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

Ref Robbins 8/e p1159; 9/e p1135 There is no histological feature that reliably predicts clinical behaviour .Infact, cellular and nuclear pleomorphism, including the presence of giant cells, and mitotic figures are often seen in benign pheochromocytoma.while cellular monotony is paradoxical associated with an aggressive behaviour. Even capsular and vascular invasion may be encountered in benign lesions . therefore the definitive diagnosis of malignancy in pheochromocytoma is based exclusively on the presence of metastasis