Why is Ir-192 commonly used in HDR afterloading brachytherapy?

Iridium-192 is commonly used in high-dose-rate (HDR) afterloading brachytherapy largely due to its high specific activity combined with lower photon energy. High specific activity means that Ir-192 can deliver a sufficient dose of radiation while being contained within a relatively small volume, making it efficient for localized treatments. Additionally, the lower photon energy of Ir-192 compared to other isotopes like cesium-137 allows for effective tissue penetration without excessive radiation exposure to surrounding healthy tissue. This characteristic is critical in ensuring that the therapeutic effects of the radiation are maximized on the target tumor while minimizing damage to adjacent normal tissues.

In contrast to the options concerning the other isotopes, Ir-192's specific activity and optimal photon energy make it particularly advantageous for HDR applications where precision in dosage is essential for effective cancer treatment.