Thymoma (WHO type B3)
Mediastinal mass for investigation.
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MRI images demonstrate a lobulated anterior mediastinal mass with increased T1 signal compared to muscle, multiple small cystic components, and patchy contrast enhancement. There is no marked diffusion restriction within its solid components. No convincing features to suggest invasion of the adjacent structures.
Macroscopy: Labelled "Anterior mediastinal tumour. Blue short stitch left inferior horn, blue long stitch right inferior horn, black short stitch right upper horn, black long stitch left upper horn". The specimen is orientated with 4 sutures as per specimen label and inked anterior blue, posterior black and a green stripe on the right side (the superior 15 mm (fibrous tissue superior to the tumour) the green stripe is on the left, anterior black and posterior blue). The inferior portion of the specimen is composed of lobulated fatty tissue up to 7 mm thick. Towards the superior end is a firm mass 90 mm left to right, 75 mm superior to inferior and 45 mm anterior to posterior with a concave surface on the superficial aspect and an overlying segment of serosa on the left/posterior aspect and 190 x 50 mm extending down to the inferior left horn.
The tumour has a partly solid and multicystic cut surface. The solid components are pale tan and up to 15 mm and are interspersed with innumerable cystic spaces ranging from less than 1 mm to 14 mm containing tan-brown fluid and blood. The lesion abuts the soft tissue resection margin and immediately underlies the serosal surface but does not macroscopically extends through it. The tumour is present 15 mm from the superior aspect and greater than 100 mm to the inferior aspect. Surrounding the lesion are nodular areas of homogeneous and mid brown/tan firm solid tissue with less than 1 mm dark flecks.
Microscopy: The sections show hyperplastic thyroid tissue within which there is a neoplasm with a multinodular and multicystic architecture. The neoplasm is comprised of a proliferation of plump epithelioid cells with large round to ovoid nuclei, irregular nuclear membranes, central prominent round nucleoli and moderate amounts of pale staining, finely vacuolated cytoplasm. There is an accompanying infiltrate of small lymphocytes of mature appearance. Tumour is focally identified within an angiolymphatic space. Tumour extends beyond the lesional capsule into adjacent normal thymus but does not appear to extend beyond thymic tissue into perithymic fat. The lesion appears to lie clear of resection margins in the sections examined.
Immunohistochemistry shows immunoreactivity of the epithelial component with antibodies against p63, CK19 and CK5/6. There is an accompanying immature T cell infiltrate showing immunoexpression of CD3, CD5 and TdT. CD20-positive B lymphocytes are also present. CD57, TTF1, MUC-1, CD117 and EBV immunohistochemistry is negative within the epithelial component.
This case illustrates a thymoma in an young adult.