A Comparison of Clinical and Dosimetric Outcomes in Patients Receiving Partial Breast Irradiation with Photon-Only versus Mixed Photon/Electron Treatment Plans

Several series evaluating external-beam partial breast irradiation (PBI) have linked negative cosmetic outcomes to large normal tissue treatment volumes. We compared patients treated with PBI whose treatment plans included only photons to those whose plans incorporated electrons. Twenty-seven patients were identified: median age 67 years, pT1 82%, pN0 56%, margin negative 100%. All received 38.5 Gy using 3-5 noncoplanar photon beams (6-15X). Electrons (9-20 MeV) were included in 59%. Median follow-up was 22 months. Ninety percent experienced good/excellent cosmetic outcomes. Two patients had fair cosmesis, and both were treated with a mixed photon/electron approach. Median conformity index for photon-only treatment plans was 1.7 (range, 0.9-2.0) and for photon/electron plans, 1.0 (0.3-1.4). Median percent ipsilateral breast volume receiving 100% and 50% of prescription dose was 19 and 50 for photon-only plans vs. 10 and 38 for photon/electron plans (p < 0.05). Median percent target volume receiving 100% and 95% of prescription dose was 93 and 98 for photon-only plans vs. 75 and 94 for photon/electron plans (p < 0.05). A mixed photon/electron, noncoplanar technique decreases the volume of treated normal breast tissue at the cost of slightly decreased tumor bed coverage. Further study is needed to determine whether this results in a more favorable therapeutic ratio than photon-only approaches.