Thymic neuroblastoma in adults: report of three cases with special emphasis on its association with the syndrome of inappropriate secretion of antidiuretic hormone.

We report three cases of neuroblastoma arising within the thymus of elderly patients. All tumors consisted of primitive neuroblasts showing focal gangliocytic differentiation within nests of neuropil. All stained for neuroendocrine markers but were negative for cytokeratins and for the MIC2 gene product. One tumor was associated with the syndrome of inappropriate secretion of antidiuretic hormone, an endocrinopathy we found in three of five case reports of thymic neuroblastoma in adults. Immunohistochemical stains confirmed production of antidiuretic hormone by this tumor. One patient died of progressive disease, one patient is disease free at 18 months, and the other patient died of unrelated causes, a spectrum that reflects the variable clinical behavior others have reported. The possible histogenesis of these purely neural tumors includes malignant transformation of a mediastinal teratoma, aberrantly located sympathetic ganglia, neuroectodermal cells native to the normal thymus, and precursors of thymic epithelial cells that have differentiated along neural lines.