Which of the following is most reliable feature of malignant transformation of pheochromocytoma?

- There is no histologic feature that reliably predicts malignant pheochromocytoma. - In fact, cellular & nuclear pleomorphism including presence of giant cells & mitotic figures are often seen in benign pheochromocytomas, while cellular monotony is paradoxically associated with aggressive tumors. - Even capsular and vascular invasion may be encountered in benign lesions. - Therefore, the definitive diagnosis of malignancy in pheochromocytomas is based exclusively on presence of metastases.