OVARIAN SMALL CELL TUMORS
G. Richard Dickersin and Robert E. Scully
Department of Pathology, Massachusetts General Hospital

Small cell tumors of the ovary are uncommon but represent an important group to recognize in the differential diagnosis of primary and metastatic ovarian neoplasms. In some cases the correct diagnosis cannot be confidently made on the basis of clinical setting, routine light microscopy and immunohistochemistry, and electron microscopy may be supportive or definitive in establishing cell-type. The cell-type is often important in choosing optimal therapy and in predicting prognosis.

We have had the opportunity to perform electron microscopy on a moderate number of ovarian small cell tumors and in this presentation shall describe and illustrate the diagnostic features of representative examples of various types. The ultrastructural features of the metastatic tumors, such as embryonal rhabdomyosarcoma, neuroblastoma and melanoma, are identical to those of their respective primary tumors, are well known and usually pose no problem in diagnosis. On the other hand, the ultrastructural features of some primary ovarian small cell tumors may present a more difficult differential diagnosis, because they have features that are subtle and/or in common. Exemplary of tumors in this category are small cell carcinomas of the hypercalcemic and pulmonary (oat cell) types. Distinguishing between them may be difficult but still possible. For example, prominent rough endoplasmic reticulum is a moderately reliable feature of the hypercalcemic type tumor and an absent one in the pulmonary type. Whorls of cytoplasmic filaments are present in some hypercalcemic type tumors, whereas tonofibrils may be present in some pulmonary type small cell carcinomas. Dense core granules are absent in the hypercalcemic type but may be present in small numbers in the pulmonary type.

Diffuse adult granulosa cell tumor, ovary. A group of diffusely arranged neoplastic cells have a high nucleocytoplasmic ratio, indented euchromatic nuclei, a noderate number of mitochondria and focal collections of filopodia (arrow). Magn. x7,100

Another example of an ovarian small cell tumor that may be difficult to identify by its ultrastructural features is endometrioid stromal sarcoma, as well as metastatic endometrial stromal sarcoma, in which tumor cells often have a nondescript appearance; or they may have features of poorly differentiated fibroblasts and myofibroblasts. However, the absence of specific features of other small cell tumors is also often useful in supporting a "consistent with" level of diagnosis.

Endometrial stromal sarcoma, metastatic to lung from uterus: The neoplastic cells are oval, have a high nucleocytoplasmic ratio and have nuclei with a moderate amount of heterochromatin and prominent nucleoli. Mitochondria (arrows) are the main cytoplasmic organelle in some cells. Magn. x7,500

Overall, an accurate diagnosis of specific types of small cell tumors of the ovary may be enhanced in a significant percentage of cases if electron microscopy is available to supplement diagnostic information obtained from the clinical presentation, light microscopy, immunohistochemistry and, in some tumors, cytometric analysis of these neoplasms.

Small cell carcinoma, hypercalcemic type, ovary: A group of oval and polygonal neoplastic cells with oval and irregularly shaped nuclei and dilated cisternae of rough endoplasmic reticulum (arrows). Magn. x5,000.

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