For brachytherapy sources with an average photon energy greater than 200 keV, why does the dose fall off with respect to distance greater than 1 cm from the source?

The correct answer is based on the understanding of how radiation interacts with matter, particularly in the context of brachytherapy. For brachytherapy sources that emit photons with an average energy greater than 200 keV, both attenuation and scatter play significant roles in how the dose diminishes as one moves away from the source.

Attenuation refers to the reduction in intensity of radiation as it passes through material. Higher-energy photons, while they are less attenuated compared to lower-energy ones, still experience some degree of energy loss when interacting with tissue. Scatter refers to the deflection of photons as they collide with atoms in the tissue, which can redirect some energy away from the line of travel toward the detector or patient volume.

In this situation, at distances greater than 1 cm from the source, the effects of attenuation and scatter can roughly compensate for each other, creating a more gradual dose fall-off rather than a sudden drop-off. This balance allows for some sustained dose levels even as distance increases, due to the high-energy photons able to travel further through tissue.

Options focusing on collisional energy loss and inverse square law dynamics are not the primary factors here. For high-energy photons, collisional losses are relatively low. While the inverse square law does