A 28 year old lady with a pituitary mass.

We present a case of an atypical pituitary adenoma with prominent lymphocyte infiltration and germinal center formation. Histologically, the tumor revealed clusters of large pleomorphic cells with vesicular nuclei, prominent nucleoli, smudgy chromatin, and abundant eosinophilic cytoplasm. There was heavy lymphocyte infiltration and prominent germinal center formation. Mitoses were rare, and the Ki-67 index was 15%. The lymphocytes were composed of both CD3-positive T lymphocytes and CD20-positive B-cells. The diagnosis was atypical pituitary adenoma. Germinal centers have not previously been reported in these tumors. It is important to correlate histology and immunohistochemistry to differentiate this adenoma from other pituitary lesions with prominent lymphocyte infiltration, such as germinoma and lymphocytic hypophysitis.