PURPOSE: The placement of breast implants in a pre-pectoral compared to sub-pectoral plane has become increasingly popular in breast reconstruction, though data are limited on how this affects radiation delivery for locoregional disease control in women with breast cancer. This study aims to assess the dosimetric implications of implant location in women undergoing post-mastectomy radiation therapy (PMRT) after immediate breast reconstruction. METHODS: Patients with breast cancer treated with mastectomy, implant breast reconstruction, and PMRT between 2014-2022 at a single center were included. All patients received either intensity-modulated or 3-D conformal radiation therapy over an average of 5 weeks. Demographic, clinical and treatment data were collected through chart review. Patients were matched 1:1 on cancer sidedness, use of boost, fractionation and extent of radiation fields. Groups were compared using Mann-Whitney U or t-tests for continuous variables, and chi-square or Fisher’s Exact tests for categorical variables, as appropriate. RESULTS: Twenty-seven patients with pre-pectoral and 26 patients with sub-pectoral implants were included. Patients with pre-pectoral reconstruction had lower average implant volumes (452cc vs 503cc) prior to radiation. There was no difference in chest wall coverage (p=0.107), internal mammary node coverage (p=0.215), heart dose (p=0.627) or lung V20Gy (p=0.774). CONCLUSION: In this assessment of radiotherapeutic dosimetry in patients undergoing immediate breast reconstruction, we found no significant difference in target coverage or normal tissue dose based on pre- versus sub-pectoral implant location. This suggests that implant placement can be selected to optimize reconstructive outcomes, without concern for compromise to the oncologic quality of PMRT.