ESTRO 36 Abstract Book

S803 ESTRO 36 _______________________________________________________________________________________________

Calculated and measured doses are the average doses of symmetrical angles from 180º. Reference dose without Fraxion was the average dose at 0º, 90º, and 270º. 100 MU were delivered at each angle. All measured doses were compared with the ones calculated with Monaco. To measure the skin dose and the dose distribution in the Build-up region, several radiochromic Films EBT3 were placed at linac CAX between the slabs of a RW3 phantom placed over Fraxion (SSD= 90 cm) and read using FilmQA Pro software. Films were situated at the surface, 0.5 cm, 1.5cm depth and the linac isocenter. 200 MU were delivered for 10x10 and 5x5 open field sizes and 0º gantry angle. Once irradiated and removed, another set of films were placed under the phantom, in contact with Fraxion, and at 0.5 cm and 1.5 cm from Fraxion, as well as at the linac isocenter. Additional films were located 1 cm away from CAX as in this section Fraxion is wider. Same field sizes and MU at 180º were employed. Results Table 1 shows the comparison between measured and calculated transmitted dose with and without Fraxion in the calculation. Measurements show a 1% attenuation for 180º gantry angle as stated on the Fraxion manual, but this attenuation can be as high as 5 % (5x5 open field) or 6 % (10x10 open field) for 150º gantry angle, as with this angle, the beam traverses the thickest part of the Fraxion. If Fraxion is not included in the calculation, Monaco calculation can result in a 7 % difference between measured and calculated doses, while with Fraxion in the calculation, the maximum difference is 1.5% (10x10, 150º). Table 2 shows the evaluated skin dose increment caused by Fraxion, and compares calculated and scanned values. Fraxion increases 3.8 times the surface dose, and by 17% at 0.5 cm depth, which can be calculated by Monaco with a difference lower than 1% if Fraxion is included in the calculation.

Conclusion It has been shown that the attenuation varies with gantry angle. The inclusion of Fraxion in Monaco improves the calculation from 7% difference to 1% in the worst case (150º, 5x5 open field), furthermore, the skin dose increment and the dose in the build-up region are correctly calculated. EP-1498 IMRT and VMAT commissioning for Versa HD linear accelerator using AAPM TG-119 S. Sharma 1 , V. Subramani 1 , P. Kumar 1 , S. Bhaskar 1 , S. Pathy 1 , S. Thulkar 1 , M. Sairem 1 , A. Binjola 1 , P. Agarwal 1 , N. Dhayanethi 1 , P. Kumar 1 , S. Chander 1 1 All India Institute of Medical Sciences, Radiation Oncology, New Delhi, India Purpose or Objective The purpose of the study is to evaluate the end to end commissioning accuracy of intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) for Versa HD linear accelerator using AAPM TG-119 protocol. Material and Methods Phantom with contoured structure set was downloaded from AAPM website provided with the TG119 report and above structure sets were used as the patient for all plans created in the study. IMRT [step and shoot (SMLC) and dynamic (DMLC)] and VMAT plans were created for TG119 test cases. All the plans were generated using Monaco 5.1 treatment planning system (TPS) for Elekta Versa HD (Crawley UK) linear accelerator. All plans were created using 7-9 beams for IMRT (as per TG119) and single arc for VMAT for energy 6MV, 6MV-FFF & 10MV-FFF (FFF- Flattening filter free). Prescription and planning goals were as kept as per TG119. For point dose measurement CC01 (0.01cc) ion chamber was used and measurements were carried out as per TG119 specified points in high and low dose gradient regions. Point dose difference was calculated as ratio of difference between measured and planned dose with prescription dose. Similarly for planar dose measurement I'matriXX (IBA, Scanditronix Wellhofer, Germany) along with multicube-lite phantom was used and measurement plane was kept at 11cm depth. Planned and measured dose planes were compared using gamma index criteria (dose difference/distance to agreement) of 3%/3mm and 2%/2mm. All measurements were performed by keeping phantom on couch at gantry angle zero. Confidence limit calculation was done as specified in TG119.