Tumor-like lesions of the mediastinum consisting exclusively of mature pancreatic tissue are rare. Most authors regard these lesions as developmental abnormalities. We report the case of a 17-year-old male who presented with progressive dyspnea due to a large mediastinal tumor with accompanying pericardial effusion. Percutaneous core needle biopsies showed differentiated pancreatic tissue with chronic inflammation and cystic transformation. On this basis, heterotopic pancreatic tissue of the mediastinum, a lesion occasionally reported in the literature, was suspected and the lesion was excised. The histopathological workup of the surgically excised lesion, in the first place, supported the primary diagnosis. However, after extensive sampling of the lesion, the diagnosis had to be changed to mature teratoma with subtotal unidirectional pancreatic differentiation. We hypothesize that at least some of those cases reported in the literature as "heterotopic pancreatic tissue of the mediastinum" may be in fact unidirectionally differentiated teratomas and should therefore be regarded and treated as true neoplasms rather than developmental abnormalities.
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