Rebound enlargement of an ectopic cervical thymus mimicking relapse of lymphoblastic lymphoma
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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 × 40 × 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 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.
The thymus gland is known for its variability in size and configuration in both health and disease. The thymus is the most sensitive organ in the body in terms of its atrophic reaction to stressful factors, such as starvation, fever, and chemotherapy. The thymus can also regrow to its original size after atrophy or, at times, rebound to a larger size. Reactive thymic hyperplasia following chemotherapy for malignant tumors may be misdiagnosed as residual tumor or disease relapse, leading to unnecessary chemotherapy. In the present patient, the ectopic thymus disappeared after intensive induction chemotherapy and re-emerged after maintenance chemotherapy was started. The re-enlargement of the thymus may have represented a rebound phenomenon due to decreased treatment intensity.
Administration of exogenous glucocorticoids leads to apoptosis of thymocytes and thymic shrinkage, which is useful for differentiation between physiological thymic enlargement and pathological lesions. However, in patients with steroid-responsive lymphoma and leukemia, prednisolone administration may result in misdiagnosis. Recently, FDG-PET has become a widespread tool for the staging of cancers, including lymphoma. However, tumors may be indistinguishable, as FDG uptake is increased in the thymus in both patients after chemotherapy and in normal children. Surgical biopsy is necessary for confirmation of the diagnosis of ectopic thymus, especially when malignancy is possible.