ESTRO 2021 Abstract Book

S1590

ESTRO 2021

Materials and Methods Both Seq-IMRT and SIB-IMRT plans were created for 50 patients with locally advanced HNC. The Seq-IMRT was planned in 3 phases, phase I (low-risk region) with 44 Gy in 22 fractions; phase II (intermediate-risk region) with 12 Gy in 6 fractions; and phase III (high-risk region) with 14 Gy in 7 fractions in sequence. The SIB-IMRT was planned with 70 Gy for high-risk region, 63 Gy for intermediate-risk region, and 56 Gy for low-risk region, all in 35 fractions thus prescribing 2 Gy, 1.8 Gy and 1.6 Gy respectively. Both plans were optimized with progressive resolution optimizer algorithm for volumetric modulated arc therapy within Eclipse version 11. Dosimetric comparison was done between the two plans. Following factors were compared: target coverage, doses to various organs at risk (OARs), conformity index (CI), heterogeneity index (HI) and integral dose (ID). Results Dosimetric comparison is shown in table. Table: Dosimetric comparison of various factors between the two plans

Conclusion Seq-IMRT is comparable with SIB-IMRT with regard to target coverage. Hot-spot, ID and doses to OARs are significantly lesser in Seq-IMRT. Cold spot is lesser in SIB-IMRT. Whether these differences translate into clinical meaningful differences in disease outcomes and toxicity remains to be tested in clinical study. PO-1866 Clinical electron beam modulation by plastic samples produced with 3D printing approach S. Stuchebrov 1 , A. Bulavskaya 1 , Y. Cherepennikov 2 , A. Grigorieva 1 , D. Kokontsev 3 , A. Loginova 3 , I. Miloichikova 2,4 1 Tomsk Polytechnic University, Research School of High-Energy Physics, Tomsk, Russian Federation; 2 Tomsk Polytechnic University, School of Nuclear Science & Engineering, Tomsk, Russian Federation; 3 Dmitry Rogachev National Medical Research Center of Pediatric Hematology, Oncology and Immunology, Radiotherapy department, Moscow, Russian Federation; 4 Cancer research institute of Tomsk national research medical center of the Russian academy of sciences, Radiotherapy department, Tomsk, Russian Federation Purpose or Objective Nowadays, modulation of the electron beam depth dose distributions in radiotherapy is provided by sets of tissue- equivalent plates or individually shaped boluses. In this study, we propose to use means of 3D-printing for making samples for depth dose distribution modulation in electron radiotherapy. This approach allows producing patient-specific boluses and compensators. The aim of this study is to prove the possibility of electron beam dose modulation by plastic samples produced with 3D-printing approach. Materials and Methods A 3D printed test sample made of ABS-plastic is used for study. This sample has three different functional areas: for partial beam absorption (2 cm thicknesses) with two collimation holes of 0.5 cm and 1 cm diameters; the wedge filter with thickness varied from 5 cm to 0 cm, and semicircle-shaped absorber of 5 cm radius. The experimental study was performed at the Dmitry Rogachev National Research Center of Pediatric Hematology, Oncology and Immunology (Moscow, Russia) using Elekta Synergy medical linear accelerator as a source of 6 and 12 MeV electron beams. The modulated beam shape is registered by Gafchromic EBT3 dosimetry film and MatriXX universal detector array at dose maximum depth in tissue- equivalent miniPhantom. Experimental studies are performed for two geometries: with test sample placed in applicator and on the surface of phantom. The first geometry simulates the use of compensator, while the second one simulates the use of bolus. Results In this study, we prove the possibility of electron beam modulation by a plastic sample produced by fused deposition modeling. The 1 cm collimation hole allows circular field shaping. It was found that collimator positioning in applicator causes broadening of the shaped radiation field because of the influence of electrons scattering in the air. Propagation of