ESTRO 2025 - Abstract Book

S2654

Physics - Detectors, dose measurement and phantoms

ESTRO 2025

3836

Digital Poster In-vivo assessment of doses delivered to out-of-field lymphoid organs during head and neck radiotherapy using radiophotoluminescent glass dosimeters Meïssane M'hamdi 1 , Pauline Maury 1 , Julie Colnot 1 , Mesut Aydin 2 , Marc André Boivin 2 , Najma Douir 2 , Eric Deutsch 1 , Charlotte Robert 1 , Ibrahima Diallo 1 1 UMR1030, Gustave Roussy, Villejuif, France. 2 Radiation Oncology, Gustave Roussy, Villejuif, France Purpose/Objective: In external beam radiotherapy (EBRT), doses are inevitably delivered outside the treatment field. Those low doses may impact the probability of radiation-induced toxicities such as second cancer or lymphopenia [1]. The mechanisms by which EBRT may induce radiation induced lymphopenia are not known, but one hypothesis could be the non negligible distant doses delivered to lymphoid organs. To our knowledge there are no published datasets of doses received by these lymphoid organs during EBRT treatment. Radiophotoluminescent glass dosimeters (RPLGD) present interesting properties for the determination of whole-body in-vivo measurement thanks to their low detection limit (30 µGy) and their small size [3]. The objective of this study is to establish a methodology for estimating the orders of magnitude of doses absorbed by lymphoid organs during treatment in head and neck (H&N) patients, using in-vivo measurements obtained with RPLGD. Material/Methods: A solid phantom was assembled using water-equivalent plates (PlasticWater®–TheOriginal; 30x30 cm 2 ). We positioned RPLGD 302M on the surface of the phantom at various distances from the central beam axis (10, 20, 30, 40 cm) and out-of-field dose measurements were performed for different field sizes (5x5; 10x10; 15x15 cm 2 ). Under the same experimental conditions, measurements were also performed using an ionization chamber (PTW 30010), placed successively at depths of 1 cm, 10 cm, and 15 cm within the phantom. Irradiations were performed using different Linacs: a Varian Clinac 2300CD 6MV and an Elekta VERSAHD 6MV. In parallel, in-vivo dose measurements were conducted for 8 patients undergoing H&N treatment of 30 or 33 fractions. Detectors were placed on the skin of the patient at various anatomical locations, including the forehead, sternum, left and right axillary regions, spleen, umbilicus, right and left inguinal ares, left knee, and right foot. To estimate the orders of magnitude of the depth doses from RPLGD surface measurements, all doses measured with th ionization chamber were plotted as a quadratic function of th RPLGD readings. Results: The fitting results are shown in Figure 1, demonstrating a limited dependency on the depth of the organ. The fitting parameters yield R 2 values ranging from 0.989 to 0.992. Preliminary doses at the organs were evaluated to range from 0.2 to 19.0 mGy (Table 1).