Abstract book - ESTRO meets Asia

S112 ESTRO meets Asia 2018

PO-273 FFF for lung SBRT-comparison of two beams for dosimetric accuracy and delivery efficiency M. Mundayadan Chandroth 1 , A. Venning 1 , B. Chick 1 , B. Waller 1 1 North Coast Cancer Institute, Radiation Oncology, Port Macquarie, Australia Purpose or Objective Flattening filter free (FFF) beams have shown to improve treatment delivery efficiency in stereotactic body radiation therapy (SBRT). Our clinic is equipped with 6MVFFF and 10MVFFF beams to be used predominantly for hypo-fractionated treatments. Treatment planning is done on a commercial treatment planning system (TPS) which uses a Monte Carlo (MC) dose calculation engine. This study evaluated the accuracy of beam modelling, dose calculation accuracy of the TPS in the presence of lung equivalent media, treatment delivery times with the two beams and the suitability of these beams for lung SBRT treatments. Material and Methods 6MVFFF and 10MVFFF beams were commissioned on Elekta® linac and were modelled in Monaco® TPS for Monte Carlo (MC) dose calculation algorithm. The basic validation of the beams is performed in a stack of water equivalent plastic. To test the accuracy of dose calculation in low density materials like lung, tissue equivalent slabs and phantoms are used. First, a set of treatment plans with simple geometries were created and dose calculated, for both dose to medium (Dm) and dose to water (Dw). Monaco reported radiological depths are compared to calculated depths. Then, five complex clinical 6MVFFF SBRT lung VMAT plans were recalculated on a CIRS E2E SBRT phantom®. By retaining the segments and MU, the plans were recalculated for 10MVFFF. The same plans were recalculated again by scaling the MU to match the dose of 6MVFFF plans. Dose measurements were performed using an ion chamber and EBT3 films in both cases and compared against Dm and Dw calculations. Peripheral dose to the tumour was assessed using normoxic polymer gel and EBT3 films. The treatment times for 6MVFFF and 10MVFFF plans were compared. Results Monaco reports radiological depths accurately with inhomogeneous beam path. In water, calculated dose (Dc) agreed to delivered dose (Dd) for fields down to 2x2cm 2 . With decrease in the electron density of the medium and increase in beam energy, the dose agreement becomes poor, about 15% for 10XFFF for 2x2cm 2 (Table 1). For the SABR plans, point dose at the centre of the lung tumour in E2E SBRT phantom agrees better for 6XFFF (3.1%) compared to 10XFFF (5.5%). On the periphery, the films and GEL shows that Dc overestimates the Dd, more for 10XFFF than 6XFFF. This is due to the effect of inaccurate modelling of the secondary dose build-up from lung tissue to tumour. On an average, 10XFFF plans deliver faster by 2.7sec/Gy of prescription (Table 2) compared to 6XFFF plans. Conclusion This study shows that, for VMAT lung SABR treatments, 6XFFF plan have better dosimetric accuracy compared to 10XFFF plans but delivers slower. The minimal treatment time advantage of 10XFFF beam should be weighed against the uncertainty on the tumour peripheral dose for lung SBRT. Careful consideration should be given in the selection of beam energy while planning SBRT lung treatments, especially with low density targets. PO-274 Do small (within tolerance) uncertainties matter? J. Shakeshaft 1 1 Gold Coast University Hospital, Radiation Oncology Centres, Gold Coast, Australia

middle thorax EC by 14.7%(241s vs. 282s), 23.6%(237 s vs. 310 s) and 20.8%(246s vs. 311s), respectivly. Conclusion For the the middle thorax EC V-IMRT is better than E- IMRT,not only in treatment time but also in the PTV dose; while for the lower thorax EC E-IMRT is the first choice for better CI; for the other part of the EC usually E-IMRT can increase the CI, while V-IMRT can reduce treatment time, can be selected according to the actual situation of the patient treatment. PO-272 Validation of beam matching accuracy between beam- matched linacs and its impacts on IMRT Delivery C. Krishnappan 1 , C. Krishnappan 2 , C. Anu Radha 2 1 Apollo CBCC Hospitals, Department of Medical Physics, Gandhinagar, India 2 VIT University, School of Advanced Sciences, Vellore, India Purpose or Objective To evaluate the dosimetric variation between two beam matched linacs from the same vendor at the levels for IMRT plan delivery. Material and Methods Two beam matched Varian machines of one Varian Clinac iX, and one 600 C/D is taken for the evaluation. Sixty IMRT fields of different treatment site at different gantry angles which were earlier treated on 600 C/D machine at our hospital are chosen. For all sixty fields plans the portal predication are created by using the Eclipse TPS. The generated QA plans were deliver with the same 600 C/D and evaluated the maxiumum gamma, average gamma and area gamma less than one are calculated and analyzed. The same plans are delivered at Clinac iX by using the option of machine over ride. The 600 C/D Vs 600 C/D and 600C/D Vs Clinac iX are compared and evaluated. The results between the two evaluations are compared with t- distribution tests. Results The average of maximum gamma, average gamma and area gamma less than one pass criteria between the two machines is 0.37±0.29, 0.06±0.05 and 0.87±0.86. The results shows that there is no statistical significant between the delivery at both the machines. Conclusion Our overall results shows that dosimetric variation for swapped plans between the two machines at IMRT plans are in the range of clinically accepted limits.