Osteoid osteoma is a benign bone tumor, which represents approximately 10% of all benign bone tumors. When localized to the carpus, osteoid osteomas are most often seen in the scaphoid and capitate. Rarely, these tumors can also be observed in the trapezium. Given the infrequency with which osteomas are located in the trapezium and often nonspecific presenting symptoms, diagnosis of this tumor can be challenging and requires a high index of suspicion.

Here we report a case of renal oncocytoma in a 68 year-old male. The diagnosis was initially made on a needle biopsy 6 years prior to the partial nephrectomy. The case is unique that in addition to the gross and microscopic features commonly seen in renal oncocytomas, both lymphovascular invasion and prominent intracytoplasmic vacuole-like spaces are also present in this tumor. Although vascular invasion is increasingly recognized as compatible with renal oncocytoma, intracytoplasmic vacuoles are a rare and unusual finding that may lead to diagnostic difficulty. The diagnosis of renal oncocytoma was confirmed after immunohistochemistry was performed to argue against succinate dehydrogenase deficient renal cell carcinoma (RCC) and chromophobe RCC. This case highlights the importance for practicing pathologists to recognize the rare co-occurrence of lymphovascular invasion and large intracytoplasmic vacuole-like spaces in renal oncocytoma. Other differential diagnoses may include emerging renal tumor entities, such as the recently-proposed eosinophilic vacuolated tumor.

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Primary mediastinal germ cell tumors (GCTs) account for a small subset of all GCTs. Teratoma is the most common GCT of the mediastinum and usually occurs in adults from 20 to 40 years of age. Malignant somatic transformation is a rare phenomenon that has been described in both primary and metastatic mediastinal GCTs. The most common types of malignant somatic transformation described in mediastinal GCTs are sarcomas and carcinomas, with very few cases of melanoma found in the literature. We report the case of a 21-year-old male patient who presented with anterior mediastinal mass that on initial biopsy and workup revealed a mediastinal mixed GCTwith teratoma and yolk sac components. Following chemotherapy, the resected mass demonstrated residual teratoma with malignant transformation into melanocytic, neuroectodermal, and undifferentiated sarcomatous elements. The patient rapidly developed metastases to the lung, liver, spleen, and spine. Biopsy of the bone marrow and liver confirmed metastatic dissemination by melanocytic transformed GCT. Molecular analysis for BRAF mutation was negative. The rapid multiorgan pattern of metastatic spread indicates a very aggressive phenotype. This case represents the second reported case of malignant melanocytic tumor as part of multilineage malignant differentiation arising from mediastinal GCT.

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