Abstract
Background/Purpose: Radiopharmaceutical therapy (RPT) with alpha-emitting radionuclides, such as 225Ac, offers highly localized dose delivery due to its short particle range and high linear energy transfer (LET). However, unlike external beam radiotherapy (EBRT) and brachytherapy, which have traceable absorbed dose standards, RPT currently lacks a standardized absorbed dose measurement framework. This study aims to quantify the absorbed dose to air from a 225Ac source using an extrapolation chamber, supported by Monte Carlo (MC) simulations, to establish a robust methodology for dose validation of computational methods.
Methods: An extrapolation chamber was used to measure the absorbed dose to air from a drop casted 225Ac source, with source activity determined using a Low-Energy Germanium (LEGe) detector. High-resolution 2D imaging characterized the spatial distribution of deposited activity, enabling precise source geometry modeling for MC simulations. Self-attenuation effects were quantified using alpha spectrometry, and automated voltage control improved measurement repeatability of the extrapolation chamber. Absorbed dose calculations were compared between experimental and MC results across multiple air gaps.
Results: Experimental and simulated absorbed dose values were in strong agreement, with experimental measurements consistently 1–2% higher than MC predictions across all air gaps. The ionizing-radiation Quantum Imaging Detector (iQID) system provided activity mapping for source characterization, reducing uncertainties in MC modeling. The integration of automated chamber voltage control enhanced measurement precision, while uncertainty analyses highlighted activity determination and alignment as key contributors to variability.
Conclusions: This study establishes a validated methodology for quantifying 225Ac absorbed dose using extrapolation chamber measurements. The findings support the development of traceable absorbed dose standards for RPT and highlight the need for further refinement in alignment protocols and activity quantification. Future work should explore comparisons with time-integrated activity (TIA)-based absorbed dose calculations to align experimental methodologies with clinical RPT dosimetry practices.
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