ESTRO 2025 - Abstract Book

S317

Brachytherapy - Physics

ESTRO 2025

324

Digital Poster Dosimetry of Ru-106 ophthalmic applicators with a diamond detector calibrated in the electron beam Assi Valve 1,2 , Vappu Reijonen 1 , Anna Rintala 1,2 , Satu Strengell 1 , Katri Nousiainen 1 , Mikko Tenhunen 1 1 Helsinki University Hospital, Comprehensive Cancer Centre, Helsinki, Finland. 2 Department of Physics, University of Helsinki, Helsinki, Finland Purpose/Objective: Ruthenium-106 (Ru-106) applicators (Eckert & Ziegler Strahlen und Medizintechnik AG, Berlin, Germany) are used to treat uveal melanomas. The clinical user needs to verify the reference absorbed dose-rate at a reference point [1 2]. We have applied a PTW (PTW-Freiburg, Freiburg, Germany) MicroDiamond detector for the quality assurance (QA) of Ru-106 applicators. We calibrated the detector and defined the angular sensitivity using an electron beam from a clinical linear accelerator in a clinically feasible set-up. Material/Methods: Ru-106 applicator was modelled as a concave surface with a constant surface activity concentration. For a point beta source, the dose D(r) was modelled with the Vynckier-Wambersie approximation [3]. Dose D APPL along the applicator central axis at the effective point E (seen in Fig 1) of the detector was calculated as

where angle θ L defines the edge of the applicator, angle θ a point on the applicator surface at distance r to the effective point and R is the inner radius of the applicator. The detector has a variable sensitivity in mixed-electron beam as a function of angle of radiation incidence α and detector axial rotation β but the latter averages out due to the cylindrical symmetry of measurement geometry. The sensitivity F was modelled only as a function of α : F = F ( α )= a+(1- a) cos α . The diamond detects the dose signal D D

The geometry correction factor C G for correction of measured value was

The diamond was calibrated using a nominal 4 MeV electron beam at the depth of 9 mm to approximate electron energy spectrum of Ru-106 (mean E 0 =1.41 MeV). The angular sensitivity determination using the same energy gave F ( α )= 0.77+0.23 cos α. The dose-rate of the applicators was measured in PTW MP3 water phantom with 1 mm steps between 2 mm – 6 mm and compared with the reference values from the manufacturer.