Risk Factors for Fat Necrosis after Stereotactic Partial-Breast Irradiation (S-PBI) for Early-Stage Breast Cancer on a Phase I Clinical Trial

PURPOSE This study reports predictive dosimetric and physiologic factors for fat necrosis after Stereotactic-Partial Breast Irradiation (S-PBI). MATERIALS AND METHODS Seventy-five patients with ductal carcinoma-in-situ or invasive nonlobular epithelial histologies stage 0, I, or II, with tumor size <3 cm were enrolled in a dose escalation Phase I S-PBI trial between January 2011-July 2015. Fat necrosis was evaluated clinically at each follow-up. Treatment data was extracted from the Multiplan® Treatment Planning System (Cyberknife, Accuray). Univariate and stepwise logistic regression analyses were conducted to identify factors associated with palpable fat necrosis. RESULTS With a median follow up of 61 months (range: 4.3-99.5 months), eleven patients experienced palpable fat necrosis, five cases of which were painful. The median time to development of fat necrosis was 12.7 months (range: 3-42 months). On univariate analyses, higher V32.5-47.5Gy (p<0.05) and larger breast volume (p<0.01) were predictive of any fat necrosis; higher V35-50Gy (p<0.05), receiving two treatments on consecutive days (p=0.02), and higher Dmax (p=0.01) were predictive of painful fat necrosis. On multivariate analyses, breast volume larger than 1063cc remained a predictive factor for any fat necrosis, receiving two treatments on consecutive days and higher V45Gy were predictive of painful fat necrosis. Breast laterality, PTV volume, race, BMI, diabetic status, and tobacco/drug use were not significantly associated with fat necrosis on univariate analysis. CONCLUSIONS Early stage breast cancer patients treated with breast conserving surgery and S-PBI in our study had a fat necrosis rate comparable to other APBI modalities, but S-PBI is less invasive. To reduce risk of painful fat necrosis, we recommend to not deliver fractions on consecutive days, limit V42.5<50cc, V45<20cc, V47.5<1cc, Dmax ≤48Gy and PTV<100cc when feasible and counsel patients about the increased risk for fat necrosis when constraints are not met and for those with breast volume greater than 1000cc.