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.

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.

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.