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July 5, 2013 | Improving iodine-125 calibration for more effective brachytherapy

CEA LIST’s Laboratoire National Henri Becquerel has developed a new method to more accurately determine the iodine-125 dosages delivered to cancer patients during brachytherapy.

Brachytherapy, which entails placing radiation sources like iodine-125 as close as possible to the tumor being treated, is currently one of the most commonly-used techniques to treat ophthalmic and prostate cancers. One of the key factors in treatment efficacy is knowing the exact dosage delivered by each radiation source placed in the patient’s body. CEA LIST’s Laboratoire National Henri Becquerel has developed a new method using an air-wall ionization chamber to better calibrate the radiation sources.

Measuring photon spectra

To determine the measurements of absorbed-dose-to-water at specific geometries, a number of calculated correction factors must be applied. Therefore, the energy of the X and γ photons emitted by the source must be accurately known. This is particularly true for low-energy radiation sources like iodine-125 (maximum energy 35 keV). In this case, small variations in energy result in large variations in photon-matter interaction probabilities. Finally, the confinement of the radioactive iodine in seeds generates other photons resulting from fluorescence and diffusion.

Knowing the detector’s response

To effectively measure the energy of the photons, the lab used a high-purity germanium (HPGe) detector. And, to take into account the phenomena that occur inside the detector (distorting the spectra), the researchers developed a correction method. They studied the detector’s “response” with a source of photons of the same energy (and whose value is accurately known). The experiment was repeated, varying the energy over a range covering the detector’s scope of use. This required using specific radiation sources: the LNHB Solex source (6 keV to 17 keV) and the ESRF’s ID17 source (30 keV to 60 keV). Based on the data obtained, the researchers developed mathematical models to calculate the “spectra” of energy obtained with the HPGe detector.

Reconstructing the actual spectrum of iodine seeds

Thanks to the new technique, it is now possible to match a spectrum of photons (such as that emitted by an iodine-125 seed) to the spectrum as it would be “seen” by the HPGe detector. By comparing the spectra “seen” by the detector (obtained by the measurement and reconstruction process), it is possible to extract the spectra actually emitted by the iodine-125 source.

spectre
The spectrum of an iodine-125 seed measured using a HPGe detector and corrected for distortion caused by the detector