Why is the TAR concept usually not used with accelerator photon beams?

The correct response highlights that the concept of Tumor Dose-Absorbed Radiation (TAR) is often not applied to accelerator photon beams due to the challenge in defining the dose in free air. In radiation therapy, TAR is a ratio that compares the dose at a point in the patient or tissue to the dose in free space. However, in the context of photon beams produced by accelerators, the concept of free air dose becomes complicated.

Photon beams from accelerators are influenced significantly by factors such as scatter, attenuation, and varying energy levels. These beams do not deliver a uniform dose in free air, making it difficult to establish a reliable measurement of dose for TAR calculations. In clinical practice, the actual dose delivered to the target volume needs to account for tissue heterogeneity, overlying structures, and other variables, which are not adequately captured by TAR when applied to accelerator photon beams.

This makes the use of TAR less relevant and practical in this scenario. Instead, more accurate methods, like the use of Monte Carlo simulations or direct measurement techniques, are preferred to assess the dose distribution and ensure precise treatment delivery.