Histologically, pheochromocytoma is composed of large pleomorphic chromaffin cells. Electron microscopy reveals electron-dense neurosecretory granules. Approximately 10% of pheochromocytomas are considered to be malignant. No histologic features exist that distinguish benign from malignant tumors. Microscopic evidence for local invasion of tissue or blood vessels, however, suggests malignancy.[1] Criteria based on tumor size, mitotic index, and DNA ploidy have been reported to be helpful in some series, though they are not always reliable predictors of biologic behavior.[2,3,4] Because the distinction between benign and malignant tumors cannot be made with certainty, careful surveillance is needed for a prolonged period after the initial surgical resection.

References:

1. Manger WM, Gifford RW: Pheochromocytoma. New York: Springer-Verlag, 1977.
2. Amberson JB, Vaughan ED Jr, Gray GF, et al.: Flow cytometric determination of nuclear DNA content in benign adrenal pheochromocytomas. Urology 30 (2): 102-4, 1987.
3. Hosaka Y, Rainwater LM, Grant CS, et al.: Pheochromocytoma: nuclear deoxyribonucleic acid patterns studied by flow cytometry. Surgery 100 (6): 1003-10, 1986.
4. Capella C, Riva C, Cornaggia M, et al.: Histopathology, cytology and cytochemistry of pheochromocytomas and paragangliomas including chemodectomas. Pathol Res Pract 183 (2): 176-87, 1988.