TAR can be directly measured or derived from which data type?

Tissue-air ratio (TAR) is an important concept in radiation therapy dosimetry that allows for the determination of dose distribution within a patient's tissue and surrounding air as a treatment is administered. TAR can be derived from percentage depth dose (PDD) data because both measurements relate to how radiation behaves in tissue compared to air.

PDD data provides information on the percentage of the dose delivered to a point within a given medium (such as tissue) at a certain depth relative to a reference depth, typically at the surface. TAR specifically compares the dose in a given tissue to the dose in the air at the same depth, making it fundamentally linked to the information provided by PDD measurements.

When analyzing how radiation interacts with tissue and air, PDD data can be used to derive TAR values by adjusting for the varying densities and composition of the materials involved. This is why the correct choice is the percentage depth dose data, as it serves as a crucial input for calculating the tissue-air ratio. Other types of data, while important in general dosimetry practices, do not directly lead to the derivation of TAR in the same manner as PDD data does.