Micronodular PEComas of the appendix

Perivascular epithelioid cell tumours (PEComas) of the appendix have been reported very rarely. In this study, we describe three cases of a distinctive micronodular proliferation in the appendix consistent with a variant of PEComa. Although known as 'granular degeneration of smooth muscle' in prior reports, we reappraise its clinicopathological, immunohistochemical and ultrastructural features which support a change in classification.

Patients were two females (aged 33 and 41 years) and one male (aged 41). None had a history of tuberous sclerosis. Histologically, each case demonstrated a multifocal nodular proliferation towards the distal tip of the appendix, composed of epithelioid cells with abundant granular eosinophilic to clear cytoplasm. By immunohistochemistry, the lesional cells were positive for muscle markers [smooth muscle actin (SMA) and desmin], melanocytic markers (HMB45, melan A), cathepsin K and the lysosomal marker NKI-C3 in each case. MITF was positive in two of three cases. None expressed S100 protein. Electron microscopy in one case revealed striated electron-dense structures consistent with pre-melanosomes. Follow-up, available in one case, showed no recurrence at 5 years.

We propose the term 'micronodular PEComa' for this appendiceal lesion to reflect more accurately its histological and immunohistochemical characteristics, which include consistent positivity for both muscle and melanocytic markers. Micronodular PEComa seems to follow an indolent course, consistent with its uniformly low-grade histological features, and appears to be unassociated with tuberous sclerosis.