LAUSR

Mixed tumour of the skin is a rare, benign skin adnexal tumour presenting as a firm, intradermal or subcutaneous, asymptomatic, slow-growing nodule. It is characterized by the coexistence of epithelial and mesenchymal features, and is usually classified as apocrine or eccrine variants [1]. The apocrine variant, also termed apocrine mixed tumour (AMT), is more common; this variant exhibits differentiation in the epithelial, myoepithelial, and stromal components, and often shows decapitation secretion. The predilection sites of AMT are the head and neck regions (especially the nose, upper lip, and cheeks, where folliculosebaceous-apocrine units are well developed); development in other areas is rare. Here, we report a case of AMT on the right side of the abdomen, which is an extremely unusual site of occurrence. A 61-year-old woman presented with a painless, slowgrowing mass on the right abdomen, which had developed over a five-year period (figure 1A). Physical examination showed a firm, solitary, mobile, painless nodule of 20 mm in diameter. The overlying skin was normal. The lesion was surgically excised. Histopathological examination showed a well-circumscribed tumour invading the subcutis, which had the characteristic structure of a solid area surrounded by large multiple cysts (figure 1C). The solid area was composed of several parts that exhibited multilineage differentiation into cartilage, tubular structures, and myxoid stroma (figure 1D). The cysts contained notably tubular structures with decapitation secretion (figure 1E). Immunohistochemically, duct epithelial cells stained positive for CAM5.2, epithelial membrane antigen, and gross cystic disease fluid protein 15 (figure 1F). Spindle cells were positive for alpha-smooth muscle actin, S-100 protein, CAM5.2, vimentin, and p16. Oestrogen receptor, progesterone receptor, and Ki-67 (figure 1G) were negative, both in the ductal epithelial cells and the spindle cells. The tumour was not associated with mammary gland tissue and showed no malignancy features. A diagnosis of AMT was made. No relapse was observed at a one-year follow-up visit. AMT accounts for less than 0.098% of all primary cutaneous tumours [2]. Based on a survey of the literature, 219 AMT lesions were identified in which the location was specified; the majority (92%) were on the head and neck region, and a small minority (2.3%) on the trunk [3]. Therefore, our case on the right abdomen is extremely unusual. AMT is a benign tumour, but malignant variants have been reported. Watarai et al. identified 43 cases of malignant mixed tumours of the skin. Only 28% of these malignant cases were on the head and neck, and they occurred more frequently on the lower limbs (35%), upper limbs (23%), and trunk (14%) [4]. Therefore, complete excision of AMT, especially when located on areas other than the head or neck, is the best therapeutic option. Close follow-up is recommended. AMT is considered analogous to pleomorphic adenoma of the salivary glands, and similar neoplasms also exist in the breast [5]. Characteristic findings of the present case are the location of the lesion and the prominent apocrine differentiation, which suggests an association with mammary gland A B