ESTRO 2020 Abstract Book

S800 ESTRO 2020

Results Both photon and proton plans provided excellent target volume coverage, homogeneity and conformity. With both SFO and MFO, there was a reduction of the V20 and V30 to the organs at risk compared to VMAT (table below).

electron beam percentage depth dose (PDD) in 3D-printed samples with different material filling is performed. Material and Methods To optimize the manufacturing of plastic shaping samples, numerical simulation of electron beam PDD in 3D-printed samples is carried out in 100-75% material filling (k) range with the 5% step. For these purposes, the cubic PLA samples (50% C, 5.6% H, 44.4% O) are created with UP! Plus 2 by fused deposition modelling. The measured densities are varied from 1.2 g/cm 3 (k=100%) to 0.9 g/cm 3 (k=75%). Geant4 toolkit is used for clinical electron beam model creation. The radiation parameters are selected according to real 6 MeV extracted electron beam of ONCOR Impression Plus medical linear accelerator. The statistical error for the calculated results is less than 1%. Results The electron beam PDD in 3D-printed samples with 100- 75% material filling is simulated using a created model. It is shown that the depth of the dose maximum (R Dmax ) is shifted with material filling factor changing. R Dmax values equals to 12 mm (k=100%), 14 mm (k=90%), 16 mm (k=80%) and 17 mm (k=75%). The half-value depth (R 50 ) equals to 18 mm (k=100%), 21 mm (k=90%), 23 mm (k=80%) and 24 mm (k=75%) accordingly. The plastic sample thickness required for electron beam total absorption (R max ) are 25 mm (k=100%), 28 mm (k=90%), 30 mm (k=80%) and 32 mm (k=75%). Conclusion The obtained results allow us to conclude that electron beam shaping devices can be produced using fused deposition modelling with variable material filling of sample volume. It is shown that for k=90% the main parameters of electron beam PDD (R Dmax , R 50 , R max ) are shifted to 3 mm and for k = 80% to 5 mm. This does not significantly increase the samples printing time to achieve a given sample thickness. At the same time, the manufacturing quality of such samples significantly higher in comparison with 100% filling. This work is supported by the Russian Science Foundation, project No. 19-79-10014. References 1. Miloichikova I. et al. (2019). Physica Medica , 64, 188- 194. 2. Miloichikova I. et al. (2019). Radiotherapy and Oncology , 133, S1008-S1009. PO-1487 Proton versus photon therapy in locally advanced cervical cancer; a dosimetric study D. Abdulwahid 1 , D. Lines 2 , P. Stich 2 , J. Lee 2 , L. Barraclough 1 , P. Hoskin 1 1 The Christie NHS trust, clinical oncology, Manchester, United Kingdom ; 2 The Christie NHS trust, Physics, Manchester, United Kingdom Purpose or Objective To investigate whether protons deliver comparable target dose whilst reducing organs at risk (OAR) doses for the external beam component of cervical cancer treatment which could allow dose escalation for the brachytherapy component and improve toxicities. Material and Methods Five patients who received radical chemoradiation with a Volumetric Modulated Arc Therapy (VMAT) technique using the EMBRACE II protocol for definition of planning volumes and constraints. The prescribed dose to the planning- target volume primary (cervix, uterus, pelvic ± para-aortic lymph nodes) was 45 Gy in 25 fractions. The SIB dose for the involved nodes was 55 Gy in four patients. They were retrospectively replanned with single field (SFO) and multi-field (MFO) optimised proton plans. The lower target constraints were 95% of the prescribed dose in 95% of the target volume. For every patient, target parameters as well as V20, V30,V40 to the organs at risk (bladder and rectum) were evaluated and compared between the two modalities .

Conclusion Photon and proton plans were dosimetrically adequate in terms of target coverage, conformity and homogeneity. Protons offered the best sparing of bladder and rectum at low dose levels but were similar for dose volume parameters >V30. This could contribute to a significant reduction of acute and late toxicity in cervical cancer treatment. Further validation with clinical endpoints will determine whether there is a role for proton therapy instead of photon therapy to improve the side effect profile of patients with locally advanced cervical cancer. PO-1488 Emerging method of beam shaper with high density silicone bolus for electron beam radiotherapy N. López-Martín 1 , F. Derecho-Torres 1 , G. Muñiz-Romero 1 , S. Velázquez Miranda 1 1 Hospital Universitario Virgen del Rocio, Medical Physics, Sevilla, Spain Purpose or Objective Beam shaper can be made it with high density silicone bolus. The aim of this study is evaluating the performance of this high density silicone bolus for conformal electron radiotherapy and preparing it for clinical applications. Dosimetric characteristics for different combinations of field size were determined by ionometric dosimetry and film dosimetry. This high density bolus could be place on the skin to shield normal tissue areas. It is essential to calculate the half value layer (HVL) and the tenth value layer (TVL) for 6MeV energy of an electron beam. In addition, the thickness necessary to reduce at 99 % the dose is determined (CVL). Material and Methods 3mm of high density silicone bolus (HDSB) sheets are used to calculate HVL, TVL and CVL. A VARIAN TrueBeam linac and an IBA PPC40 detector are employed. The detector is put into a solid water phantom at source-surface distance (SSD) of 100cm. The detector effective point is 1.5cm from the phantom surface. A 6x6cm 2 field is chosen with a dose rate of 600MU/min and 500MU are delivered. Different HSDB thicknesses are put on the phantom surface and the charge variation is measured. Five measurements per thickness are repeated. A curve “HDSB thickness vs. Charge” is obtained and fitted to a six grade polynomial function with R=1. Solving the equation for Q o /2, Q o /10 and Q 0 /100; HVL, TVL and CVL are obtained, respectively. Furthermore, a toroidal HDSB bolus, Gafchromic films