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SU‐GG‐J‐34: Comprehensive Clinical Commissioning and Quality Assurance Procedures of a Big Bore CT Simulator in a Radiation Oncology Department

Purpose: To perform acceptance testing and clinical commissioning of a 16‐slice big bore CT‐simulator (85 cm) and comprehensive quality assurance (QA) procedures for radiation treatment planning using several phantoms. Method and Materials: This study focused on the performance evaluation of this CT simulator in a radiation oncology environment through the acceptance testing and commissioning for clinical use. Parameters derived from commissioning were used as reference values for a comprehensive QA program. Several phantoms were used in this project to evaluate the CT simulator image quality, like ACR phantom, Vendor (Philips) provided phantom and CATPHAN 504. CTDI phantom was used to evaluate the radiation dose to patients during scans. RMI phantom was used to evaluate the CT number linearity for use in the treatment planning system for density correction. Results: For clinical commissioning, the radiation dose reported in this study is lower compared to published data based on CTDI measurements. The results from ACR phantom showed that image quality specifications were met. The highest spatial frequency for two parts of the ACR phantom; abdomen and chest, there were 8 and 9 lp/cm; respectively visualized. The manufacture specification is 5 lp/cm. CT number linearity and low contrast resolution tests were also within acceptance requirements. Image quality evaluation parameters from different phantoms were compared. These showed good self consistency using this CT simulator. Conclusion: A comprehensive analysis of the test results indicates consistent and reproducible operation of the big bore CT‐simulator. This big bore CT simulator is well suited for use in a radiation oncology setting.

SU‐E‐J‐108: Quantitative Analysis of Longitudinal Cognitive Impairment Due to Radiation Therapy Based on Automatic Segmentation of Hippocampus and Subcortical Structure

PURPOSE In this study, we developed a quantitative analysis tool based on patients longitudinal MR images to 1) measure the radiation dose received by each subcortical structure, 2) follow the change of volume and shape of each structure longitudinally. This tool provides a systematic approach to study the radiation therapy (and subsequent chemotherapy) associated with cognitive impairments. METHODS MRI scans of one patient taken before and after radiation therapy are demonstrated in this study. 3D Conformal radiation therapy was performed on RapidArc™. An open source MRI analysis tool, FMRIBs Integrated Registration and Segmentation Tool (FIRST), was used for segmentation. The images are registered to a standard template with expert-defined labeling for all sub-cortical structures, and the labeling of each structure is mapped back to the individual MRI space for segmentation. After the segmentation, the radiation dose map was coregistered to the MRI space to calculate the dose received by each structure. RESULTS For the structure that is contained within the radiation zone, we can calculate the total dose based on the volumetric distribution of radiation dose. For the structure that is outside the radiation field, we can calculate the distance from the radiation zone. We have demonstrated in this work that the analysis can be done for all segmented sub-cortical structures. The change of volume before and after radiation treatment can be analyzed, and the results can be correlated with the change of cognitive performance over time. CONCLUSIONS We presented an automated tool for efficient, quantitative and user-independent measurements of radiation dose in subcortical structures. The obtained results can be correlated with the cognitive test score and the clinical outcome to evaluate radiation and the subsequent chemotherapy induced changes in brain structures and functions.

SU-FF-T-104: Simultaneous Boost and Skin Dose Toxicity Reduction for Breast Cancer Treatments Using IMRT and RapidArc

Purpose: To reduce the overall treatment fractions by about a week and minimize the skindose toxicity by treating stage I‐II breast cancer patients with IMRT and eventually RapidArc both, incorporating a simultaneous boost to the tumor bed. Background:Radiation therapy is prescribed to breast cancer patients after receiving a lumpectomy or mastectomy to remove canceroustissue. In the past, treatments using two 6MV tangential fields with wedges followed by an electron boost to the tumor bed have often resulted in unnecessary dose to unaffected tissue. As a consequence, patients experienced edema, pain, desquamation, discoloration, and other negative side‐effects. IMRT and RapidArc are treatment techniques where the dose can be tailored to the shape of the target, reducing the exposure of adjacent tissues to unnecessary radiation and potential damage. Methods and Materials:Skindose measurements using TLDs from 35 patients receiving conventional radiotherapy were collected over a period of 9 months. Seven 3×3×1 mm3TLD chips were placed on the 4 quadrants of a patients breast and 3 on the inframmary fold. Same TLD arrangement was used for patients receiving IMRT or RapidArc which, in addition, incorporated a simultaneous boost of an additional ∼10Gy delivered to the tumor bed, within the regular 28 treatment sessions. Results: Preliminary data indicates percent differences of 4–20% less skindose on the IMRT/RapidArc cases, primarily in the inframammary fold region. Furthermore, our skindose data on the IMRT/RapidArc treatments indicates a strong correlation to the size and shape of the breast under treatment suggesting that patients with large breast may experience little skin sparing — confirming observations reported elsewhere. Dose to heart, lungs and contralateral breast were kept to a minimum. Conclusion: This work continues with more breast patients receiving IMRT or RapidArc, and thus far the data continues to support our preliminary observations.

SU-F-T-568: QA of a Multi-Target Multi-Dose VMAT SRS

PURPOSE To, experimentally, corroborated the prescribed doses utilizing dosimeters (e.g. films and TLDs) that can provide high spatial resolution, allow dose measurement of multiple targets at once, and provide accurate dosimetric results. METHODS A single-isocenter 6FFF SRS VMAT plan consisting of one 358° arc at 0° couch angle and four 179° arcs at 30°, 60°, 330° and 300° couch angles respectively, was generated in ECLIPSE v.11 using a Rando-Alderson anthropomorphic head phantom CT study. This plan was a reproduction of a clinical plan generated for a stage-IV melanoma patient diagnosed with 19 intracranial lesions. The phantom was loaded with axially mounted (between phantom slabs) Gafchromic EBT3 film and TLDs strategically positioned within various target volumes. Film and TLDS were calibrated according to established protocols. Target prescription doses were 16 Gy (3cc≤, 3 lesions), 18 Gy (∼1-3cc, 10 lesions) and 20 Gy (≤1cc, 6 lesions). Phantom setup was verified through CBCT imaging prior to irradiation. Gafchromic films were scanned in transmission mode and TLDs were read, respectively, ∼24 hrs after irradiation. RESULTS Dose calibrated Gafchromic film data were compared to the ECLIPSE calculated data using a 3% / 3mm gamma function analysis. Results for the gamma values were 96-99% in agreement with the calculated data and with 84-90% of the film pixels within the 3% dose difference. TLD data showed a dose difference of 0.4-8% while the film data for those same locations yielded a difference of 0.4-4%. It was observed that the highest dose discrepancies correlated with the location of the small volume targets. CONCLUSION Overall this study corroborated that a VMAT SRS treatment, employing various treatment table rotations and arcs, to multiple intracranial lesions with multiple dose prescriptions can be delivered accurately with the existing radiotherapy technology.

SU-E-T-216: Comparison of Volumetrically Modulated Arc Therapy Treatment Using Flattening Filter Free Beams Vs. Flattened Beams for Partial Brain Irradiation

Purpose: Flattening Filter Free (FFF) beams offer the potential for higher dose rates, short treatment time, and lower out of field dose. Therefore, the aim of this study was to investigate the dosimetric effects and out of field dose of Volumetric Modulated Arc Therapy (VMAT) plans using FFF vs Flattening Filtering (FF) beams for partial brain irradiation. Methods: Ten brain patients treated with a 6FF beam from a Truebeam STX were analyzed retrospectively for this study. These plans (46Gy at 2 Gy per fraction) were re-optimized for 6FFF beams using the same dose constraints as the original plans. PTV coverage, PTV Dmax, total MUs, and mean dose to organs-at-risk (OAR) were evaluated. In addition, the out-of-field dose for 6FF and 6FFF plans for one patient was measured on an anthropomorphic phantom. TLDs were placed inside (central axis) and outside (surface) the phantom at distances ranging from 0.5 cm to 17 cm from the field edge. Paired T-test was used for statistical analysis. Results: PTV coverage and PTV Dmax were comparable for the FF and FFF plans with 95.9% versus 95.6% and 111.2% versus 111.9%, respectively. Mean dose to the OARs were 3.7% less for FFF than FF plans (p<0.0001). Total MUs were, on average, 12.5% greater for FFF than FF plans with 481±55 MU (FFF) versus 429±50 MU (FF), p=0.0003. On average, the measured out of field dose was 24% less for FFF compared to FF, p<0.0001. A similar beam-on time was observed for the FFF and FF treatment. Conclusion: It is beneficial to use 6FFF beams for regular fractionated brain VMAT treatments. VMAT treatment plans using FFF beams can achieve comparable PTV coverage but with more OAR sparing. The out of field dose is significant less with mean reduction of 24%.

SU-C-137-01: Out-Of-Field Fetal Dose Measurement From a Head-And-Neck Treatment with VMAT: An Anthropomorphic Phantom Study

PURPOSE The irradiation of pregnant patients is an uncommon occurrence but if discovered requires accurate fetal dose estimates. For this purpose, we measured the out-of-field dose to the fetal region from the primary dose given to a head-and-neck tumor(s) irradiated with a volumetric-modulated arc therapy (VMAT) technique. METHODS A VMAT-RapidArc plan was generated in ECLIPSE for a head-and-neck case extrapolated to an anthropomorphic phantom. The clinical plan, including target and OAR contours was co-registered to the phantom CT study. The phantom received a total dose of 60 Gy in 2 Gy/fx using a 6 MV photon beam from a Varian Trilogy irradiated using two 358o axial arcs. 108 TLDs and 6 Gafchromic films were mounted outside and inside the phantom to measure the out-of-field surface and internal scatter dose. Films were mounted axially in the phantom so that ∼60% of the area provided internal scatter and ∼40% leakage dose data. RESULTS At 9 cm inferior to the primary treatment site, the leakage dose (∼150 cGy) was about two times higher than internal scattering (∼80 cGy). At 30 cm, the leakage (∼40 cGy) was 2-3 times higher than the internal scatter dose (∼15cGy). The highest dose measured by TLDs at 25 cm away from the target was 20 cGy (0.3 % of total). Although, the data showed higher dose contribution from leakage than internal scattering, the combined amount was small with respect to the primary dose. The estimated fetal dose from these measurements amounted to 0.1-0.3 cGy (at 45-60 cm from the target) which could be slightly higher compared to the established NCRP #91 recommendation (0.5 mSv). CONCLUSION Fetal dose measurements at different stages of pregnancy using a VMAT-RapidArc technique are underway. This data should provide updated and comprehensive information to complement TG-36.

SU‐GG‐T‐109: Comprehensive RapidArc™ Treatment Planning and Quality Assurance for Head and Neck Cancers

Purpose: To assess the dosimetric quality of a two‐arc RapidArc™ plans for the treatment of head and neck cancers and present corresponding quality assurance (QA) results. Method and Materials: Fifty three patients (male=32,female=21, = 61.9 years (range: 25–87 years) treated for nasopharynx, oropharynx, base of tongue and laryngealcancers were included in this study. Treatment doses ranged from 12Gy to 70.4Gy with many cases requiring irradiation of the cervical nodes as well as the primary site. RapidArc™ plans were generated using Varian Eclipse™ 8.6 and consisted primarily of two 358‐degree arcs delivered counterclockwise and clockwise, respectively with a ±5‐degree couch rotation for one of the arcs. Based on the treatment site and target location, there were instances where a 358‐degree arc and a partial arc with 90‐degree couch rotation were used. QA plans were generated and delivered to a solid‐water phantom with a Mapcheck™ detector array centrally mounted between the solid‐water slabs. Results: The RapidArc™ treatment plans were evaluated based on RTOG conformality index (CI), RTOG homogeneity index (HI), monitor units (MUs) and beam‐on time. The primary targets (PTVs) had mean±SD CI and HI values of 0.93±0.04 and 1.10±0.03, respectively. The average number of MUs was 565 (range: 350 – 2144) and beam‐on times ranged from 3 to 5 min. The average %PASS for the plan QA was 99.3% (range: 97.9% – 100%) using the 3% /3mm plan evaluation criteria in Mapcheck™. Conclusion: This dosimetric analysis indicates that a two‐arc RapidArc™ plan provides highly conformai dose distributions to head and neck treatments. QA results have shown that dose calculation and measurement are in good agreement for these plans. Short beam‐on times and few MUs have reduced the overall treatment time (setup + beam‐on times) by ∼40% per patient making RapidArc™ a more efficient delivery technique than multifield IMRT.

SU‐GG‐T‐154: Special Radiation Treatment Procedures Using RapidArc™

Purpose/Objective(s): To report on the capabilities of RapidArc to deliver a conformai dose to a highly irregular target and its capability of sparing nearby organs‐at‐risk (OARs). Material/Methods: RapidArc™ consisting of 2 and 3‐arc plans were generated to treat highly irregular targets on five patients (male=1, female=4, =68 yrs) diagnosed with inflammatory chestwall, angiosarcoma of the scalp, melanoma of the scalp, cancer to the left axilla and cancer to the left orbit respectively. The average target volume was 1019.86 cc (range: 50.7cc ‐ 3039.9cc) with the smallest target attributed to the left orbit and the largest to the inflammatory chestwall. Treatmentdoses ranged from 30Gy to 60Gy and were delivered in fractions of 1.8, 2.0 and 3.0Gy. Full (360‐deg) and partial arcs as well as 90‐deg couch rotation were used to treat these irregular sites. With the exception of the axilla case, all patients were treated with bolus of 2.5mm or 5mm thick. RapidArc™ plans were generated using Varian Eclipse™ 8.6 treatment planning software which allows for multiple arcs in a plan. Daily kV/kV and weekly CBCTimaging were performed prior to treatment for verification of patient positioning. Results: Beam‐on times ranged from 3–6 min and the average number of monitor units was 814 (range: 350 – 1907). Dosimetric evaluation of the plans was performed based on RTOG conformality index (CI) and homogeneity index (HI). The overall mean±SD values for CI and HI were 0.93±0.05 and 1.18±0.06 respectively. Remarkable OAR sparing was observed in all cases. Dose verification plans were generated and delivered to a solid‐water phantom comprising a Mapcheck unit. Conclusion: RapidArc™s capability of delivering multiple coplanar and non‐coplanar arcs to irregularly shaped targets with short beam‐on times have resulted in very conformai doses with great OAR sparing.

SU‐FF‐T‐260: In‐Vivo Dosimetry Verification of a 3D Treatment Plan Prescription Dose at a Depth Beyond Dmax Using Diodes

Purpose: To provide an in‐vivo verification using diode detectors of a patient prescription dose calculated by a three‐dimensional treatment planning algorithm. Method and Materials: Three models of diodes specially designed for high‐energy photon beams (6‐MV Isorad and QED, and the 20‐MV QED diode detectors manufactured by Sun Nuclear Corporation, Melbourne, Florida) were used to measure the percent depth dose (PDD) of a 6‐MV and 20‐MV photon beams produced by a Varian 2300 C/D Linac. These diodes were not water proof and the measurements were performed at multiple depths in a solid‐water phantom. Results: The measured PDD curves agree within 2% with clinical PDD curves scanned in a water tank to depths up to 25 cm. This allows scaling of the diode dose reading measured at the patient skin to a depth inside the patient which is usually the prescription point or the isocenter for a multiple beam SAD treatment as defined in a 3D treatment plan. In‐vivo measurements were performed on real patients. The doses scaled to the measurement depth were summed for all beams and compared with the calculated value from the treatment planning system (TPS). The treatment sites and agreement are listed below. ‐ 10 head and neck cases thus far, 0.4%–6.7% difference between calculated and measured doses, ‐ 8 pelvis cases thus far, 1.9%–6.5% difference between calculated and measured doses, ‐ 6 breast cases thus far, 2.3%–5.4% difference between calculated and measured doses, 4 chest cases thus far, 1.3%–6.7% difference between calculated and measured doses. Conclusion: An in‐vivo dosimetric method has been developed to measure delivered dose to the prescription point at a depth within the patient. Overall agreement between the measured and TPS‐calculated dose to the prescription point falls within ± 7% as recommended by the TG‐62 protocol.

SU‐FF‐T‐438: Using Diode Dosimeters to Characterize Dose in the Buildup Region of High‐Energy Photon Beams

Purpose: For external photon beams, the depth dependence of the stopping power ratio between air and water in the buildup region is difficult for accurate quantitative characterization. Qualitatively the depth dependence increases as the photon energy increases above the cobalt‐60 energy. As a result, the depth dependence is often ignored and the “uncorrected” percentage depth dose curves are being used in many clinics. However, accurate knowledge on dose in the buildup region is important for applications such as the tangential beam setup. This work is to look for a dosimeter that measures with ease the correct photon beam PDD curves for all depth in phantom including the buildup region and beyond. Method and Materials: The PDDs of a 10 MV photon beam at field size of 10×10 cm2 and 100 cm SSD was scanned in a water‐tank with a cylindrical (Scanditronix, RK) ion chamber, a parallel‐plane (NACP) ion chamber, and a diode dosimeter (Scanditronix, designed for photon field with back‐scatter absorber). The raw scan readings were corrected only for the effective point shifts as defined for depth beyond dmax. The Monte Carlo code EGS was used to generate the “true” PDD curve. Results: The ion chambers, diode and EGS show good agreement beyond the dmax region. In the buildup region, the diode matches the EGS curve within 3% while the two ion chamber curves differ from the EGS curve by 5% to 20%. Conclusion: A scanning diode dosimeter can be designed to generate true PDD curves, with a simple effective point correction, for both the buildup region and beyond for high energy photon beams. The ease of use afforded by such diode dosimeters would mean more availability of true PDD curves at clinic, including the dose buildup region.