AJNR Case Collection

Discussion
The imaging findings in our patient are nonspecific but those of an aggressive and presumably malignant mass lesion due to the presence of skull base bony erosion and intermediate T2 signal. Considering the patient's age, tumor location, and imaging findings, the differential diagnosis includes non-Hodgkin lymphoma (NHL), rhabdomyosarcoma (RMS), and nasopharyngeal carcinoma (NPC), which are, respectively, the most common tumors affecting the nasopharynx in childhood. While NPC is seen between the ages of 10 and 19 in early adolescence, RMS has a bimodal distribution in children, occurring in those aged 2–4 years and 12–16 years but mostly in those younger than 6 years. Lymphoma may be seen in all age groups in children, but NHL primarily affects children aged between 2 and 12 years. These nasopharyngeal tumors may be asymptomatic or with common benign disease symptoms for months, and, consequently, result in a delayed diagnosis. NPC is endemic in Southeast Asia and closely linked to Epstein-Barr virus (EBV) infection. Also, although the pathogenesis of extranodal NHL is unclear, immunocompromised children exposed to oncogenic infections such as EBV are at particularly high risk for this disease. MR signal characteristics and contrast enhancement, with aggressive features such as adjacent bone destruction/erosion, intermediate T2 signal, and skull base invasion, can be similar for all 3. They may also appear as hypermetabolic masses with increased FDG uptake on PET-CT. They may be indistinguishable at conventional imaging alone, so the final diagnosis requires confirmation by biopsy. Additionally, NHL and NPC often present with cervical lymph node metastases, which are less common in RMS. In this patient’s MRI, there was no lymphadenopathy, and in situ hybridization for EBV performed on biopsies of nasopharynx and adenoids was negative. RMS may be considered first in the differential diagnosis as the lesion was EBV-negative, and there was no lymphadenopathy, but the patient's age is not typical for RMS. Then, the patient underwent a nasal endoscopy with a biopsy of the nasopharynx, and the pathology was consistent with Rosai-Dorfman disease (RDD), a.k.a. sinus histiocytosis with massive lymphadenopathy.

RDD is a rare and non-neoplastic disease that is classified as a non-Langerhans cell histiocytosis. It can be seen in any age group, mostly children and young adults. It can affect many organ systems, including nodal (57%) or extranodal (43%) sites. Extranodal involvement can occur anywhere, including the skull base. The clinical presentation and radiologic imaging findings of extranodal RDD of the skull base overlap significantly with other neoplastic processes of this location. On MRI, the lesions are isointense to brain parenchyma on T1-weighted and T2-weighted images and demonstrate typically uniform contrast enhancement. The literature has reported that they may have hypointense areas on T2-weighted images, which are postulated to reflect the release of free radicals by inflammatory macrophages. However, this is not specific to this disease and can be seen in other inflammatory diseases and neoplasms. Because of all these overlaps, the definitive diagnosis of RDD is based on pathologic examinations. PET-CT, in particular, is very helpful in evaluating the extent of the disease, and when there is involvement of multiple subsites, it can help increase the specificity of diagnosis.

Teaching Points:

• The most common tumors affecting the nasopharynx in childhood are NHL, RMS, and NPC, respectively.
• There is a need for a biopsy performed on nasopharyngeal masses in children because the clinical presentation and radiologic imaging findings of extranodal RDD of the skull base overlap significantly with other neoplastic processes of this location.
• When an aggressive nasopharyngeal mass is present in children, although the diagnosis is challenging to make prospectively, including RDD in the differential when appropriate may help guide workup.