Partial derivative.

Top Line: Does hypofractionated partial breast irradiation (PBI) reduce chronic induration compared to whole breast radiation?

The Study: The DBCG PBI trial was a randomized, phase 3 trial of 865 patients with early stage, low risk breast cancer. It is very similar to the UK IMPORT LOW trial in comparing whole breast RT and partial breast RT using the same dose-fractionation. Eligible patients had hormone receptor positive, HER2 negative, grade 1-2 invasive ductal carcinoma measuring <2 cm with >2 mm surgical margins. All patients were treated with 40 Gy in 15 fractions, but they were randomized to receive either WBRT or PBI. The PBI target was the tumor bed plus a 1.5 cm CTV margin (confined to the breast CTV) and a 5mm PTV margin (cropped 5 mm from the skin surface). In both arms, tangent fields and a forward-planned field-in-field technique were used. The trial was designed to show that PBI resulted in non-inferior breast induration. Indeed, grade 2-3 induration 3-years after PBI was not only non-inferior but significantly less than after WBRT (5.1% v 9.7%). Overall, larger breast volume was associated with a higher rate of breast induration (13% v 6%). However, this was mainly driven by an increased rate of induration in those with large breasts who had WBRT (13% v 5%). Induration was similar for large and small breasts treated with PBI (6% v 5%). Put another way, those with large breasts appeared to derive the greatest reduction in induration from the use of PBI. At 5 years, there was no difference in local recurrence with PBI (1.2% v 0.7%) nor was the difference significant at 9 years (3.1% v 1.7%).

TBL: For the same dose, fractionation, and planning technique, partial breast irradiation results in less chronic induration than whole breast radiation without a significant increase in local recurrence at 3-5 years for those with low risk disease. | Offerson, J Clin Oncol 2022