Rebound hyperplasia of ectopic cervical thymus following cessation of cytotoxic therapy

Thymic hyperplasia following cessation of chemotherapy is a well-documented phenomenon. It occurs mostly in children independent of the tumor type and the chemotherapy regimen used, and is difficult to differentiate from tumo r recurrence, especially in the mediastinum. Diagnostic imaging studies usu ally fail to differentiate benign thymic hyperplasia from recurrent disease and the diagnosis can only be made either by biopsy or by serial radiologi cal monitoring of the mass for increase in size. We report a 10-year-old boy who was diagnosed with B-cell acute lymphoblast ic leukemia (B-cell ALL). Four months following cessation of the chemothera py he developed a mass in the upper right cervical area. Excisional biopsy of the cervical mass revealed ectopic normal thymic tissue with no evidence of malignancy. Hyperplasia of ectopic thymus should be considered in the differential diag nosis of a cervical mass occurring in a patient following cessation of chem otherapy.