Abstract
241244
Introduction: A recently developed radionuclide therapy employs sources coated with α-emitting radium-224, delivered by intratumoral insertion for the treatment of cutaneous, mucosal, or superficial soft tissue neoplasia. The release and diffusion of daughter products provides regional irradiation. Techniques for imaging and quality control measurements of these sources were evaluated using equipment standard for a clinical nuclear medicine department.
Methods: Activities of applicators containing 1, 3, and 5 Alpha-DaRT sources (Alpha Tau Medical, MA) were measured with a dose calibrator (CRC-25R, Capintec, NJ) at its setting for 224Ra without daughter products. The setting was then adjusted to include daughter products and decay corrected for comparison with the manufacturer’s reference values. A radiation survey meter (PDR2 NE Technology Ltd., UK) provided field measurements. Uptake probe (Captus 3000, Capintec, NJ) measurements provided spectral peaks. The 80 and 240 keV photopeaks with 20% windows were employed for planar imaging on an Anger γ-camera (Discovery 670, GE Healthcare, Israel) using both low energy high resolution (LEHR) and medium energy general purpose (MEGP) collimators. The ratio of counts from each applicator imaged provided relative quantitation. For single photon emission computed tomography (SPECT) acquisitions with and without water as scattering material, 5 applicators of 2 sources each were mounted in a cylindrical phantom.
Results: Activity measurements at the dose calibrator setting for 224Ra without daughter products were 133 times higher than the nominal activities. Bateman equation analysis showed that 86 % (114/133) of this difference could be attributed to the γ-emissions of daughter products. Adjusting the setting to CAL# 797 yielded an error of <11% at 56 kBq, <3% at 174 kBq, and <1% at 288 kBq. A further difference of −12.3±1.5%/day (95% confidence interval), for <3 days duration was noted, likely because this was before transient equilibrium was reached. Survey meter fields at 20 cm were low: <0.6 μSv/h/source or <0.011 μSv/h/kBq. The uptake probe showed photopeaks corresponding to the initial 224Ra decay (240 keV measured), and to daughter products 212Pb (77 and 240 keV measured) and 208Tl (510 and 585 keV measured). Planar image quality was best for the MEPG collimator, resolving 5 sources in one applicator and giving a relative quantitation to within 18%. Relative quantitation using the LEHR collimator was <20%, with poorer image quality. Individual applicators were clearly visualized using SPECT, but in no case were two sources spatially resolved.
Conclusions: A dose calibrator provides sufficient quality control checks of 224Ra source activities. A γ-camera with an MEGP collimator can provide optimal images using combined 80 and 240 keV photopeaks, thus opening the possibilities for theranostic approaches.
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