In which configuration is the TAR measured during calibration?

Tissue Air Ratio (TAR) is measured in a specific configuration during calibration to ensure consistency and accuracy in dosimetry. The correct configuration utilizes a 10cm x 10cm field at a source-to-surface distance (SSD) of 100cm. This setup is standard because it provides an adequate size for the radiation field while being close enough to the source to minimize variations in radiation response due to the inverse square law.

The choice of a 10cm x 10cm field creates a homogenous dose distribution, making it easier to measure the ratio of the dose in the tissue to the dose in air. At a distance of 100cm, the conditions are representative of commonly used treatment setups in clinical practice. This calibration ensures that the dosimetric data can be reliably applied to the larger clinical scenarios encountered in radiation therapy.

In contrast, the other configurations have variations in field size or SSD that may not reflect typical treatment conditions, potentially leading to discrepancies in the measured TAR value. Hence, the first choice adheres to established protocols for accurate calibration.