Alpha DaRT source activity confirmation using a reentrant well-type ionization chamber

Alpha-emitting radionuclides enable precise cancer therapy through high linear energy transfer and limited tissue penetration, damaging tumor cells while sparing healthy tissue. Diffusing Alpha-emitters Radiation Therapy (Alpha DaRT) features Ra-224 sources that are implanted directly into the tumor and emit alpha particles during radioactive decay. Alpha DaRT has demonstrated efficacy and safety in preclinical and early clinical trials across multiple tumor types, including skin, head and neck, and pancreatic cancers.

Reliable and efficient methods for verifying Alpha DaRT source activity prior to treatment can help support accurate and consistent radiation delivery. The direct measurement of alpha particles from sources within an Alpha DaRT applicator is impractical due to their short range; however, gamma emissions from the Ra-224 sources can be used to infer radioactivity. This study established a protocol for verifying the source activity within Ra-224 Alpha DaRT applicators using a reentrant well-type ionization chamber, providing users with a practical method for detecting errors in source manufacturing or certificate paperwork without compromising applicator sterility.

Ra-224 Alpha DaRT sources in sterile packaging (Flex and Needle applicators with 1-4 sources) were assessed. Source energy spectra and activities were verified using a high-purity germanium (HPGe) radiation detector. Calibration factors (kBq/pA) were established using an IVB1000 well-type ionization chamber with measurements conducted by placing single applicator sterile packages into the chamber with sources centered in the chamber's sweet spot and corrected for temperature, pressure, and leakage current. Quality assurance was performed on 26 Flex applicators using the established calibrations before the first clinical procedure.

HPGe measurements agreed with vendor-stated activities. The average calibration coefficient using the IVB1000 chamber was 233 ± 3 kBq/pA for Flex and 597 ± 7 kBq/pA for Needle applicators. Calibration coefficients were consistent across two IVB1000 chambers. Source number dependence was observed, with calibration factors increasing by 1.7% ± 0.7% per source (Needle) and 2.7% ± 0.6% per source (Flex). Measurement repeatability was 3.3%. Applying the calibration to 26 Flex applicators before the first patient treatment yielded a 1.1 ± 5.8% (range: -7.5% to 11.4%) difference relative to the vendor's stated activity.

A reentrant well-type ionization chamber is suitable for pre-treatment quality assurance of Ra-224 Alpha DaRT applicators, enabling verification of the vendor-stated activity while maintaining sterility within sealed packaging.