Concepts for critical organ dosimetry in three-dimensional image-based breast brachytherapy

Purpose To investigate dose–volume histogram (DVH) parameters for organs at risk (OARs) in sectional three-dimensional image-based accelerated partial breast irradiation. Methods and Materials Skin, lung, and ribs were defined as OARs and the heart was discussed. Two different skin-contouring methods were tested on phantom before applying to the patient cohort. First, an inside skin wall contour was delineated with three different wall thicknesses to demonstrate the influence of contouring on DVH parameter. Second a structure was defined, delineated outside of the phantom surface, such that a three-dimensional skin volume was extended like a virtual bolus contour. Point dose values and DVH parameters were reported for 25 patient cases. Results The DVH parameters D0.1 cc = 65 ± 21, D1 cc = 45 ± 8, and D10 cc = 30 ± 4 cGy/pulse for the outside skin structure around the breast corresponded to skin surface areas of 1 ± 1, 6 ± 3, and 6 ± 11 cm2. Lung volume receiving 20 Gyαβ3 was 8.3 ± 11 cm3. D0.1 cc and D2 cc in the most exposed rib were 0.7 ± 0.3 and 0.4 ± 0.2 Gy/pulse, respectively. Conclusion In accelerated partial breast irradiation treatment planning, dose to OAR can be reported in a more sophisticated way with DVH parameters than using points only. The suggested method of skin delineation using the defined outside structure allows calculating reliable and reproducible DVH parameters.