The diagnosis of MLL continues to be challenging, in particular for pathologists. Awareness of this entity, clinical correlation and gross pathological correlation are essential in the separation of this distinctive pseudosarcoma from its various morphological mimics.

Several studies have now examined the cDNA expression profiles of healthy myometrium, leiomyomas (LM), and leiomyosarcomas (LMS). This has produced a list of candidate genes that might be useful tools for distinguishing these entities from each other. The potential candidates identified from this body of research include insulinlike growth factor 1, h-caldesmon, cytokeratin 18, and the cyclin-dependent kinase 4 inhibitor, p16. To determine whether the immunohistochemical expression of these proteins could aid in the diagnosis of LMS and LM variants, we constructed a tissue microarray consisting of cases of healthy myometrium (n = 10), LM (not otherwise specified and variants; n = 47), and LMS (n = 8), and then measured the immunoreactivity of each of these proteins. The cases were scored on the basis of staining intensity (weak, moderate, or strong) and extent (focal or diffuse) and were assigned a final score from 0 to +3. Immunostaining for p16 was statistically stronger in LMS than in LM and its subtypes (P < 0.001). Specifically, the p16 immunostaining score in LMS cases (n = 8) was at least +2, whereas the p16 immunostaining scores in all LM cases (n = 47) were either 0 (n = 35) or +1 (n = 12). The expression of the remaining antibodies did not show a statistically significant difference between the 2 groups. Furthermore, none of the markers studied showed any differences among the LM variants. The results of this study confirm the overexpression of p16 in LMS and suggest that p16 can serve as a reliable immunohistochemical marker in distinguishing uterine LMS from LM and its benign variants.

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Ewing's sarcoma/primitive neuroectodermal tumor family of tumors is part of a rare group of malignant neoplasms with small round-cell morphology. We describe a 24-year-old woman who presented with non-specific back pain. A chest radiograph and magnetic resonance imaging demonstrated an extraosseous, dumbbell-shaped mass of the posterior mediastinum with extension into the spinal canal. The patient underwent a left posterolateral thoracotomy and a T3-5 laminectomy with subsequent multi-agent chemotherapy. Histopathologic examination of the tumor demonstrated sheets of primitive small round malignant cells that showed no visible differentiation. Neoplastic cells were strongly immunoreactive for CD99 and vimentin and were negative for chromogranin, synaptophysin, CD31, CD34, calcitonin, desmin, low-molecular weight cytokeratins, wide-spectrum cytokeratins, leukocyte common antigen, S-100 protein, and thyroid transcription factor-1. The neoplasm was diagnosed as a Ewing's sarcoma/primitive neuroectodermal tumor, and cytogenetic studies confirmed a t(11;22)(q24;q12) chromosomal translocation and an associated trisomy of chromosome 2, supporting the histologic diagnosis. Extraskeletal Ewing's sarcoma/primitive neuroectodermal tumors are rare neoplasms that should be distinguished from other small round-cell tumors by morphology and ancillary laboratory techniques. Although rare, they need to be considered in the differential diagnosis of primary mediastinal tumors.