Chemical-shift MRI of rebound thymic hyperplasia with unusual appearance and intense 18F-FDG uptake in adulthood: report of two cases

The paediatric thymus: recognising normal and ectopic thymic tissue

The appearance of the paediatric thymus changes as the normal process of thymic involution occurs. Thymic tissue may be orthotopic within the anterior mediastinum or ectopically located along the course of its embryological development. The variable appearance of orthotopic and ectopic thymic tissue in children on imaging studies may lead to misinterpretation of the normal thymus as pathology. Recognition of normal thymic tissue can mitigate unnecessary further diagnostic testing and patient anxiety. In this review, we discuss the embryological development and anatomical variants of normal thymus, and demonstrate the multimodality imaging features of the normal thymus in children, including positron-emission tomography, and diffusion-weighted imaging and in- and opposed-phase imaging on magnetic resonance imaging. We demonstrate the normal thymus mimicking pathological processes and discuss features that distinguish normal thymus, including thymic rebound hyperplasia, from pathology.

Thymic Granuloma Mimicking Malignancy on 18F-FDG PET/CT

We present a case with an unexpected nodule in the thymus showing increased F-FDG uptake on PET/CT. The nodule was surgically removed. Histopathologic examination revealed granuloma in the thymus. This case indicates physicians and radiologists must be aware that although thymic granuloma is rare, it should be included in the differential diagnosis of the thymic lesions with F-FDG accumulation along with neoplastic and nonneoplastic conditions.

[True thymic hyperplasia : Differential diagnosis of thymic mass lesions in neonates and children]

Reactive and neoplastic thymic pathologies are the main considerations in the case of masses in the anterior and middle part of the mediastinum, while neurogenic tumors are predominant in the posterior mediastinum (which are not dealt with here). In neonates and infants, the commonest pathologies in the anterior mediastinum comprise germ cell tumors (mainly teratomas), congenital thymic cysts and true thymic hyperplasia (TTH). In toddlers, teratomas, yolk sac tumors and cysts predominate. In children over 5 years of age, lymphomas are the commonest mass lesions whereas thymomas and thymic carcinomas are rare. In addition, inflammation-linked hyperplasia in myasthenia gravis and rebound thymic hyperplasia after chemotherapy must be considered. Although rare at all ages, sarcomas must be considered in the differential diagnosis from birth onwards and throughout adolescence. Based on the report of a rare case of recurrent TTH, the differential diagnosis of this benign but potentially life-threatening condition is discussed.

Rebound Thymic Hyperplasia after Chemotherapy in Children with Lymphoma

BACKGROUND

Development of mediastinal masses after completion of chemotherapy in pediatric patients with malignant lymphoma is worrisome and challenging to clinicians.

METHODS

We performed a retrospective review of 67 patients with lymphoma treated at our hospital from January 1, 2001 to June 1, 2013. Patients who received at least two chest computed tomography (CT) examinations after complete remission (CR) was achieved were further analyzed. Gallium-67 scans and positron emission tomography (PET) were recorded and compared between these patients.

RESULTS

Sixty-two of 67 patients reached CR, of whom 31 (22 male, 9 female) were patients that received at least two chest CT examinations after CR. Rebound thymic hyperplasia (RTH) was diagnosed in 21/31 patients (67.7%), including 14/23 (60.9%) and seven out of eight (87.5%) with non-Hodgkin's lymphoma and Hodgkin's lymphoma, respectively. Ages ranged from 3 years to 18 years (median 10 years). Increased radioactivity uptake of the anterior mediastinum in gallium scans was found in nine out of 20 patients (45%) with thymic rebound. PET was performed in six out of 21 patients. Increased fluorodeoxyglucose (FDG)-avid uptake in the anterior mediastinum was observed in four of six patients (66.7%) by PET. One patient received thymectomy. No patients with RTH had lymphoma relapse within the median follow-up period (5 years). Relapse was statistically significantly different (p = 0.001) between patients with and without RTH.

CONCLUSION

RTH developed in 67.7% of pediatric patients with lymphoma in CR after chemotherapy. The association of RTH development and lowered relapse rates has yet to be determined. Awareness of this phenomenon is important in the prevention of unnecessary surgical intervention or chemotherapy.