Purpose: Breast radiotherapy using tangents is delivered in the supine or prone position. Tumor bed boosts, however, are typically performed in the supine position with electrons, requiring an additional simulation CT scan for patients initially treated prone. Boost delivery in the prone position with mini-tangent photon beams is an alternative; however, the reliability of daily set-up is less certain. Using daily CBCT to verify adequacy of daily target coverage we evaluate the reliability of our daily set-up protocol for prone mini-tangent boosts. Methods: A retrospective study of 13 patients treated with prone mini-tangent boost with a prescription dose of 10Gy. Patients’ daily setup protocol consisted of standard SSD verification and lateral light fields with weekly port films and daily post-setup CBCT. Accuracy of daily treatment delivery was assessed as a qualitative metric of treatment delivery accuracy by using BEV to determine how often the surgical bed was completely encompassed by both treatment portals, and how often the edge of the surgical bed extended to within 7mm of the closest field edge. As a quantitative metric of treatment delivery accuracy, we used the CBCT to simulate the dose distribution of delivered radiation for each treatment fraction, keeping treatment parameters identical to those used in the delivery. Dose-volume histograms from each fraction were used to calculate dosimetric parameters. Results: Characterization of the tumor bed location indicated that in a total of 390 evaluated cases, the tumor bed was within the boost field 77% of the time. Dosimetric analysis showed no statistical difference between the planning CT and the averaged contoured tumor beds with respect to Maximum Dose, Mean Dose, and D90. Conclusion: We believe that the initial setup may not deliver the dose to at least 90% of the tumor bed as seen by Patients 5 and 8 who had poor initial setups for 3 out of the 5 treatment fractions. All other patients had an accurate initial setup as shown by our results. Even when initial treatment delivery does not look accurate, CBCT images help improve treatment localization.