Defining the clinical target volume for patients with early-stage breast cancer treated with lumpectomy and accelerated partial breast irradiation: a pathologic analysis.

BACKGROUND: This pathologic analysis was conducted to help define the clinical target volume (CTV) for partial breast irradiation (PBI) by analyzing the amount and distance of residual disease found at reexcision after an initial lumpectomy.
MATERIALS AND METHODS: The study population consisted of 441 patients derived from a dataset of 607 consecutive cases of Stage I and II breast cancer (reviewed by one pathologist) who underwent reexcision (after lumpectomy) before radiation therapy (RT) as part of their standard breast-conserving therapy (BCT). The assumption in this analysis was that the maximal measured extension distance from the initial excision specimen margin (in the reexcision specimen) represents the minimum distance that needs to receive full-dose RT for PBI to be successful. In 333 of the 441 cases, it was possible to measure this distance. Margins were classified as negative (carcinoma > 1/2 low-power field [LPF] from the margin), near (< 1/2 LPF from the margin), or positive. The amount of carcinoma near the final margin was quantified as the width of invasive carcinoma and number of ductal carcinoma in situ (DCIS) ducts near the margin and divided into three groups: least, intermediate, and greatest amount.
RESULTS: Of the 333 cases, 119 (35.7%) had no residual carcinoma in the reexcision specimen, 67 (20.1%) had maximum extension (invasive carcinoma or DCIS) distances of >0<5 mm beyond the initial excision cavity edge, 83 (24.9%) extended 5 to <10 mm, 34 (10.2%) extended 10 to <15 mm, and 30 (9.0%) extended > or =15 mm. In 90% of 134 patients with negative initial lumpectomy margins (per National Surgical Breast and Bowel Project criteria) at lumpectomy, if any residual disease was present (38.2% of cases), it was limited to <10 mm from the edge of the original lumpectomy margin. The initial lumpectomy margin status was then combined with the invasive carcinoma: specimen maximum dimension ratio to determine if these two criteria (when combined) could better identify patients with residual disease limited to <10 mm from the initial margin. Analyzed in this fashion, all 13 of the reexcision specimens (9.7%) with >10 mm of maximum extension by carcinoma beyond the edge of the initial excision specimen cavity could be identified.
CONCLUSIONS: A margin of 10 mm around the tumor bed should be adequate in covering disease remaining in the breast after lumpectomy in >90% of patients treated with PBI. However, it is possible to accurately identify all patients with disease extending beyond 10 mm using more restrictive pathologic selection criteria. These results can also be used as a guide for defining the CTV for boost treatment after whole-breast RT and the amount of breast tissue to remove at reexcision.