Effect of External Boost Volume in Breast-Conserving Therapy on Local Control With Long-Term Follow-Up

Purpose: To determine the effects of boost volume (BV) in relation to margin status and tumor size on the development of local recurrence with breast-conserving therapy. Methods and Materials: Between 1983 and 1995, 1,073 patients with invasive breast cancer underwent 1,101 breast-conserving therapies. Of these 1,101 BCTs, 967 were eligible for analysis. The BV was categorized into tertiles: <66 cm³ (n = 330), 66-98 cm³ (n = 326), and >98 cm³ (n = 311). The median follow-up was 141 months. Separate analyses were done for women ≤40 years and >40 years. Results: No significant difference in local recurrence was shown between the tertiles and the recurrence site. The 15-year local recurrence-free survival rate was 87.9% for the first tertile, 88.7% for the second, and 89% for the third. For women ≤40 years old, the corresponding 15-year local recurrence-free survival rate was 80%, 74.5%, and 69.2%. For women >40 years old, the corresponding rate was 88.7%, 89.5%, and 90.9%. At 5 years, women >40 years old had significantly more local failures in the first tertile; this difference disappeared with time. A test for trend showed significance at 5 years (p = 0.0105) for positive margins for ductal carcinoma in situ in women >40 years of age. Conclusion: The results of this study have shown that the size of the external BV has no major impact on local control. For women >40 years old, positive margins for ductal carcinoma in situ showed a trend with respect to BV at 5 years. The BV had no influence on local control in the case of positive margins for invasive carcinoma.