ESTRO 2025 - Abstract Book

S2686

Physics - Dose calculation algorithms

ESTRO 2025

247

Digital Poster Impact of Ra-224 progeny desorption on absorbed dose in diffusing alpha-emitters radiation therapy (DaRT)

Ahtesham Ullah Khan, Sean Jollota, Jeff Radtke, Keith Kunugi, Larry DeWerd Department of Medical Physics, University of Wisconsin-Madison, Madison, USA

Purpose/Objective: Absorbed dose deposited in tissue from a DaRT seed is dependent on the desorption of the 220 Rn and 212 Pb progeny off the surface of the seed [1]. The desorption probabilities are tied to the depth of 224 Ra in the source needle, which may vary based on the construction of the source [2]. The aim of this work was to investigate the impact of the inter-seed desorption variability on absorbed dose for a DaRT treatment. Material/Methods: A Monte Carlo (MC) model of a DaRT seed was constructed with the 224 Ra depth of 0.0-1.0 nm with 0.2 nm increments. For each source depth, 220 Rn and 212 Pb desorption probabilities were calculated using alpha and gamma emissions, respectively [3]. A finite element analyzer (COMSOL Multiphysics) was utilized to calculate the cumulative absorbed dose to water over a 30-day treatment period by solving the diffusion-leakage (DL) model with the MC-calculated desorption probabilities [4]. The differences in the absorbed dose were accentuated. Results: While changing the 224 Ra source depth from 0 nm to 1.0 nm, the 220 Rn desorption probability decreased from 0.49 to 0.05 and the 212 Pb desorption probability decreased from 0.69 to 0.27. Using the MC-calculated desorption probabilities, the cumulative absorbed dose was found to increase with increasing desorption. With radial dose profiles normalized to the source depth of 0 nm, significant dose differences of over 80% were found. For every 0.2 nm increase in source depth, a reduction of over 12% in absorbed dose was noted. Despite the large dose differences observed for the investigated desorption probabilities, the 10-Gy prescription isodose lines were found to be within 0.7 mm of each other indicating a spatial shift in the dose distribution.