MammoSite excision volume as a predictor for residual disease

The MammoSite catheter is a brachytherapy device used for accelerated partial breast irradiation. Currently, it is available as 2 spherically shaped balloons meant to fill 70-cc and 120-cc cavity volumes. This study was designed to define the relation of these excision volumes to the likelihood of microscopically detectable, residual disease based on tumor size, margin status, patient age, and histology.

The study data base was comprised of 531 patients with Stage 0, I, and II breast carcinoma (using American Joint Committee on Cancer staging criteria) who received breast-conserving therapy and underwent surgical reexcision. Patients in the data base were stratified based on the volume of their initial excision: < or = 70 cc versus > 70 cc and < or = 120 cc versus > 120 cc.

Surgical margin size was found to be a strong predictor of residual disease both for patients with smaller excision volumes (P = 0.0014) and patients with larger excision volumes (P = 0.0003); histology (extensive intraductal component [EIC] or pure ductal carcinoma in situ [DCIS]) also was a strong predictor. Tumor size was significant only for the larger volume group (P = 0.029). On multivariate analysis, only histology and initial margin status were significant correlates with residual disease. The adjusted odds ratio for residual disease with pure DCIS was 0.79, and the adjusted odds ratio for invasive ductal or lobular carcinoma (IDC/ILC) without EIC was 0.44 relative to IDC/ILC with EIC (P = 0.008). The adjusted odds ratio for residual disease with a positive initial margin versus a negative initial margin was 2.65 (P < or = 0.0001).

For excisions amenable to use of the MammoSite catheter, a margin > or = 1.0 mm appeared to afford at most a 35% risk of microscopically detectable residual tumor. Evidence of EIC on excision of IDC/ILC connoted a significantly higher risk. Age did not appear to be predictive for residual disease.