[Difficult diagnosis of a rapidly developing post-traumatic tumor of the thymus region. Encysted hematoma or intracystic hemorrhage?].

We report a case of rapidly developing (subacute) post-traumatic mediastinal haematoma which initially suggested a malignant tumour of the thymus region. The lesion, discovered on the basis of non-specific signal symptoms 2 months after a mild chest injury, consisted of a voluminous tumour of the thymus region associated with pericardial effusion. At angiography, the mass proved to be extravascular. CT scans of the thorax showed that it surrounded, but did not compress, the mediastinal vessels. Surgery revealed an organized encysted haematoma of the thymus region that had ruptured into the pericardial cavity containing a blood collection. The possibility of an intracystic haemorrhage in a cystic tumour preexisting to the trauma is discussed, but a more likely diagnosis would be chronic haematoma, especially since a similar case independent of any injury has already been published.