Analysis of the impact of wedge filters on the dose profile symmetry in external radiotherapy

Context A bibliographic review, following a symmetry defect discovered on a radiation therapy accelerator, reveals a lack of results on the analysis of asymmetries created on the depth dose profiles with wedge filters. To deal with this issue, we propose an experimental and theoretical study. Material and methods Measurements of the dose profiles for different wedge angles (15° to 60°) are performed in the reference conditions (SSD = 90 cm, SAD = 100 cm, 10 × 10 cm 2 field), for a 18 MV photon beam with a VARIANO 2100C accelerator. Two types of detectors are used: 0.3 cc Semiflex ionization chambers in a PTW MP3 water tank, and a PTW 2D-Array seven29 detector array. These profiles are compared with the data of both TPS Oncentra Master Plan and the Monte-Carlo simulations results, performed with the BEAMnrc code. The combination of multiple wedges is also analysed. Results The dose profile symmetry differs according to the nature of the filter (physical or dynamic) and depends on experimental conditions, such as the measurement depth or the field size. The combination of two wedges in series (physical and dynamic) has also been modelized. It enables to check that the ‘‘equivalent” wedge angle resulting from these two filters is not a linear combination of these two angles. Conclusion The results obtained in reference conditions, which should be complemented by a dosimetric evaluation, were used to quantify the symmetry defects in output beams.