Pitfalls in the diagnosis of ectopic hamartomatous thymoma.

We describe four cases of ectopic hamartomatous thymoma with unusual morphological features. In cases 1 and 2 we found a dysplastic glandular component which was often arranged in a cribriform pattern bridging several glandular spaces. In case 2 we found solid adenocarcinomatous areas with infiltrative edges. Case 3 showed widespread smooth muscle differentiation within the spindle epithelial cell component, the presence of multiple clear cell glands which contained a distinct myoepithelial layer and an overall picture which vaguely resembled a sclerosing adenosis of the breast. Case 4 contained frequent foci of multinucleated giant epithelial cells within the markedly cellular spindle cell component: it also showed multiple, hypocellular, myxoid areas containing eosinophilic cords and pearls of squamous cells, and resembled squamous cell carcinoma with a sarcomatoid spindle cell component. We could not find any evidence of a thymic origin or of thymic differentiation in our four cases or in any other reported cases of ectopic hamartomatous thymoma. We believe that ectopic hamartomatous thymomas manifest salivary gland differentiation.