ESTRO 36 Abstract Book

S823 ESTRO 36 _______________________________________________________________________________________________

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 C. Cases 1 , A. Latorre-Musoll 1 , P. Carrasco 1 , N. Jornet 1 , T. Eudaldo 1 , A. Ruiz-Martínez 1 , M. Lizondo 1 , P. Delgado- Tapia 1 , M. Ribas 1 1 Hospital de la Santa Creu i Sant Pau, Radiofisica i Radioprotecció, Barcelona, Spain Purpose or Objective For SBRT lung treatments accurate 4D dose calculations, accounting for heterogeneities and respiratory motion, are crucial to determine an optimal ITV beyond a purely geometric ITV. We propose a model to predict an optimal ITV from a single figure computed from the Probability Density Function (PDF) of the breathing waveform. Material and Methods We used the QUASAR Respiratory Motion Phantom (Modus Medical Devices) with a cylindrical mobile wood insert as lung substitute and an inner 30mm diameter sphere as tumour substitute (GTV). We acquired 21 independent scans (CT z ) by axially shifting the mobile insert from z = – 10 to z = 10mm in 2mm steps. We generated 6 ITV: from ITV 0mm (static case, equal to the GTV of CT 0mm ) to ITV 10mm (overlap of all GTV from CT –10mm to CT 10mm ). We planned a SBRT treatment collimating to each ITV (from PLAN 0mm to PLAN 10mm ) in Varian Eclipse (AAA v13.5) using a 6MV non- coplanar 3DCRT technique. Due to the in silico nature of the study we added no extra margins to the ITV. We considered 3 breathing patterns: sinusoidal (provided by QUASAR software), free and trained (obtained by the Varian RPM from real patients). We rescaled every

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).

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