'Organoid' thymoma: a well-differentiated variant with distinctive clinicopathological features.

The present paper describes the clinical, histological and immunohistochemical features of five cases of 'organoid' thymoma. The histological hallmark of this lesion is the prominent and diffuse 'organoid' pattern, defined by the presence of several areas of medullary differentiation. These areas, which are strictly reminiscent of the medullary area of the normal thymus, are scattered within a neoplastic tissue resembling the thymus cortex, the overall appearance mimicking that of normal thymus. All cases shared common clinical features; they were non-invasive or minimally invasive tumours arising in young or middle-aged female patients. Although the incidence of 'organoid' thymoma is low (5.2% in our series), our morphological, immunohistochemical and clinical data suggest that this peculiar tumour may represent a well-differentiated variant of thymoma, with low-grade aggressiveness and a distinct clinicopathological profile.