The dependence of percentage depth dose on the source-to-skin distance for kilovoltage x-rays

The variation of percentage depth dose (PDD) with source-to-skin distance (SSD) for kilovoltage X-rays used in radiotherapy has been investigated. Based on physical parameters of photon fluence, absorption and scatter during interaction of radiation with tissue, a mathematical model was developed to predict the PDDs at different depths of treatment for various field sizes and at selected SSDs for radiation qualities Half Value Layers of 1 mm Al and 8 mm AI, using 10 cm SSD input data. Higher values of PDD were predicted for increasing SSD as compared to benchmark PDD data, and the differences were more pronounced at larger depths and lower PDDs. Within the range of clinical significance of 90 - 100 % PDD, the discrepancies between the calculated values and benchmark data were ≤ 0.5 % when the SSD was varied by 50 %, and increased to a maximum of 2.2 % with SSD difference of 200 %. With 1.0 % error acceptable in radiotherapy dosimetry, the model was found to be efficient, especially for applications involving small differences in SSD. (au)