ESTRO 2022 - Abstract Book

S100

Abstract book

ESTRO 2022

The Exradin A26 chamber, IBA RAZOR Nano chamber and PTW microDiamond detector have been characterized in terms of their energy (6 keV to 70 keV) response to X-ray beams in air at both “N” and “TW” PTB series (18 qualities in total). The angular dependence of these detectors was also measured at intervals of 10° in a 180° range with the INTRABEAM at 50 kV and reported relative to the aligned incidence. The procedure to determine the energy response can be expanded to other energy ranges and to other detectors and may lead to the creation of a detector-wise library avoiding the need of the user to re-calibrate the instrument for every new application and energy range. The three detectors are promising candidates for 3D dose measurements of eBT sources in water. In particular, the A26 chamber exhibits an excellent angular response and a more flatter energy response to X-ray beams from above 20 keV compared to the other two detectors. For the conversion of the detector’s response in terms of absorbed dose to water, MC simulations are currently been done by other partners of 18NRM02 PRISM-eBT project (see below). Even when the response is far from flat, the knowledge of a reliable energy dependence curve allows the use of these detectors in the dosimetry of low photon energies, in eBT or in other applications of low energy X-ray conventional beam (as cell culture or small animal irradiators). This research was accomplished within the framework of project 18NRM02 PRISM-eBT that has received funding from the EMPIR programme co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme.

OC-0124 Skin dose study under rpm marker BLOCKS IN BREAST TREATMENTS

N. Garcia Apellaniz 1 , N. Jornet 1 , J. Pérez-Alija 1 , S. Olivares 1 , C. Ansón 1 , F. Suriñach 1 , P. Gallego 1 , H. Vivancos 1 , A. Ruiz 1 , M. Barceló 1 , F. Leo 1 , P. Carrasco 1

1 Hospital de la Santa Creu i Sant Pau, Servicio de Radiofísica y Radioprotección, Barcelona, Spain

Purpose or Objective After including in our routine Varian’s new RPM marker block, we have seen an increase in skin toxicity on the area where it is positioned. The aim of this study is to assess the dosimetric effect of the new Varian marker block. Materials and Methods Unlike Varian’s previous marker block which has 6 dots, the new one has 4 dots and a posterior wall that is in contact with the patient, potentially increasing the skin dose due to a bolus effect. Phantom measurements were performed for a 6MV X-ray beam from a Varian TrueBeam linac (20x20 cm2 field size and SSD=100) using EBT3 films to check the surface dose with and without the marker block and the attenuation at 3 cm depth beneath. EBT3 films were scanned using an EPSON EXPRESSION 10000XL scanner and FilmQAPro Software. The measured doses were compared to dose calculations on the same phantom setting using Eclipse TPS vs 15.6 (AAA calculation algorithm-grid 0.2 mm). A total of 55 breast patients were included in a prospective study to assess skin dose in vivo with EBT3 films of 2x2 cm2 inside sleeves of low-density polyethylene of 50 µ m thickness. Films were positioned medially inside the internal tangential beam under the marker block, if any. Different techniques with the 6 dots, 4 dots and no marker block were included. Most treatments only use 6MV X-ray beams, 10 MV X-rays are used only for low weighted beams in some 3D-CRT plans. Prescribed dose per fraction was 2.67 Gy. We used the Mann–Whitney U test to analyse the skin dose difference by technique and marker block. Results The phantom measurements showed a 3-fold surface dose increase under the block and the attenuation 3 cm beneath it was on average 1.3%, reaching 4.5% under the markers. These results agreed with TPS calculations. Figure 1 shows the in vivo skin dose measurements for the patients included in the study stratified per technique. The skin dose is higher for BH techniques irrespective of the marker block used and technique (IMRT or 3D-CRT), showing significance (p=0.032) for IMRT vs IMRT-BH (both using respiratory control). The results also indicate a significant increase of skin dose when using any of the marker blocks (p<0.002) but the difference was not significant when comparing the 4 dots marker block with 6 dots marker block (p=0.057) (Fig. 2).