Prone positioning on a breast board is a simple and effective way for improving dosimetry in patients receiving breast radiotherapy. This technique has been demonstrated to reduce the degree of moist desquamation in patients with pendulous breasts and has also been shown to significantly reduce dose to heart and lungs. We recently observed a case of unanticipated severe moist desquamation in the inframammary fold of a patient treated with prone positioning. Review of pre-treatment port-films demonstrated that the inferior edge of the board aperture was closer to the inframammary fold during treatment than planned. We conducted a dosimetric and phantom study to investigate possible causative factors. We hypothesized that 1) proximity of the breast to the support structure at time of treatment and 2) the method of optimizing beam fluence during segment weighting both contribute to increased skin dose. A breast phantom was constructed using silicone breast prostheses affixed to a RANDO thorax and abdomen phantom. This was placed on a prone breast board and scanned in different positions: Breast centered vs Breast touching medial and inferior board edge. Plans were generated in the Treatment Planning System (TPS) using AAA. A forward planned, segment weighted photon plan was generated on the control scan (breast centered). The fluence from these plans was then deposited onto the shifted scan and dose was recalculated. The plans were then delivered onto the phantom and surface doses in the inframammary region were recorded using MOSFETs. To evaluate the impact of planning technique, the same process was also performed using a separate plan in which the inferior flash (fluence in air) was not segmented out. Surface dose in the inframammary region significantly increased with proximity to the breast board edge. With the breast centered using fully segmented plan, the calculated (TPS) and measured (MOSFET) surface doses were 83-87% and 81-84% of prescription (Rx) respectively. With the breast touching the inferior board edge, these doses increased to 105% and 101-103% of Rx. Segmentation of skin flash had minimal impact on calculated skin doses in the TPS. In contrast, lack of segmentation increased measured doses in the inframammary fold by 6-10%. Proximity of the breast to support structures at the time of treatment can significantly increase surface doses to the inframammary fold. This problem is magnified when inferior skin flash is not segmented out or blocked during the process of segment weighting. Taken together, these variables can result in increased surface doses of up to 30% over predicted and may significantly contribute to severed dermatitis. Care should be taken to ensure maximum separation of breast tissue from immobilization devices both at time of simulation and during treatment. Additionally, when optimizing plans for homogeneity with segment weighting, additional blocking should be used in areas of skin flash. While these changes in beam fluence may not significantly alter the observed dose distribution in the TPS, they can result in significant changes in surface dose during actual treatment.