Spindle cell endocrine carcinoma of the mammary gland.

We report the pathological characteristics of a variant of mammary endocrine tumour, predominantly formed from cytologically bland spindle cells. This neoplasm grows as a red, well defined mass lacking the usual macroscopical characteristics of breast cancer. Within smoothly contoured aggregates arranged in an insular pattern, delicate capillaries and collagen bundles support the neoplastic epithelial cells. Most of the tumour cells possess a slender spindle shape and form a solid or fenestrated sheet, but a few appear cuboidal and create glands. Immunohistochemical studies demonstrate that the spindle cells and the glandular cells constitute a single population. Both types of cells stain for neuroendocrine markers (chromogranin, synaptophysin, and CD 57), carcinoembryonic antigen, keratin 8/18, S-100 protein, and receptors for oestrogen and progesterone. Many of the tumour cells possess argyrophilic granules, and electron microscopy may reveal dense core granules.