Chest wall dose in MammoSite breast brachytherapy: radiobiologic estimations of late complication risk based on dose-volume considerations

PURPOSE: To estimate the risk of late effects in women treated with MammoSite brachytherapy (MBT), the balloon catheters of which were placed near the ribs. METHODS AND MATERIALS: Upon reviewing 93 plans, 16 patients (17%) treated with MBT were considered to have received a high chest wall dose (>or=120% isodose line in contact with a rib). A dose-volume histogram was generated for this rib, and its distance from the MBT balloon measured. Using the linear quadratic equation, the equivalent dose, delivered in 10 fractions, to the dose that causes a 5% and 50% risk of rib late effects at 5 years using 2Gy per fraction, was calculated to be 37 and 44Gy, respectively. The rib volume receiving greater than or equal to these doses (V37 and V44) was correlated to the balloon-to-rib distance. Chest wall signs, symptoms, and radiologic findings for all 16 patients were recorded. RESULTS: The median balloon-to-rib distance was 4.8mm. The median values of V37 and V44 were 13.5% and 3.3%, respectively. All patients with a V37>or=15% and V44>or=5% had a minimum balloon-to-rib distance of <5mm. Two patients reported treatment-related chest wall tenderness (both had balloons placed <5mm from the chest wall), but neither presented with radiologic complications. CONCLUSIONS: Sixteen patients considered to receive relatively high chest wall doses had less than one-third of their primary rib volume being exposed to the estimated TD 5/5 and TD 50/5 doses. Therefore, we estimate the risk of late effects in women treated with MBT, the balloon catheters of which placed near the ribs were negligible, and believe that MBT remains a safe and effective treatment for selected patients with early stage breast cancer.