The Influence of Margin Width and Volume of Disease Near Margin on Benefit of Radiation Therapy for Women With DCIS Treated With Breast-Conserving Therapy

In Brief Objective and Summary Background Data: There remains variation in the use of radiation therapy (RT) in women with ductal carcinoma in situ (DCIS), despite prospective randomized trials documenting its benefit in reducing the risk of ipsilateral breast tumor recurrence (IBTR). Methods: Patients with DCIS treated with excision alone or excision plus RT from 1991 to 1995 were identified. Margin width, number of involved ducts at closest margin, age, presence of palpable mass, presence of lobular neoplasia, nuclear grade, and necrosis were tested in uni- and multivariate analysis for association with risk of IBTR and added value of RT. Results: Two hundred ninety-four patients with a median follow-up of 11 years had actuarial 10- and 15-year overall IBTR rates of 22% and 29%, respectively. For lesions excised with margins of <1 mm, 1 to 9 mm, and ≥10 mm, the actuarial 10-year IBTR rates were 28%, 21%, and 19%, respectively. RT reduced adjusted IBTR rates by 62% (P = 0.002) for all patients; 83% for lesions with <1 mm margins (P = 0.002), 70% for 1 to 9 mm (P = 0.05), and 24% (P = 0.55) for ≥10 mm. After adjustment for other variables, higher volume of disease near the margin was associated with risk of IBTR in the no RT group (HR = 3.37, P = 0.002) and greater benefit of RT (HR 0.14; P = 0.004). Conclusion: Effect of RT on IBTR risk is influenced by both margin width and number of involved ducts at nearest margin. Patients with higher volume of disease near the margin derive a greater benefit from the addition of RT. Despite margins of ≥10 mm, the risk of IBTR remains substantial in patients with DCIS. Current lack of consensus regarding radiation (RT) use in ductal carcinoma in situ reflects the belief that risk/benefit ratio does not favor RT for all patients. Patients with higher disease volume near margin derive greater benefit from addition of RT. Despite ≥10 mm margins, local recurrence risk remains substantial in ductal carcinoma in situ patients.