613 Background: Local recurrence (LR) of ductal carcinoma in situ (DCIS) is reduced by whole breast irradiation following breast conservative surgery (BCS). The benefit and indication for adding a radiotherapy boost to the surgical cavity for DCIS is unclear. Methods: 220 consecutive cases of DCIS treated at the McGill University Health Centre (MUHC) with BCS and radiotherapy between January 2000 and December 2006 were reviewed to assess the impact of the radiotherapy boost on local control. All patients were CT-simulated and received whole breast irradiation ranging from 42.4-50.4 Gy in 16-28 fractions. 36% of the patients received a radiotherapy boost of a median dose of 10 Gy (range: 9-16 Gy) to the surgical cavity. Median follow-up was same for Boost and No-Boost groups (46 months). Descriptive statistics and time to event analyses was carried out using Kaplan-Meir survival analyses. Results: Compared with the No-Boost group, patients who received the boost had more positive (6% vs. 1%) and < 0.1 cm margins (51% vs. 8%) (p < 0.0001), and were in higher risk categories as defined by the Van Nuys Prognostic (VNP) index (p = 0.015). Despite being in higher risk for LR, none (0/79) of the patients who received a boost recurred locally while 8/141 patients who did not receive a boost suffered an in-breast LR (Log-rank p = 0.03). Univariate analysis of prognostic factors (age, tumor size, margin status, histological grade, necrosis, and VNP risk category) revealed only the presence of necrosis to significantly correlate with LR (Log-rank p = 0.003). All 8 LR occurred in DCIS with necrosis. No significant difference in LR rate was observed between whole breast irradiation dose and fractionation schedule. Conclusions: Our results suggest that use of a radiotherapy boost significantly impacts local control in DCIS. The use of a boost may outweigh the poor prognostic effect of necrosis. No significant financial relationships to disclose.