Breast skin dose in intraoperative radiotherapy using spherical applicators: A Monte Carlo study

Introduction and aim: In the targeted IORT booklet, consideration of minimum 5-10 mm distance between the skin and surface of the spherical applicators with emphasize on at least 5 mm, to avoid severe fibrosis which happens with the threshold dose of 13-14 Gy, have been remarked. However, there is no an accurate method of estimating the breast skin dose which may also be affected by applicator size. This work aims to calculate the skin dose for different applicator sizes and treatment depths using Monte Carlomethod Methods: MCNP code is used to simulate the IORT X-ray source (XRS) and its spherical applicators. Monte Carlo simulation provided the realistic model of the IORT radiation field to obtain photons spectrum at various situations and calculate the dose at any position. Simulation results were validated first by comparison with reference depth doses. EBT3 film was then used for the verification of surface dose simulations Results: Dose to skin dermal layer was slightly higher than epidermal layer. Both doses significantly increased when decreasing the distance from 10 mm to 5 mm. The ratio of skin dose to prescription dose and hence the overdosed area of the skin (> 6 Gy) increased with applicator size insofar as skin close to applicator receives 12 Gy in the case of 5 cm applicator and 0.5 cm treatment depth. Conclusion: Consideration of 5 to 10 mm distance between skin and applicator surface does not guarantee skin safety and overdosing is more significant in the case of larger applicators