# Advancing towards cancer theragnostic by probing the 225Ac decay chain with ultra-high-resolution metallic magnetic calorimeter based detectors

## TL;DR

Researchers used a special detector to track decay products of 225Ac, a promising cancer treatment, with high precision, which could improve its medical use.

## Contribution

This is the first demonstration of detecting most 225Ac decay products using ultra-high-resolution metallic magnetic calorimeters.

## Key findings

- MMC detectors can identify most 225Ac decay products with high sensitivity and energy resolution.
- X-ray fluorescence lines enabled differentiation of multiple decay products like 221Fr and 213Bi.
- Particle-induced X-ray emission from 225Ac was also detected for the first time.

## Abstract

225Ac is one of the most promising short-lived radionuclides for application in targeted alpha therapy (TαT). However, quantitative detection of 225Ac and its daughter radionuclides by γ-spectroscopy has so far been unattainable. Achieving this would enable precise organ dosimetry and better targeting of the therapeutic potential of the 225Ac decay series.

In this study, an X-ray and γ-spectrum of an 225Ac sample (89 kBq) was recorded with a Metallic Magnetic Calorimeter (MMC) detector over a wide energy range of 5 to 125 keV.

Here, we show the feasibility of detecting γ-lines of 225Ac, 221Fr, 213Bi, and 209Tl with ultra-high-resolution spectroscopy (FWHM of 23 eV @ 5.9 keV). By analyzing characteristic X-ray fluorescence lines, it is also possible to distinguish 221Fr, 217At, 213Bi, 213Po, and 209Pb in this sample. Additionally, particle induced X-ray emission from 225Ac is found.

To the best of our knowledge, this first MMC based X-ray and γ-spectrum of 225Ac demonstrates the detector’s potential, as it enables the separate detection of most nuclides in the 225Ac decay chain (all except 221Ra and 217Rn) with high sensitivity, excellent energy resolution, and precise energy calibration, paving the way for key technological advancements in research and clinical applications in nuclear medicine.

225Ac is a radioactive material. It releases tiny high-energy particles that can destroy cancer cells very precisely, and because of that, it can be used for a cancer treatment called “targeted alpha therapy.” During targeted alpha therapy, 225Ac can decay into various other products that can distribute throughout the body and cause side effects. It is important to be able to differentiate these decay products from the intended 225Ac, but current methods can only detect two of the many 225Ac decay products. In this study, we analyzed 225Ac using a special measurement device so called “Metallic Magnetic Calorimeters.” Using this method allows us to identify 225Ac and more decay products than previously reported, and therefore further allows us to track the products of the main decay chain of 225Ac. Continued work in this area could support future research and improve medical uses of 225Ac.

Maurer, Unger, et al. measure actinium-225 samples using a Metallic Magnetic Calorimeter (MMC) based detector across 5–125 keV. All decay products apart from Radium 221 and Radon-217 are identified with very high energy resolution, demonstrating potential for the use of MMC based detector systems in medical applications.