Functional representation of tissue phantom ratios for photon fields

The tissue phantom ratio (TPR) is a common dosimetric quantity used in photon dose calculations. For small photon fields with side lengths less than 4 cm, TPR data hardly exist in literature. In this work, a self-contained functional representation of TPR is proposed, valid for the whole range of clinically relevant depth and field sizes. This is especially useful for small fields shaped by multileaf collimators.

TPRs were measured for quadratic fields with side lengths between 0.4 and 18 cm. The measured data were fitted to a physically meaningful function taking electron buildup, buildup of scattered photons, beam attenuation, and beam hardening into account. The achievable accuracy was tested against measurement and data from the literature.

A set of parameters for the proposed function was derived for 6 and 10 MV beams. The comparison of the calculated and the measured data generally yielded a difference of less than 1%. For field sizes below 2 cm, a systematic discrepancy between the author's data and those from Cheng et al. [Med. Phys 34, 3149-3157 (2007)] was found.

With the proposed model, TPRs can be calculated for the full range of field sizes and depths required by treatment planning system algorithms and monitor unit check programs with very high accuracy. The method is also useful in detecting and reducing errors in measurement.