Depth dose in filtered fields for cobalt-60 γ-rays and 10 MV X-rays

Wedge filters and missing tissue compensators are frequently used in radiotherapy to obtain the optimum dose distribution. In general, a dose calculation with filtered fields has used depth dose data for open fields, TPR, TAR, PDD and field factors because these data between open and corresponding filtered fields are assumed to be the same or nearly equal. This study evaluated these parameters for filtered fields using cobalt-60 γ-rays and Linac 10 MV X-rays. Measured TPRs in a 10 x 10 cm field differed with a maximum deviation of 4% between open and corresponding filtered fields for typical treatment conditions, and the differences in TPRs between the two types of fields varied with depth, filter thickness and material. Field factors of filtered fields at the reference depth agreed with the maximum deviation of 10% from the corresponding open field factors. The differences of depth dose data between open and filtered fields imply beam hardening caused by modifying filters and the scatter contribution from such filters. The International Commission on Radiation Units and Measurements (ICRU) has recommended an overall accuracy in dose delivery of ±5%. Therefore, we should apply the depth dose data of filtered fields in the dose calculation when beam modifying filters are used in radiotherapy. (author)