Post-mastectomy radiotherapy: Impact of bolus thickness and irradiation technique on skin dose.

PURPOSE: To determine 10 MV IMRT and VMAT based protocols with a daily bolus targeting a skin dose of 45 Gy in order to replace the 6 MV tangential fields with a 5 mm thick bolus on alternate days method for post-mastectomy radiotherapy. METHOD: We measured the mean surface dose along the chest wall PTV as a function of different bolus thicknesses for sliding window IMRT and VMAT plans. We analyzed surface dose profiles and dose homogeneities and compared them to our standard 6 MV strategy. All measurements were performed on a thorax phantom with Gafchromic films while dosimetric plans were computed using the Acuros XB algorithm (Varian). RESULTS: We obtained the best compromise between measured surface dose (mean dose and homogeneity) and skin toxicity threshold obtained from the literature using a daily 3 mm thick bolus. Mean surface doses were 91.4 ±â€¯2.8% [85.7% - 95.4%] and 92.2 ±â€¯2.3% [85.6% - 95.2%] of the prescribed dose with IMRT and VMAT techniques, respectively. Our standard 6 MV alternate days 5 mm thick bolus leads to 89.0 ±â€¯3.7% [83.6% - 95.5%]. Mean dose differences between measured and TPS results were < 3.2% for depths as low as 2 mm depth. CONCLUSION: 10 MV IMRT-based protocols with a daily 3 mm thick bolus produce a surface dose comparable to the standard 6 MV 5 mm thick bolus on alternate days method but with an improved surface dose homogeneity. This allows for a better control of skin toxicity and target volume coverage.

Radiation exposure using the O-arm® surgical imaging system.

PURPOSE: This study was conducted to characterise the O-arm® surgical imaging system in terms of patient organ doses and medical staff occupational exposure during three-dimensional thoracic spine and pelvic examinations. METHODS: An anthropomorphic phantom was used to evaluate absorbed organ doses during a three-dimensional thoracic spine scan and a three-dimensional pelvic scan with the O-arm®. Staff occupational exposure was evaluated by constructing an ambient dose cartography of the operating theatre during a three-dimensional pelvic scan as well as using an anthropomorphic phantom to simulate the O-arm® operator. RESULTS: Patient organ doses ranged from 30 ± 4 µGy to 20.0 ± 3.0 mGy and 4 ± 1 µGy to 6.7 ± 1.0 mGy for a three-dimensional thoracic spine and pelvic examination, respectively. For a single three-dimensional acquisition, the maximum ambient equivalent dose at 2 m from the iso-centre was 11 ± 1 µSv. CONCLUSION: Doses delivered to the patient during a three-dimensional thoracic spine image acquisition were found to be significant with the O-arm®, but lower than those observed with a standard computed tomography examination. The detailed dose cartography allows for the optimisation of medical staff positioning within the operating theatre while imaging with the O-arm®.