Answer: Lymphoblastic lymphoma

Histology: The mass is composed of a relatively diffuse proliferation of small lymphoid cells. The nuclei have a rather blastic appearance and brisk mitotic rate. There are occasional histiocytes mixed within the tumor. There are no large or bizarre atypical cells with prominent nucleoli, nor does there appear to be any evidence of epithelial differentiation within the tumor.

Discussion: Based on the relatively small size of the cells, the tumor appears to be a lymphoid proliferation. The brisk mitotic rate is typical of an aggressive tumor. In general, the diffuse aggressive tumors which occur in the mediastinum would include a lymphoblastic lymphoma or a large cell lymphoma. The small size of the cells as well the lack of prominent nucleoli would rule out the possibility of a mediastinal large B-cell lymphoma. In addition, immunophenotyping would be helpful in distinguishing this tumor from other lymphomas. This tumor is a T-cell tumor that reacts with CD3 and other pan T-cell markers. In addition, it is also a TDT positive. The mediastinal large cell lymphoma usually demonstrates a B-phenotype which would be unusual for lymphoblastic lymphomas although there are some cases of pre B-cell lymphoblastic lymphomas. This tumor is easily distinguished from Hodgkin’s Disease in that it is composed of immature cells whereas the background cells in Hodgkin’s are mature T-cells and do not demonstrate the brick mitotic rate seen in this tumor. Lastly, this lesion needs to be distinguished from mediastinal thymomas. Although the background cells in thymomas are predominantly T-lymphocytes, they are usually more mature in appearance and do not demonstrate as brisk a mitotic rate as seen in lymphoblastic lymphoma. In addition, the hallmark of a thymoma is the presence of the epithelial cells which are the neoplastic component of a thymoma. If these are not readily appreciated on H&E, they can usually be appreciated using cytokeratin stains.