Why is the TMR not well defined for accelerator photon beams?

The choice regarding the dose in free air not being well defined helps to clarify the challenges associated with determining the Tissue Maximum Ratio (TMR) for accelerator photon beams. TMR is a critical parameter in radiation therapy, used to describe the depth of dose maximum in tissue relative to the dose at a reference depth.

The measurement of dose in free air is essential for establishing baseline calibration for photon beams. However, accelerator beams often exhibit complex interactions and variations due to factors such as beam quality, energy, and geometric configurations. These variations can lead to challenges in accurately defining the dose in free air, which can, in turn, complicate the assessment of TMR values.

When the dose in free air cannot be precisely measured, it directly impacts the reliability and consistency of TMR values, as these ratios rely on absolute dose measurements in both air and tissue. Therefore, the accurate determination of TMR relies upon a well-defined reference for the dose in free air, making this aspect pivotal in understanding the limitations of TMR for accelerator photon beams.