ESTRO 2020 Abstract book

S850 ESTRO 2020

and Q 0 /100; HVL, TVL and CVL are obtained, respectively. Furthermore, a toroidal HDSB bolus, Gafchromic films (EBT3) and a Euromechanics EMP 5001-424 solid water phantom are employed to verify this bolus attenuation. Bolus dimensions are 2cm thickness and 3.3cm inner diameter. Films are put on the phantom at SSD 100cm under the HDSB. The same technique explained above is used. Films are analysed with IBA OmniPRO IMRT software. Results The results obtained for HVL, TVL and CVL (figure 1) and the analysis of the film (figure 2) are shown in the following figures.

rectum) were evaluated and compared between the two modalities . 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).

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

Conclusion The HDSB necessary thickness for the first HVL in an electron beam is smaller than 5mm for a 6MeV energy and the first TVL is smaller than 8mm for the same energy. With 9mm of HDSB the charge value is reduced to a 1% of Q o . After the film dosimetry is performed, it is concluded that the HDSB can be used to modulate intensity in a 6MeV electron beam. PO-1489 Treatment Planning Study for STereotactic Arrhythmia Radioablation (STAR) of Ventricular Tachycardia I. Bonaparte 1 , F. Gregucci 1 , A. Surgo 1 , R. Carbonara 1 , N. Vitulano 2 , F. Quadrini 2 , M. Grimaldi 2 , A. Di Monaco 2 , A. Fiorentino 1 1 Miulli General Regional Hospital, Radiation Oncology, Acquaviva delle Fonti-Bari, Italy ; 2 Miulli General Regional Hospital, Cardiology, Acquaviva delle Fonti- Bari, Italy Purpose or Objective STereotactic Arrhythmia Radioablation (STAR) was recently introduced for ventricular tachycardia (VT). With precise high-dose of radiation to a well define target, STAR could become more than an option in the next future. The first STAR treatment based on linear accelerator (Linac) in our department was delivered in September 2019, thus the present analysis reported the differences of treatment plans in terms of efficiency and efficacy.