390226 Background: Radiation dermatitis (RD) is common in patients undergoing breast radiation treatment (RT), with worse RD in patients with large breasts and after mastectomy. Mepitel Film (MF), a silicone-based polyurethane film with Safetac technology, may reduce RD. We hypothesized that MF might play a role in preventing RD in this group of patients at risk. Methods: Patients were randomly assigned to receive MF or standard skin care (2:1 ratio). Patients with large breasts after lumpectomy (bra size ≥ 36 inches or cup size ≥ C) or after mastectomy were eligible. Stratification factors included surgery type (lumpectomy vs. mastectomy), dose fractionation (conventional vs. hypofractionated), and administration of boost/bolus. The primary endpoint was grade (G) 2 or 3 RD using the Common Terminology Criteria for Adverse Events (CTCAE) v5.0 during RT and within 3 months of completion of RT. Secondary endpoints included CTCAE G3, incidence of moist desquamation, use of topical antibiotic cream, and patient- (PROs) and clinician- (CRO) reported outcomes using the modified Radiation-Induced Skin Reaction Assessment Scale (RISRAS) and Skin Symptom Assessment (SSA). This study is registered with ClinicalTrials.gov (NCT04166799). Results: Between January 2020 and May 2022, 266 patients (66%) were randomly assigned to MF and 137 (34%) to standard care. After removing withdrawn patients, 376 patients were included in the modified intention-to-treat analysis. The incidence of G2 or 3 RD was significantly lower in MF patients compared to standard care (n = 39/251, 15.5%; 95% CI, 11.3-20.6% vs. n = 57/125, 45.6%; 95% CI, 36.7-54.8% respectively, odds ratio [OR]: 0.20, P < .0001). Benefits of MF over standard care remained significant in patients who developed G3 RD (n = 7, 2.8%; 95% CI, 1.1-5.7% vs. n = 17, 13.6%; 95% CI, 8.1-20.9%, OR: 0.19) and moist desquamation (n = 20, 8.0%; 95% CI, 4.9-12.0% vs. n = 24, 19.2%; 95% CI, 12.7-27.1%, OR: 0.36). When evaluating the combined patient and health care professional score using the RISRAS, the MF arm had significantly lower scores than standard care ( P < .0001); individual items on the RISRAS also favored MF for PROs (tenderness, discomfort of pain and burning sensation) and CROs (erythema and moist desquamation). Similarly, when assessed by the SSA, blistering/peeling, erythema, pigmentation, and edema in PROs, and pain/soreness, blistering/peeling, erythema, and pigmentation in CROs were significantly reduced in MF. Topical antibiotics were prescribed significantly less frequently in MF (23.1% vs. 43.2%, P < .0001). Three patients removed the film prematurely due to development of an allergic rash (n = 2) and excessive pruritis (n = 1). Conclusions: MF significantly reduces RD in breast cancer patients undergoing RT when compared to the standard care. We recommend changes to the clinical practice for RD prevention and inclusion of MF in high-risk patients in future guideline recommendations. Clinical trial information: NCT04166799.