ESTRO 36 Abstract Book

S812 ESTRO 36 2017 _______________________________________________________________________________________________

Purpose or Objective To demonstrate with end-to-end tests the ability of RayStation v5.02 (RaySearch Laboratories AB, Stockholm, Sweden) fallback planning module (RFP) to perform an accurate Helical Tomotherapy (HT) to volumetric modulated arc therapy (VMAT) plan conversion by validating the dose-mimicking algorithm used during the automatic optimization of the fallback plans. Material and Methods Thirty patient plans of various treatment sites previously treated with HT were switched to 6 MV dual-arc VMAT plans using RFP and default dose-mimicking algorithm parameters. For the purpose of this study no further optimizations were performed and delivery quality assurance (DQA) were designed for each fallback plan. DQA were delivered on a TrueBeam linear accelerator (Varian Medical Systems, Palo Alto, CA) and planar/absolute dose measurements were acquired using the ArcCHECK diode array (Sun Nuclear Corporation, Melbourne, FL) with an insert containing an Exradin A1SL ionization chamber (Standard Imaging, Middleton, WI). 3D dose distributions in the patient geometry were reconstructed within 3DVH software (Sun Nuclear Corporation, Melbourne, FL) by using ArcCHECK Planned Dose Perturbation (ACPDP). Agreement between planned and delivered dose was eventually evaluated with global and local 2D/3D gamma-index analysis (3%/3mm and 2%/2mm criteria) and DHV-based comparisons were performed using the following dosimetric parameters: quality of coverage (Q=D98%/Dref), mean dose to target (MDT=Dmean/Dref) and integral dose to organs at risks (ID_OAR=∑·Di·Vi).

Conclusion Fallback planning is an advanced RayStation feature that uses a dose-mimicking function to automatically replicate the DVH and the dose per voxel of a given plan, but for an alternative treatment machine or technique. Results presented here through a Helical Tomotherapy to VMAT plan conversion show a good agreement between planned and delivered dose for point dose measurements, gamma-index analysis and DVH-based comparisons, hence validating the dose-mimicking algorithm used during the automatic optimization of the fallback plans. EP-1531 Collimator angle influence on dose coverage for VMAT SRS treatment of four brain metastases C. Ferrer 1 , C. Huertas 1 , A. Castaño 2 , A. Colmenar 2 , R. Plaza 1 , R. Morera 2 , A. Serrada 2 1 Hospital universitaria La Paz, Radiofísica y Radioprotección, Madrid, Spain 2 Hospital universitaria La Paz, Oncología Radioterápica, Madrid, Spain Purpose or Objective To evaluate the collimator angle influence on the dose coverage of 4 brain metastases treated with volumetric modulated arc therapy (VMAT) stereotactic radiosurgery (SRS). Material and Methods Three brain metastases were prescribed to 18Gy, and a fourth one located in the cerebellar tonsil to 16Gy. Treatment was planned with Elekta Monaco treatment planning system (v. 5.00.00), and optimized using biological and physical based cost functions for mono- isocentric VMAT SRS treatment on an Elekta Synergy linear accelerator equipped with a 160-leaf Agility MLC. Five non coplanar partial arcs were used, plus a full clockwise- counterclockwise arc with 0° couch rotation to modulate only the fourth lesion with different prescription and away from the other three. Planning target volume (PTV) coverage and dose to organs at risk (OAR) have been evaluated for three different collimator angle positions, 5°, 45° and 95°. Treatment constraints were the same for the three plans, one treatment plan for each collimator angle. Results The best plan in terms of target coverage and number of monitor units was achieved with collimator angle set to 95°, with the 95% of the PTV volume receiving more than 95% of the prescription dose for the 4 lesions, with 35.8% less total MU compared with the 5° collimator angle plan (5176 MU versus 8061 MU). The target coverage for the 45° collimator angle plan was lower than for the other two plans. OAR maximal doses were similar for the brainstem, optic nerves and eye lens, but maximum dose to the optic chiasm was 42% and 49.1% lower for the 5° collimator angle plan compared with the 95° and 45° angle plan respectively. Conclusion The choice of collimator angle influences the target coverage as well as the total MU and the doses to OAR. The optimal choice of this angle in VMAT SRS treatments improves the optimization outcome. EP-1532 ITV optimization for SBRT lung treatment planning accounting for respiratory dose blurring

Results Results of point dose measurements, gamma-index analysis and HDV-based comparisons are listed in table 1. Absolute dose differences were all <1% with an average value of 0.4±0.4%. Average differences of gamma passing rate (%GP) with a low-dose threshold of 10% of the maximum dose were 99.9±0.2% and 99.2±1.2% (2D global 3%/3mm and 2%/2mm criteria), 95.9±3.4% and 89.4±6.5% (2D local 3%/3mm and 2%/2mm criteria), 99.5±0.9% and 97.0±2.9% (3D global 3%/3mm and 2%/2mm criteria), 96.9±2.8% and 89.2±6.0% (3D local 3%/3mm and 2%/2mm criteria) respectively. Finally, DVH-based comparisons between calculated and delivered fallback plans showed differences of respectively -0.5±0.8% for the quality of coverage (Q), -1.0±0.7% for the mean dose to target (MDT) and 0.3±0.9% for the integral dose to organs at risks (ID_OAR).