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BackgroundConservative surgery followed by breast and nodal irradiation is the standard loco-regional treatment for early breast cancer (BC) patients with four or more involved lymph nodes. However, the treatment strategy when fewer nodes are involved remains unclear, especially when lymphadenectomy has not been performed. Sensitive molecular techniques for nodal status assessment such as the OSNA (One-Step Nucleic Acid Amplification) assay can contribute to the definition and standardization of the treatment strategy. Therefore, the OPTIMAL study aims to demonstrate the feasibility of the incidental irradiation of axillary nodes in patients with early-stage BC and limited involvement of the SLN.MethodsBC patients with conservative surgery and a total tumour load of sentinel lymph nodes assessed with OSNA ranged between 250–15.000 copies/µL will be eligible. Patients will be randomized to receive irradiation on breast, tumour bed, axillary and supraclavicular lymph node areas (intentional arm) or only on breast and tumour bed (incidental arm). All areas, including the internal mammary chain, will be contoured. The mean, median, D5% and D95% doses received in all volumes will be calculated. The primary endpoint is the non-inferiority of incidental irradiation of axillary nodes compared with the intentional irradiation in terms of 5-year disease free survival. Secondary endpoints comprise the comparison of acute and chronic toxicity, loco-regional and distant disease recurrence rates.DiscussionStandardizing the treatment and diagnosis of BC patients with few nodes affected is crucial due to the lack of consensus on the therapeutic strategies in these patients. Hence, the quantitative score for metastatic burden of SLN provided by OSNA can contribute to improve the discrimination of BC patients with a limited nodal involvement who can benefit from incidental radiation as an adjuvant treatment strategy.Trial registrationClinicalTrial.gov, NCT02335957; https://clinicaltrials.gov/ct2/show/NCT02335957