In vivo Skin Dosimetry of Volumetric Modulated Arc Therapy to the Tumor Bed without Bolus after Breast Conserving Surgery and Whole Breast Irradiation

Background : There are no data on in vivo skin dosimetry of volumetric modulated arc therapy (VMAT) without bolus for tumor bed irradiation after breast-conserving surgery (BCS) and whole breast irradiation (WBI). The purpose of this study was to evaluate the accuracy of the expected dose to the tumor bed in patients undergoing non-bolus VMAT using in vivo skin dosimetry. Materials and Methods : In vivo skin dosimetry was performed in 78 patients who underwent image-guided helical VMAT to the tumor bed after BCS and WBI. The expected doses at 1279 points were compared with the measured doses. The relative errors and variations of the relative errors were evaluated. Results : The relative errors for 0.01–0.09 Gy, 0.10–0.49 Gy, 0.50–0.99 Gy, 1.00–1.49 Gy, 1.50–1.74 Gy, and ≥ 1.75 Gy were –0.666 ± 9.442 % (95 % CI –1.958–0.627 %), –0.265 ± 4.010 % (95 % CI –0.809–0.278 %), –0.205 ± 2.506 % (95 % CI –0.511–0.101 %), 0.207 ± 2.024 % (95 % CI –0.045–0.459 %), and –0.182 ± 1.665 % (95 % CI –0.449–0.084 %), respectively. The absolute value of the relative error was smallest for expected doses of 1.75 Gy or greater. The variation in relative errors was significantly greater at 0.01–0.09 Gy (p