A 25 year old man was referred to the thoracic surgery department of the Second University of Naples (Italy) because of a left mediastinal mass, which was discovered on a routine pre-employment physical examination. The patient was completely asymptomatic. Physical examination and laboratory tests were non-contributory. A chest roentgenogram showed considerable enlargement of the mediastinum, involving particularly the left side of the chest (fig 1A). Computed tomography showed a mixed density voluminous mass with well defined edges and non-homogeneous density, involving predominantly the left part of the mediastinum (fig 1B). A thoracotomy was performed. A large, tan, lobulated, and encapsulated mass was found in the left mediastinum and was removed completely.
Figure 1 (A) An x ray shows an enlargement of the mediastinum involving particularly the left side of the chest. (B) A chest computed tomography scan shows a voluminous mass with well defined edges and non-homogeneous density, (more ...)
The excised tumour specimen was fixed in 10% buffered formalin and paraffin wax embedded. Sections of 5 μm thickness were stained with haematoxylin and eosin, haematoxylin–van Gieson, and periodic acid Schiff–haematoxylin. Other sections were immunohistochemically stained using the standard avidin–biotin peroxidase complex method with S-100 antibody (Dako Corporation, Glostrup, Denmark). Diaminobenzidine was used as final chromogen and haematoxylin as nuclear counterstain. Negative controls were obtained by omitting the primary antibodies.
Light microscopic examination revealed a tumour divided into numerous lobules by well vascularised connective tissue (fig 2A). The tumour cells were round or polygonal, multivacuolated with centrally placed nuclei resembling lipoblasts. Immunohistochemically, the cytoplasm of these cells was stained for S-100 protein (fig 2B). Univacuolated cells with eccentrically placed nuclei resembling mature lipocytes were also present. A delicate fibrocollagenous and vascular capsule was present.
Figure 2 (A) Light microscopic examination reveals a tumour divided into numerous lobules by well vascularised connective tissue. The tumour cells are round or polygonal, multivacuolated with centrally placed nuclei resembling lipoblasts (more ...)
These macroscopic and microscopic findings were suggestive of a mediastinal hibernoma. The patient made an uneventful recovery and after a five year follow up period there has been no recurrence.
Take home messages
- Hibernomas are rare, asymptomatic, benign tumours that arise mostly in adults from the remnants of fetal brown adipose tissue, and usually affect muscle and subcutaneous tissue
- We describe only the fourth case of mediastinal hibernoma to be reported in the world literature
- We suggest that mediastinal hybernomas should be included among the differential diagnosis in every growing mass of the thorax, especially if the detection of the tumour is a fortuitous coincidence in an asymptomatic patient