Abstract
Abstract only 79 Background: Whole breast irradiation (WBI) leads to acute and late toxicities, which can be worse with plan dose inhomogeneities. This has been of clinical concern for large-breasted patients, especially with hypofractionation. Two approaches of 3D modulation of the radiation beam profile to optimize dose distribution and improve homogeneity are commonly employed. One is field-in-field forward planning wherein up to 3 or 4 subfields are generated within the initial radiation field. The other is inverse-planning IMRT, which typically utilizes 5 or more segments. In this study, we compare dosimetric parameters of WBI using the field-in-field technique with up to 3 subfields (3D-FiF) compared with inverse planning IMRT in large breasted patients. Methods: 10 large-breasted patients (planning target volume [PTV] >2500cc) treated between 2007-2013 with WBI in the prone position with hypofractionation (42.4 Gy in 16 fractions) were selected. For each, an inverse planning IMRT and a 3D-FiF plan were created for the treated breast. Plans were normalized so that V95% PTV coverage was the same. Dose-volume histograms were evaluated for volumes receiving > 105% (V105) and >107% (V107) of prescribed dose, and maximum dose (Dmax). Results: Median PTV was 3443cc (2675-3875) and median separation distance at the chestwall posterior field edge was 25.9cm (23.7-27.3). IMRT significantly (p<0.005, wilcoxan) reduced the volume receiving V105 and V107. Mean V105 and V107 for IMRT were 124cc (6-266) and 1.3cc (0-11.8), respectively. Mean V105 and V107 for 3D-FIF were 525.6cc (259-765) and 86 (6-171). IMRT also significantly reduced Dmax from a mean of 109.4% to 106.7%. 3D-FiF was able to limit max dose below 110% in 9 of 10 patients. Conclusions: 3D-FiF can achieve a maximum point dose under 110% of prescribed dose with similar target coverage to IMRT for most large breast patients. However, IMRT can significantly reduce the V105 and V107 in these women. Improved dose homogeneity is expected to provide a benefit in terms of acute skin toxicities and late breast fibrosis in such large breasted women receiving WBI with hypofractionation; however further study is needed to prove its true clinical benefit.