Abstract
Accessory mammary tissue occurs in 0.2-6% of women, and is under the same hormonal influences as breast tissue, potentially undergoing malignant transformation. Carcinomas of accessory mammary tissue account for ∼0.3% of breast cancers, 5% of which are within a supernumerary breast. Due to its rarity and the low index of clinical suspicion, it is often diagnosed at a later stage compared with breast cancer. Reports of carcinoma developing in a supernumerary breast are rare. We describe a case of a 48-year-old woman, presenting with ectopic breast carcinoma within a supernumerary breast below the inframammary fold. We describe the mode of presentation, diagnosis and treatment with reference to the available literature. The radiotherapy treatment plan is discussed in detail to provide reference for future cases as the available literature offers no formal guidance on radiotherapy dose, fractionation or treatment field.