Plexiform fibrohistiocytic tumor is a low-grade soft tissue malignancy that can at times be difficult to differentiate from the less biologically aggressive cellular neurothekeoma. The two entities, which may display identical clinical and histological features, cannot be distinguished by immunohistochemical or molecular diagnostic means. Electron microscopy may enable the accurate identification of problematic examples and thus aid in resolving these occasionally occurring diagnostic dilemmas. To illustrate typical variations in the ultrastructural appearance of plexiform fibrohistiocytic tumor, the authors present two diagnostically noncontroversial examples, and to demonstrate the potential diagnostic utility of electron microscopy in this setting, they present an example of plexiform fibrohistiocytic tumor that could not otherwise have been distinguished from cellular neurothekeoma.

Benign müllerian papilloma of the female reproductive tract is a rare childhood tumor that can easily be mistaken by those unfamiliar with the entity for botryoid rhabdomyosarcoma. Ultrastructural findings have been mentioned only in two individual case reports, and these both were issued many years ago. The aim of this update is to familiarize the reader with the clinical, light, and electron microscopic features associated with this distinctive entity, and thereby hopefully preclude the risk of making a serious diagnostic error. Two cases are illustrated, one very typical in its presentation and the other less so.

Electron microscopy was used to examine 72 cases of medulloblastoma to better characterize the ultrastructural spectrum of this tumor. Twenty-four cases showed prominent neural differentiation. Twenty-three cases showed minimal (21) or no (2) recognizable neural differentiation, and the remainder of the cases (25) showed intermediate differentiation. All 42 cases tested stained for neuron-specific enolase, 28 for synaptophysin, and 12 for neurofilament protein. All cases showed strong reactivity for glial fibrillary acidic protein (GFAP) within reactive astrocytes. Three cases showed reactivity for GFAP within tumor cells. Medulloblastoma exhibits a broad spectrum of neural differentiation, with nearly all cases showing at least some degree of this change, and it universally exhibits participation of reactive astrocytes which can create a potential for diagnostic confusion.

To assess the diagnostic potential of perivascular elastic fiber detection as an indicator of ependymoma, the authors performed ultrastructural studies on a large series of pediatric brain tumors. Elastic fibers were demonstrated by electron microscopy in 38 of 50 (76%) ependymomas, 2 of 25 (8%) choroid plexus tumors, 0 of 100 (0%) medulloblastomas, and 0 of 100 (0%) astrocytomas. In some poorly differentiated examples, elastic fibers were initially the sole indicator of ependymal differentiation. The authors conclude that the sensitivity and specificity of this feature is sufficient to render demonstration of perivascular elastic fibers a useful diagnostic indicator of ependymoma.