RDD was described as a rare, idiopathic, benign histioproliferative disease which was predominantly affects children and young adults with a slight male predominance. Extranodal involvement is documented in 43%. ¹ Intracranial involvement is rare. We report a brain MR and MRS findings of a 5-year-old boy with mass in the posterior fossa that’s diagnosed pathologically as RDD. A 5-year-old boy presented with bilateral ptosis, diplopia. MR revealed a well-defined, contrast-enhancing solid mass in the cerebellar pedincle and pons, extending into fourth ventricle. The lesion was hypo-isointense on T1, hyperintense on T2. The lesion showed diffusion restriction peripherally and hyperintense signal centrally on diffusion images and increased ADC values (Fig 1). MRS of the lesion revealed a raised Cho/NAA ratio and lactate peak (Fig 2). Pilocytic astrocytoma and glioma were considered among differential diagnoses. The patient underwent surgery. Histopathologic findings were consistent with RDD. Microscopically, the lesion consisted of an attenuated infiltrate of lymphoplasmacytic cells and scattered, often multinucleate, histiocytic cells. The lymphoplasmacytic cells were well differentiated without nuclear atypia.. The histiocytes show emperipolesis (lymphocytophagocytosis). Immunohistochemistry with positive labeling for S100 and CD68 were characteristic features. There was no staining for CD1a confirming the histiocytic cell lineage. Scattered myeloperoxidase (MPO) immunoexpressivity of neutrophils among histiocytes was not an uncommon feature of this spectacular lesion (Fig 3).

RDD predominantly affects children and young adults. Involvement of the central nervous system is rare.² 28-months-old girl with extra-axial intracranial RDD lesion, was the youngest case in the literature.¹ To our knowledge, present case is the youngest case of intraparenchymal posterior fossa location of RDD in the relevant literature.

Intracranial RDD generally presents as a solitary or multiple dural based lesions. ^{1, 3-6} Although rare, intraparencymal lesions have also been reported. ^{2, 6-8} By and large, radiological diagnosis of intraparencymal lesions are glioma, lymphoma or metastasis. ⁸ Presented case is exceptional both because of age at presentation and location of the lesion. Hitherto two cases with cerebellar RDD lesion have been reported. ^{2, 7}

Since emperipolesis is not a unique phenomenon for RDD. It is not uncommon in both normal and leukaemic processes. The cells also have phenotypic features of the macrophages; specifically they have CD68+ and alpha-1-antitrypsin+ immunoexpressivity. Moreover the cells show some of the phenotypic characteristics of the dendritic cell series, i.e., S100, cathepsin, and CD1a. This noteworthy disease can be differentiated from nervous system histiocytoses X in which eosinophils are conspicuous; histiocytes lacking emperipolesis and the lesion contain Birbeck’s granules on electron microscopy. The other uncommon nervous system lesions those should be taken into consideration are lymphoplasmacyte-rich meningioma, lymphoproliferative disorders and plasma cell granuloma.

On MR scans, the lesion of intracranial RDD is usually iso-hypointense on T1 and T2 images. ^{1, 5, 6, 8} In our case, the lesion was iso-hypointense on T1. Udono et al. suggested that hypointense signal on T2-weighted images might reflect the presence of free radicals produced by macrophages. ⁴ However, in our report the lesion was hyperintense on T2. Fukushima et al. reported frontal lobe lesion which was hyperintense on T2, as in our case. ⁸ In RDD, homogenous enhancement is typical. ^{1, 5, 6, 8} Some reports revealed that the lesion in the other extranodal sites like spine, orbit, paranasal sinus, tonsils generally showed isointense signal on T1, T2 and homogenous enhancement. ¹ Fukushima et al. reported frontal RDD lesion which showed a mild heterogeneous signal on diffusion images and increased signal on ADC. ⁸ Raslan et al. showed intracranial dural-based lesion is isointense on ADC, with someT2 shine through effect on diffusion images. ¹ However, in our case, the lesion showed diffusion restriction peripherally and hyperintense signal centrally on diffusion images and increased signal on ADC. Probably the number and the dominancy of the lymphoplasmocytic cells and cytoplasm reflect the T1 and T2-weighted image intensities and diffusion properties on MR. Symms et al. reported that the intracranial RDD lesion exhibits elevated lipid and NAA peaks on MRS. ⁶ In our case, MRS of the lesion revealed a raised Cho/ NAA ratio reflecting cellulary proliferation.

This case is the youngest case of intracerebellar location of RDD in the pertinent literature. In children, MR and MRS findings of RDD may mimic infratentorial masses like pilocytic astrocytoma, ependymoma and glioma. Thus, a definite diagnosis relies on the histological features and immunohistochemical characterization of the lesion. Certain features of brain MR, MRS and diffusion may reflect some histopathologic properties of this rare entity.