Scanning Beam Intensity Modulated Proton Therapy for Accelerated Partial Breast Irradiation
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BACKGROUND: Previous studies of accelerated partial breast irradiation (APBI) with passive scatter proton therapy have demonstrated promising dosimetric and clinical results. Scanning beam Intensity Modulated Proton Therapy (IMPT) has potential advantages over passive scatter proton therapy in regards to field selection, treatment duration, dose homogeneity and certain normal tissue sparing. This retrospective review represents the first known clinical experience using IMPT technique for treatment of breast cancer with APBI. METHODS: Between March 2014 and August 2015, 11 patients with IDC, DCIS, or ILC underwent IMPT treatment. All patients underwent CT based simulation and treatment planning and were set up supine on a breast board or in the prone position. Daily setup and localization was accomplished with 4-6 skin surface fiducial markers tracked with orthogonal x-ray pairs. Treatment was administered while free breathing in 10 M-F daily fractions over a 2 week period, with 3800-4000cGy prescribed to the operative cavity and 3400-3850cGy prescribed to the clinical target volume which was defined as 10-15mm expansion of the operative cavity respecting anatomical boundaries. Treatment was delivered with a single enface scanning proton beam. Clinical outcomes were monitored during and after treatment and later abstracted from the EMR. RESULTS: Mean total patient time in treatment room was 14.6 minutes. Enface proton beam setups included LAO, AP, LL and RAO. Maximum and mean doses are as follows: cardiac 531.4cGy/4.3cGy, ipsilateral lung 2138.0cGy/56.2cGy, chest wall 3504.0cGy/678.6cGy and skin 5mm of 4013.1cGy/964.1cGy. 9 of 11 patients experienced grade 1 dermatitis of skin overlying the treatment area and no other acute toxicity; the other 2 patients experienced no acute toxicity. With a mean follow up time of 5.6 months, 3 patients had minor dry skin in the treatment area and no other late toxicities. All patients self-reported 'good to excellent' cosmetic outcomes. No patients had evidence of local failure at the time of most recent follow up. CONCLUSIONS: Using single field IMPT is a feasible and effective approach for APBI. Improved treatment time and use of a single scanning beam helps to reduce delivery uncertainties and reduce intra-fractional motion and respiratory variance. IMPT provides excellent normal tissue sparing dosimetry and good acute toxicity profile.