The dose fall off for 125I or 103Pd sources significantly deviates from which law?

The correct answer is based on the understanding of how brachytherapy sources like 125I (Iodine-125) and 103Pd (Palladium-103) deliver radiation dose and the principles governing this process.

The inverse square law states that the intensity of radiation from a point source is inversely proportional to the square of the distance from the source. This means that as you double the distance from the source, the intensity is reduced by a factor of four. For point sources, one would expect this relationship to hold true up to a certain distance.

However, for 125I and 103Pd sources, which are commonly used in brachytherapy, the dose distribution does not follow this law due to their finite size and the way they emit radiation. The presence of the source itself alters the dose distribution in a way that creates a more complex pattern than what would be predicted by a simple application of the inverse square law, especially in the close vicinity of the source. As a result, dose fall-off characteristics are influenced by factors such as source geometry, energy spectrum, and the surrounding tissue, leading to significant deviations from this linear prediction, particularly at distances close to the source.

In contrast, the concepts related to the linear