How does dose fall off behave over a short distance of 1 to 5 cm for a pellet source of 103Pd or 125I?

The behavior of dose fall-off over short distances, specifically 1 to 5 cm, for pellet sources like 103Pd or 125I is characterized by a steep dose gradient. This steepness is primarily due to the fact that these are low-energy photon emitters, which are used in brachytherapy for localized treatment of cancer.

Unlike high-energy sources, where the inverse square law applies more predictably, low-energy sources like palladium and iodine experience greater attenuation of their radiation over short distances. As the distance from the source increases, the dose decreases more sharply than what would be predicted by the inverse square law. This is because the interaction of the emitted photons with surrounding tissue is more significant at close proximity, leading to a rapid decline in dose delivery.

Therefore, in the context of brachytherapy applications, selecting the appropriate source and understanding the steep dose fall-off is critical for achieving the desired therapeutic effect while minimizing exposure to surrounding healthy tissues. The steepness reflects the physical characteristics of the radioisotope and the need for precision in treatment planning.