3 fraction pencilbeam scanning proton accelerated partial breast irradiation early provider and patient reported outcomes of a novel regimen background and purpose to report dosimetry and early adverse effects aesthetic and patientreported outcomes of a prospective study of 3fraction pencilbeam scanning pbs proton accelerated partial irradiation apbimaterials and methods eligibility included women age50years with estrogen receptor positive er sentinel lymph node negative invasive or insitu breast cancer measuring 25cm the prescription was 219gy rbe 11 in 3 daily fractions to the postoperative tumor bed with a 1cm expansion toxicities were collected using common terminology criteria for adverse events ctcae version 40 10point linear analog scale assessment patientreported outcomes version of the ctcae and the harvard breast cosmesis scaleresults seventysix women were treated between 2015 and 2017 the median breast volume receiving 50 of prescription or more was 28 median mean heart mean ipsilateral lung and maximum skin dose were 0gy 01gy and 206gy respectively with a median followup of 12months no treatmentrelated toxicity grade2 has been observed most common grade 1 adverse events were dermatitis 68 and skin hyperpigmentation 18 at 12months the only persistent toxicities were one patient with grade 1 breast edema and one patient with a grade 1 seroma 90 of patients reported quality of life as 7 out of 10 0 indicating as bad as it can be and 10 indicating as good as it can be and 98 of patients reported excellent or good cosmesisconclusion 3fraction pbs proton apbi is well tolerated with low rates of physician and patient reported early adverse effects followup is ongoing to assess late toxicities and disease control outcomes further investigation of this novel adjuvant treatment strategy is warranted