Pigmented pulmonary carcinoid tumor. An immunohistochemical and ultrastructural study.

We evaluated three cases of pigmented pulmonary carcinoid tumors that were retrieved from the files of the Armed Forces Institute of Pathology, Washington, DC. Clinical follow-up showed no indication of tumor recurrence or metastases, nor was there evidence of malignant melanoma. All three cases exhibited histologic features of typical carcinoid tumor; there were focal oncocytic changes in two cases. Finely dispersed, brown pigment, believed to be melanin, was distributed in two different patterns: in sustentacular cells (case 1) or within the tumor cells (cases 2 and 3). Fontana-Masson stain was positive in areas of this pigment in all cases. The tumor cells showed immunoreactivity for chromogranin, synaptophysin, keratin (AE1/AE3 and CAM-5.2), and S100 protein in all cases. Focal staining for vimentin and corticotropin was seen within neoplastic cells in two cases. The pigmented sustentacular cells in case 1 showed focal immunoreactivity for S100 protein and HMB-45. Ultrastructural studies of paraffin-embedded tissues were performed in two cases. They showed well-developed melanosomes in the pigmented sustentacular cells in case 1. In both cases, cytoplasmic neurosecretory-type granules were identified in neoplastic cells. These findings demonstrate that pigmented pulmonary carcinoid tumor has an immunohistochemical profile similar to that of typical pulmonary carcinoid tumor. In some instances, pigmented pulmonary carcinoid tumors may show ultrastructural evidence of melanocytic and neuroendocrine differentiation. These immunohistologic and ultrastructural findings distinguish pigmented pulmonary carcinoid tumor from malignant melanoma and support the concept of "multidirectional cellular differentiation."