The physical penumbra depends on which of the following factors?

The physical penumbra is a critical aspect of radiation therapy and relates to the area at the edge of a radiation beam where the dose transitions from near full intensity to background levels. It is influenced by several factors, leading to a more pronounced understanding of how the radiation interacts with the tissue and devices involved in treatment delivery.

Geometric penumbra plays a significant role, as it describes the sharpness of the radiation beam's edge due to the geometry of the source, collimator, and the distance of the source from the treatment area. A broader geometric penumbra increases the physical penumbra, affecting how precisely the dose conforms to the target area.

Lateral electron transport in the patient also is a key factor. When a beam hits the tissue, secondary electrons are emitted, which can travel laterally before depositing their energy. This effect can blur the boundary of the dose distribution, thus impacting the penumbra's physical characteristics as the radiation interacts with different tissue densities.

Finally, collimator transmission contributes to the physical penumbra by determining how much radiation can pass through the collimator. Variations in collimator design can lead to changes in the penumbra region, either enhancing or reducing the spread of the dose