Paragangliomas are of two types, sympathetic and parasympathetic, depending on the type of paraganglion in which they arise.
The term pheochromocytoma is reserved for tumors arising in the adrenal medulla.
These tumours are usually fairly easy to diagnose.
However, several areas are the subject of debate, including the identification of malignant potential, the diagnosis of medullary hyperplasia, and the recognition of composite tumors.
Some histologic features can cause problems in differential diagnosis.
Paragangliomas may have spindle cell morphology or contain pigment, requiring distinction from mesenchymal tumors and melanoma, respectively.
Extensive degenerative change in pheochromocytomas may mimic adrenal cortical tumours.
In a typical adrenal pheochromocytoma there is a packeted arrangement of short spindle cells surrounded by vascular stroma.
Immunohistochemistry would show two cells type - sustentacular cells and chief cells.
Electron microscopy shows dense core neurosecretory granules. In these, noradrenaline gives a characteristic appearance of eccentrically-placed dense cores looking like a fried egg.
The incidence of malignancy in adrenal pheochromocytoma has been reported as 1-14% but it is probably truly nearer the 1% end of range due to selective reporting of malignant cases and the inclusion in some series of extra-adrenal cases which are widely recognized as having a higher incidence of malignancy.
Short of the documentation of metastases, prediction of malignancy is notoriously difficult.
General features more likely to be associated with malignancy include large tumor size (average 759gm versus 157gm for benign tumours).
Most malignant examples are functionally active and there is a male predominance.
Histological features more likely to be associated with malignancy include : Composition of small cells, extensive confluent necrosis, spindle cell formation (one study only), extensive local or vascular invasion, high mitotic count more than 3 per 30 high power field, coarse nodularity and the absence of hyaline globules and of cells with ganglion-like features.
In one statistical model (Linnoila et al. 1990):
- 71% of malignant paragangliomas had 2 or 3 of the features - Extra-adrenal location, coarse nodularity, confluent necrosis and absence of hyaline globules, while
- 89% of benign ones had one or none of these features. There was a greater than 95% probability that more than 70% of the tumours could be correctly classified.
Special techniques have limited value in predicting malignancy.
There is diminished expression of neuropeptides.
DNA flow cytometry suggests some evidence that DNA aneuploid tumours are more likely to be malignant.
p53 mutations have been found in both benign and malignant pheochromocytomas.
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