ESTRO 2021 Abstract Book

S1340

ESTRO 2021

Medical Physics Department, Poznan, Poland; 3 Poznan University of Technology, Faculty of Materials Engineering and Technical Physics, Poznan, Poland; 4 Poznan University of Medical Science, Department of Electroradiology, Poznan, Poland; 5 Greater Poland Cancer Centre, Department of Medical Physics, Poznan, Poland Purpose or Objective The first aim of this study was to validate the Monte Carlo dose calculation in the presence of very high- density materials (from 4.5 to 19.4 g/cm 3 ) using EBT 3 gafchromic films. The second aim was to build a dedicated holder for gafchromic films and metal inserts enabling percentage depth dose (PDD) measurements. Materials and Methods Constantly increasing number of patients who have a metallic implant are treated external beam radiotherapy every year. In some cases, a dose calculation near metal elements is very important and crucial for proper treatment. For this reason, a dedicated holder was projected and printed using 3D printing technology (PLA material). The holder presented on Figure 1 has ability for parallel to beam axis film placement and to place at 5cm depth a cut in half metal inserts. The inserts were made from high density materials: titan, alloy 330, alloy 600 and tungsten and has physical densities 4.5, 7.9, 8.5 and 19.4 g/cm 3 respectively. Monte Carlo simulation was performed using Geant4 package by Cern. The phase space data of TrueBeam machine was provided by the manufacturer. For the validation of a Monte Carlo calculation in presence of very high-density materials a four metal inserts were placed in the holder transversely to beam axis at 5 cm depth. Films were placed parallelly and were irradiated with 900 MU on TrueBeam (Varian, Palo Alto) with 6 MV photon beam. Two extra films for calibration without inserts were irradiated with 900 and 1600 MU. Six films (four with different inserts and two without inserts for calibration) were scanned and analyzed. The EPSON PERFECTION V750 PRO scanner was used. For the analysis a Film and Image Analysis software (PTW, Freiburg) was used.

Results All films were analyzed and compared with Monte Carlo calculation. All percentages depth doses were normalized to the depth of maximum dose achieved without presence of metal inserts – 1.5 cm. Monte Carlo calculation were smoothed using rolling average. All measurements were in good agreement with MC calculation. All percentage depth doses in the range between 0 and 10 cm are presented on figure 2. The biggest differences were observed for buildup region. To avoid saturation of films caused extra dose from back scatter radiation at the water metal junction an extra calibration points obtained from 1600 MU irradiated film were used. For the materials maximum dose achieved 114, 116, 118 and 160% of the predicted dose.