Rebound enlargement of an ectopic cervical thymus mimicking relapse of lymphoblastic lymphoma

Abstract

A previously healthy 6-year-old boy was referred to our hospital with a tumor in the right upper mediastinum on chest X-ray. Physical examination showed no abnormal respiratory sounds, hepatosplenomegaly, or palpable tumor. On MRI, a right mediastinal tumor, 37 9 38 9 25 mm in size, with low intensity on T1-weighted images and high intensity on T2-weighted images was seen. A left-sided cervical mass, 27 9 29 9 10 mm in size, and several small lesions were seen in the liver and kidneys; all were of intensities similar to those of the mediastinal tumor (Fig. 1a, b). On histopathological examination of the biopsied samples from the mediastinal tumor, diffuse proliferation of medium-sized atypical lymphocytes with scant cytoplasm and fine chromatin was seen (Fig. 2); immunohistochemistry was positive for CD79a, CD10, and CD34, and negative for CD3, CD7, and CD20. Bone marrow aspiration showed a normocellular marrow with 5.4% lymphoblasts, suggesting involvement of lymphoma cells. These findings led to a diagnosis of precursor B-cell lymphoblastic lymphoma (LBL) stage IV. After the induction phase of chemotherapy (vincristine, cyclophosphamide, daunorubicin, L-asparaginase, and prednisolone), all of the tumors disappeared on MRI (Fig. 1c, d), and lymphoblasts were undetectable in the bone marrow. The patient subsequently received chemotherapy that included an early intensification phase, a central nervous system prophylaxis phase, a late intensification phase, and a maintenance phase. Follow-up MRI revealed no tumor at the beginning of maintenance chemotherapy, but the cervical mass re-emerged at the ninth month (Fig. 1e, f), at which time the mass was 35 9 40 9 11 mm in size and larger than at the onset of LBL. An ultrasound of the neck detected a lobulated, low-density structure with hyperechoic lines. The patient was asymptomatic. The reappearance of the enlarged cervical mass was initially suspected to be a relapse of LBL. An open biopsy was performed. On histopathological examination of the mass, normal thymic tissue with no evidence of lymphoma cell involvement was seen (Fig. 3). Flow cytometry confirmed cells positive for CD3, CD4, CD7, and CD8, and negative for CD10, CD19, and CD20. The cervical mass was diagnosed as an ectopic thymus. In accordance with the protocol, chemotherapy was continued for the following 6 months. The patient achieved complete remission, and the ectopic thymus remained unchanged in size 7 months after chemotherapy. The MRI and ultrasound can be used to diagnose ectopic thymus. MRI is the most accurate diagnostic method, showing slightly higher signal intensity than that of muscle on T1-weighted images and signal intensity close to that of fat on T2-weighted images. Ultrasound is the most convenient and non-invasive diagnostic modality. Ultrasound examination of normal thymic tissue shows multiple echogenic linear structures and foci. In the present patient, the ultrasound characteristics were potentially compatible with those of cervical ectopic thymus. However, it was difficult to differentiate a relapse of LBL from a non-neoplastic lesion as MRI revealed close similarities between the mediastinal LBL and the cervical mass. Therefore, a biopsy was performed to allow histopathological confirmation and surface marker analysis. T. Matsubayashi (&) H. Kitazawa R. Matsubayashi Department of Pediatrics, Seirei Hamamatsu General Hospital, 2-12-12 Sumiyoshi, Naka-ku, Hamamatsu, Shizuoka 430-8558, Japan e-mail: matr@sis.seirei.or.jp