Watchful waiting for some children with a mediastinal mass: the potential role for ¹⁸F-fluorodeoxyglucose positron emission tomography: a case report and review of the literature

Abstract

Background: Benign hyperplastic thymus is a rare but important differential diagnosis of anterior mediastinal lesions. Histological and radiological criteria are used to distinguish this benign condition from other malignant diseases but have their limitations, and biopsy of mediastinal masses can be risky. We report for the first time the diagnostic value of fluorodeoxyglucose 18F positron emission tomography for patients with incidentally identified anterior mediastinal masses to avoid biopsy in some cases.

Case presentation: A 2 year old girl presented with new onset of emesis and constipation leading to the incidental discovery of an anterior mediastinal mass on radiograph. Chest computed tomography revealed cystic components within the mass concerning for a malignancy. Biopsy of the lesion and bone marrow aspiration and biopsy were negative but there was concern that the mediastinal biopsy may have missed the malignant component of the lesion. Hence, a positron emission tomography scan was obtained that showed mild homogeneous fluorodeoxyglucose 18F avidity within the mass similar to that of normal thymus. The diagnosis of benign hyperplastic thymus was made.

Conclusion: The differential diagnosis of an incidentally found anterior mediastinal mass includes malignancy, but benign lesions such as benign hyperplastic thymus must also be considered, particularly when the complete blood count and biochemical profile are normal. Fluorodeoxyglucose 18F positron emission tomography can help guide a clinician's decision for further interventions and treatment.

Figure 1

Diagnostic imaging studies of the presented patient. (A) Anteroposterior chest radiograph reveals a large predominately right-sided mass. (B) PET image shows that activity within the mass (black arrow) is indistinguishable from that in the normal thymus (white arrow). (C) Fused transaxial ¹⁸ F-FDG PET/CT shows only low-intensity, homogeneous ¹⁸ F-FDG avidity within the mass, with photopenia corresponding to the cystic/necrotic area (white arrow).

Figure 2

Flow chart for the workup of a mediastinal mass in an otherwise asymptomatic child.