MASSIVE THYMIC HYPERPLASIA - CHARACTERIZATION OF A RARE MEDIASTINAL MASS

Thymic hyperplasia is a rare cause of an anterior mediastinal mass in children. True thymic hyperplasia is characterized by massive thymic h ypertrophy with retention of normal thymic architecture, and must be d istinguished from more commonly seen tumors of the anterior mediastinu m. Previous reports of thymic hyperplasia primarily have been descript ive, with minimal analysis of the cellular characteristics of the tumo r. To better describe the cellular characteristics of thymic hyperplas ia, the authors report on a 10-year-old boy found to have an asymptoma tic left paracardiac mass during cardiac evaluation. The mass enlarged rapidly during the following 2 weeks and filled the entire left side of the chest. An open biopsy specimen showed normal thymic architectur e. Because of the size and rapid growth of the mass, the authors decid ed to resect it. Cellular analysis of the mass did not show difference s from normal thymic elements. There was conserved thymic architecture . The expression of cell surface markers and the proliferative respons e of thymocytes to cytokines appeared to be normal. Similarly, a long term cell culture of the thymocytes did not result in clonal prolifera tion. Immunohistologic staining of stromal cell's showed no striking d ifferences from normal thymic elements. Moreover, immunologic analysis of the patient by standard hematological parameters, lymphocyte subse ts, quantitative immunoglobins, and immunoelectrophoresis Showed no ab normalities except for lymphocytosis, which resolved after tumor resec tion. The authors conclude that thymic hyperplasia occurs in immunolog ically normal children and may exhibit (1) rapid growth with minimal a ssociated symptoms, (2) normal thymic architecture, (3) normal thymocy te and stromal growth characteristics in vitro, and (4) a normal thymo cyte response to cytokine stimulation. These data confirm, at a cellul ar level, the clinical impression that thymic hyperplasia represents b enign hypertrophy of the thymus rather than a thymic neoplasm.