Comparison of Treatment Outcome Between Invasive Lobular and Ductal Carcinomas in Patients Receiving Partial Breast Irradiation With Intraoperative Electrons.

PURPOSE: To investigate the local outcome of patients after accelerated partial breast irradiation with intraoperative electrons (IORT) for invasive lobular carcinoma (ILC) compared with invasive ductal carcinoma (IDC).

METHODS AND MATERIALS: From 1999 to 2007, 2173 patients were treated with breast-conserving surgery and IORT (21 Gy/1 fraction) as the sole local treatment: 252 patients with ILC (11.6%) were compared with 1921 patients with IDC in terms of local control.

RESULTS: Compared with the IDC subgroup, patients with ILC had a low-risk profile and were more hormone responsive. The 5- and 10-year in-breast tumor reappearance (IBTR) rates were 5.5% and 14.4%, respectively, for the IDC group and 7.5% and 21.8%, respectively, for the ILC group (log-rank P=.03). The excess risk of IBTR associated with ILC was particularly high for small tumors (≤1 cm: hazard ratio [HR], 2.24; 95% confidence interval [CI], 1.03-4.85), elderly patients (60-69 years: HR, 2.27; 95% CI, 1.11-4.63; ≥70 years: HR, 3.28; 95% CI, 1.08-10.0), low-grade tumors (grade 1: HR, 3.50; 95% CI, 1.05-11.7), and luminal A molecular subtype (HR, 3.18; 95% CI, 1.49-6.77). Among the ILC histologic variants, no difference between classic and nonclassic subgroups was observed, although the signet ring cell and solid variants had the worst local control.

CONCLUSIONS: Despite a favorable tumor profile, accelerated partial breast irradiation with IORT led to a higher incidence of IBTRs in patients with ILC compared with those with IDC. Our institutional experience emphasized the importance of the size of the irradiation field, pointing to the use of larger collimators, even when dealing with small tumors, to improve local control.