An absorbed dose-based source strength determination for diffusing alpha-emitters radiation therapy (DaRT)...

Diffusing alpha-emitter radiation therapy (DaRT) is currently being evaluated in several clinical trials as an interstitial temporary brachytherapy source. However, determining source strength solely based on gamma spectrometry measurements of ²²⁴Ra activity has limitations. This method does not account for changes in the desorption of ²²⁴Ra progeny from the seed's surface or the therapeutically relevant alpha emissions. Variations in the source construction, especially ²²⁴Ra depth in the seed, significantly impact the ²²⁴Ra progeny desorption probabilities which leads to changes in the dose deposited in the tumor. Therefore, an improved source strength specification for DaRT seeds is needed, one that accounts for variations in source construction that affects the absorbed dose delivered to the tumor.

There were two aim of this work. The first aim was to investigate the impact of ²²⁴Ra distribution depth on progeny desorption probabilities and correspondingly absorbed dose to water using both Monte Carlo (MC) simulations and the diffusion-leakage (DL) model. The second aim was to propose an absorbed dose-based source strength specification and assess the sensitivity of the proposed standard to variations in the desorption probabilities.

A MC model of the DaRT seed was constructed with varying ²²⁴Ra distribution depths ranging from 0 to 14 nm. The corresponding ²²⁰Rn and ²¹² Pb desorption probabilities were calculated using alpha and gamma emissions originating inside the source, respectively. The calculated desorption probabilities were utilized as input parameters to the DL model, which was solved using a finite element analyzer, in order to calculate the cumulative absorbed dose to tumor over the treatment period. The design considerations for the proposed absorbed dose standard were outlined along with a MC study of the expected signal from a 3 µCi DaRT seed. The sensitivity of the proposed source strength to varying desorption probabilities was assessed.

The progeny desorption was found to be highly sensitive to the distribution depth parameter with a sharp reduction in desorption probabilities with increasing distribution depth. The cumulative absorbed dose to tumor was found to vary drastically with the desorption probabilities with dose differences of up to 80%. Despite large dose differences, the 10-Gy prescription isodose line was found to be within 1 mm of each other indicating a spatial shift due to differences in desorption. The absorbed dose standard was found to have a signal > 7 pA with high sensitivity to the changes in the desorption probabilities. The MC-calculated correction factors were found to be < 3% with negligible dependence on the changes in desorption.

This work investigated and reported a large dependence of absorbed dose to water on desorption of ²²⁴Ra progeny. An absorbed dose-based source strength specification was proposed with a preliminary design of the apparatus. Using MC methods, the proposed instrument was deemed suitable for source strength measurements with a high sensitivity to changes in the DaRT seed construction.