Application of gamma criteria for FIF therapy for wide breast size range

Abstract

Evolution of complex planning techniques, such as IMRT and VMAT, has set high requirements for labor intensive quality assurance procedures. For the evaluation of intensity modulated radiotherapy plan, gamma evaluation method is widely used for accurate quantitative comparison of measured dose distribution and desired dose distribution from treatment planning system (TPS). Field – in – field (FIF) 3D conformal planning technique, widely used now in breast cancer treatment. FIF is the forward planning technique, where one or more subfields are introduced to achieve higher dose homogeneity in treatment volume. Since FIF is based on the same intensity modulation principles as IMRT, it is possible to use gamma evaluation method for FIF breast fields. The aim of the present paper is to determine whether gamma criteria pass/fail rate is treatment volume dependent. For this study, four patients with different breast volumes (1115.4 cc, 1716.46 cc, 2531.65 cc and 3276.27 cc) were examined. Calculation was performed using treatment planning system XiO, where reference fields were exported to phantom and artificial shifts of the field in step of 1 mm were introduced. The maximum shift for one direction (lateral or longitudinal) does not exceeded 1 cm. The FIF segments were shifted relative to base fields in eight directions: lateral „+”, lateral „-”, longitudinal „+”, longitudinal „-”. All uncertainties related to LINAC and MLC were excluded because of TPS versus TPS comparison method. For acceptance criteria ΔDM=3% dose-difference and ΔdM=3 mm distance-toagreement (DTA) were used and 95% of all pixels should be within this criterion. Results demonstrates, that, independently on the breast size, gamma criterion failed to detect internal field displacement up to 5 mm