6–7 for 10–24 Gy. Larger PTV did not affect the difference in the total MU. FFF required a short beam‐on time and the ratio of FFF and non‐FFF was 0.34 to 0.88 for 7‐ and 3‐arc, respectively. For larger PTV, the ratio increased to 0.45–0.90. Ratio of total MU for large PTV was 3–8% lower in the non‐FFF plans. Although the small difference in MU, beam‐on time was 1.1 to‐1.6 times longer in the 3‐ and 7‐arc non‐FFF plans. Plan verification showed the similar gamma index passing rate.Conclusion:While total MU was similar with FFF and non‐FFF modes, the beam‐on time was shorter in the FFF treatment. The advantage of FFF was greater in treatments with high dose per fraction using more arc numbers. For dose less than 10 Gy, using FFF and non‐FFF modes, tumor size did not affect the relationship of total MU, beam‐on time.

SU‐F‐T‐624: Dose Sparing to Normal Brain Tissue for Stereotactic Radiosurgery Utilizing Negative Margins

Purpose:The volume of normal brain tissue exposed to doses above 12 Gy (V12) strongly predicts the risk of radionecrosis. A negative margin MLC aperture results in fields smaller than the PTV. The aim of this study is to investigate the relationship between V12 and the negative margin MLC while keeping adequate PTV coverage with proper dose normalization.Methods:Spherical PTVs were contoured with a center of mass in the frontal and left temporal lobe ranging in size from 1 cm to 4 cm in diameter. A field arrangement was determined for each PTV location using five dynamic conformal arcs for the frontal PTV and three dynamic conformal arcs for the left temporal PTV. Treatment plans for each PTV were generated using a MLC aperture with a positive margin of 1 mm and dose normalized so that the prescribed 18 Gy covers 95% of the PTV. New plans were generated with MLC aperture margins reduced in 1 mm increments, until the max point dose reached is greater than 200%. The volume of dose outside the PTV to 12, 9 and 5 Gy was analyzed.Results:Depending on the PTV size, the greatest negative margin planned was between −4 mm and −2 mm. Negative margins of −1 mm yielded the lowest significant volumes to all doses. Reductions in V12 in relation to the PTV volumes were between 15% and 31% for PTV sizes of 4 and 1 cm, when compared to the 1 mm positive margin plan.Conclusion:Doses to normal tissue were reduced most with margins of −1 and −2 mm for all PTV sizes. In general the −2 mm margin was minimally superior, so a −1 mm margin can be followed in most circumstances to limit maximum PTV dose.