Merkel cell carcinosarcoma: Merkel cell carcinoma with embryonal rhabdomyosarcoma-like component.

Merkel cell carcinoma is an uncommon primary neuroendocrine neoplasm of the skin that may exhibit divergent differentiation. However, rhabdomyosarcomatous differentiation has only been rarely described and takes the form of isolated rhabdomyoblasts. We describe a case of cutaneous Merkel cell carcinoma with biphasic morphology imparted by discrete patches of embryonal rhabdomyosarcoma-like spindle cells alternating with islands of neuroendocrine small round cells, justifying a designation of "Merkel cell carcinosarcoma." The former component showed positive immunostaining for desmin and myogenin; and the later component, pan-cytokeratin, cytokeratin 20, synaptophysin, and chromogranin. The patient was an elderly man who presented with a temporal skin mass, and the biphasic morphology was evident in the recurrence and metastasis that developed 2 months after incomplete excision of the skin lesion.

ALK+ histiocytosis: a novel type of systemic histiocytic proliferative disorder of early infancy.

We report 3 cases of a previously uncharacterized form of histiocytosis presenting in early infancy and showing ALK immunoreactivity. The patients presented with pallor, massive hepatosplenomegaly, anemia, and thrombocytopenia. Liver biopsy showed infiltration of the sinusoids by large histiocytes with markedly folded nuclei, fine chromatin, small nucleoli, and voluminous lightly eosinophilic cytoplasm that sometimes was vacuolated or contained phagocytosed blood cells. One patient developed cutaneous infiltrates that morphologically resembled juvenile xanthogranuloma. The histiocytes were immunoreactive for histiocytic markers (CD68, CD163, lysozyme), S100 protein, ALK (membranous and cytoplasmic pattern), and dendritic cell markers (fascin, factor XIIIa), but not CD1a and langerin. One case successfully analyzed by molecular techniques revealed TPM3-ALK fusion. Thus the spectrum of diseases exhibiting ALK translocation should be expanded to include ALK(+) histiocytosis. The disease in the 3 patients (2 having been given chemotherapy) resolved slowly over many months.

Microthymoma: definition of the entity and distinction from nodular hyperplasia of the thymic epithelium (so-called microscopic thymoma).

We report 2 cases of microscopic-sized thymoma, which probably represents the earliest phase of thymoma development. The 2 patients presented with pure red cell aplasia and myasthenia gravis, respectively. The thymectomy specimens did not reveal tumor on gross examination, but histologically each contained small thymomas measuring 5 mm and 7 mm in largest dimension, respectively. One of the tumors was unencapsulated and involved a single lobule only, and the other was encapsulated and comprised two lobules. The tumors consisted of ovoid epithelial cells with pale nuclei and distinct nucleoli, scattered in a background of small lymphocytes. Foci of medullary differentiation and perivascular space were identified in the 2 cases, respectively. The lymphocytes were confirmed to be immature T cells on immunohistochemical studies (CD3+, TdT+). Except for the microscopic size, the morphology of the two tumors conforms to conventional type B1/B2 and type B2 thymoma, respectively. We propose calling such incidental small tumor "microthymoma" to distinguish it from the so-called microscopic thymoma, which is composed of small thymic epithelial nests and probably more appropriately termed "nodular hyperplasia" of the thymic epithelium.