Practical approach to diagnosis of breast lesions with heterologous components

The presence of ectopic/heterologous tissue within the breast has been recognised. A number of reactive, benign and malignant lesions may harbour ectopic tissue such as bone, cartilage, myxoid and mucinous matrix and squamous epithelium. In the workup of such lesions, it is important to ascertain the presence and type of the heterologous element, assess the nature of the associated lesion (i.e., benign versus malignant) and assess whether the lesion is of primary breast origin or metastatic. In malignant breast lesions with prominent heterologous morphology, it is important to sample extensively to look for DCIS/epithelial differentiation or biphasic pattern. This review aims to summarise the current state of knowledge and highlight different benign and malignant breast lesions that may feature a heterologous component, including extremely rare examples, with emphasis on approach to diagnosis.