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Breast cancer (BC) is a heterogeneous disease including multiple histological subtypes with different biological behavior and clinical outcome for the patient. Due to its heterogeneity, BC is classified into different groups, providing possibilities for target therapies and improving the outcome – histological, TNM classification, molecular classification. The molecular classification, proposed at the beginning by Perou et al in 2000, divides BC into 5 subtypes - Luminal A, Luminal B, HER 2 enriched, claudin-low and basal like. More attention should be paid to the further classification of basal like BC. This is also a heterogeneous disease including triple negative breast cancers (TNBC). They show no expression of ER, PrR or HER2 and are considered difficult to treat. Despite the bad prognosis, some of these patients demonstrate great benefit from the traditional neoadjuvant chemotherapy. Lehman et al. identified 6 molecular subtypes of TNBC, using gene expression profiling – basal like 1 (BL1), basal like 2 (BL2), mesenchymal (M), mesenchymal stem-like (MSL), immunomodulatory (IM) and luminal androgen receptor (LAR). However, after histopathological quantification and laser-capture microdissection they refined their classification and in 2016 the new molecular classification of TNBC excluding the IM and MSL types was published. The working group has concluded that the characteristics of the tumors in these two groups were due to tumor-infiltrating lymphocytes and tumor-associated stromal cells and not to the tumor cells. The refinement of the classification of TNBC provides a better distribution of the patients into the different groups and better response to the applied therapy.

Introduction Low-grade adenosquamous breast cancer (ASBC) is a rare histological subtype of metaplastic breast cancer. Even tough it is a triple negative cancer, it is characterised by a good prognosis, low probability of distant metastases and higher risk of local relapses. Clinical Case This is a clinical case of a 65-year-old woman with a retroareolar nodule on the left breast. After histological examination a diagnosis of low-grade adenosquamous breast cancer was established. Further examination of lymph nodes showed regional metastasis of ASBC. Discussion Due to its specific growth patterns the diagnosis could easily be mistaken, mostly on core biopsy samples. Vast differential diagnosis is required. The definitive diagnosis could be given on histological examination of surgical sample. Therefore, the recommended treatment is surgical removal of the lesion until more follow-up data is available in literature.