SU‐E‐J‐145: Dosimetric Impact of Patients Set‐Up Effect in Radiation Therapy Planning System for Breast Cancer Patients with Conventional Wedge and Field‐In‐Field Techniques

Purpose: To evaluate the dosimetric effects of patients setup errors and breathing movement on whole breast irradiation with conventional hard wedge (HW) and field‐in‐field (FIF) techniques. Methods: A total of 10 patients who received whole breast irradiation were chosen for this study. We performed radiation treatment planning for whole breast irradiation with two techniques; conventional hard wedge (HW) and field‐in‐field (FIF). On the subject of each patients, ‐ original plan‐was created using the general clinical planning technique and then we generated ‐ isocenter shift plan‐and ‐ gantry angle shift plan‐to evaluate dose variation according to change of isocenter and gantry angle. In our study, the isocenter was shifted by 0.6 cm for the right‐left, 0.5 cm for superior‐inferior, and 1.4 cm for anterior‐posterior direction refer to Kinoshita et al. study. Assuming that breast shape changes subtly, the gantry angle was shifted from 1° to 5° each tangential fields. To analysis the plan quality this study calculated a dose‐volume histogram (DVH) and the dose indices that are mean dose of CTV, CTV D95, CTV V95, lung V20, heart V40 and PTV dose improvement (PDI) index. Results: When isocenter of lung and heart shifted in the right‐left and superior‐inferior direction, ‐ isocenter shift plan‐showed no significant variation from ‐ original plan‐for most of the indices. However, a significant difference was found in V95 of CTV with isocenter shift in anterior‐posterior direction by assuming the patient shifted in the posterior direction. Shifts in gantry angle slightly affected lung and heart, and smaller changes were seen in PTV. Conclusions: This study evaluated the dosimetric effects of gantry angle and isocenter shift for simulating patients setup errors and breathing motion. The effects of patient and breast changes during treatment could have influence on dose distribution of OARs as well as that of PTV.