Phaeochromocytomas may arise as one or more discrete neoplasms within the adrenal medulla and particularly in MEN syndromes there may be surrounding diffuse or nodular hyperplasia.
The tumour mass varies widely but on average is approximately 100 gms. Large tumours may be encapsulated by connective tissue, and may contain areas of haemorrhage, necrosis or cyst formation. The cut surface may be grey or brown in colour, and the tumour due to its stored catecholamines reacts histochemically to dichromate producing a brown-black colouring.
Tumour cells resemble those of the normal adrenal medulla by being arranged in nests. There may be mitotic figures or nuclear pleomorphism, and tumour cells may be found in capillaries or sinusoids. None of these features however indicate that the tumour is one of the ten percent of phaeochromocytomas that are malignant. Malignancy may only be proved by the finding of distal metastases.
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