RDD was originally described as SHML.This rare benign disorder is characterized by the proliferation of non-Langerhans histiocytes that exhibit emperipolesis, S100+, CD68+, CD30+, CD1a- and Langerin-. Most cases of RDD occur in children and adolescents [9].The etiology of the disease remains unknown. Some studies have suggested that immunological dysregulation or infections with human herpes virus (HHV)-6, HHV-8, parvovirus B19, and Klebsiella are suspected causes. However, the evidence remains inconclusive [10].
The typical symptoms include painless enlarged bilateral cervical lymph nodes with fever, neutrophil elevation, elevated erythrocyte sedimentation rate, hyperglobulinemia, and other clinical symptoms [10]. However, our patient did not exhibit any symptoms, and the lesion was detected on a chest CT scan performed during a routine health checkup. This feature makes the diagnosis more challenging.
Cervical lymphadenopathy is a hallmark of RDD in 95% of the patients. Other nodal groups may also be involved, such as the inguinal (44%) and axillary (38%) lymph nodes [11]. However, RDD, is not limited to lymph nodes and extranodal organ involvement including that of the skin, bone, central nervous system, and orbital tissues [6], may be observed in approximately 23–40% of cases, with a particular preference for various sites in the head and neck region [3]. Extranodal involvement may be an initial or an only presentation of this disease. RDD is rarely restricted to the thymus. Currently, few cases of RDD of the thymus have been reported, and its pathogenesis remains unclear [12–13]. Our patient was initially misdiagnosed with thymoma because a routine chest CT revealed a solitary anterior mediastinal mass in the thymus with multiple patchy fat depositions in the lesion. This case enriches the clinical data of the disease and will help improve our understanding of the disease.
Jorge et al. reported a case of RDD that presented as a solitary anterior mediastinal mass with calcifications [8]. The patient had a suspected history of pulmonary tuberculosis.Therefore, they could speculate that calcification may have been present in the adenopathy before the development of the RDD. However, in our case, we identified numerous patchy fat depositions in the thymic RDD. To our knowledge, this case report is the first to describe fat in thymic RDD, either in the mediastinal or other nodal groups. Our patient was a 50-year-old man whose normal thymic tissue had been substantially replaced by fat. We hypothesized that the fatty tissue within the lesion was degenerative to the thymus rather than emerging from the lesion itself, suggesting gradual progression of the condition. Pathologic examination confirmed the diffuse proliferation of large histiocytic cells forming a discrete mass surrounded by thymic tissue, displaying fatty involution.
Due to the presence of fat in the anterior mediastinal mass, the differential diagnoses in our case included thymoma, lymphoma, teratoma, and Castleman disease. One should keep in mind that although RDD is a rare disorder, it is an important entity in that it may mimic other diseases.
RDD is thought to be a benign, self-limiting disease with spontaneous remission reported in up to 50% of cases [14].The same finding was observed in our case, in which the lesion slightly reduced in size after half a month. A percentage of patients still experience prolonged disease or multiple organ invasion[15]. Currently, surgery is the primary treatment option for RDD, where total excision of the lesion is performed to promote complete remission. Refractory or nonresectable extranodal disease has historically been treated using various therapeutic approaches, such as corticosteroids, chemotherapy, low-dose interferon, antibiotics, and radiotherapy. However, the response remains highly variable, and no established standard of care for treatment is present [16].
In conclusion, the extranodal presentation of RDD poses a diagnostic challenge. Histopathological examination and imaging play important roles in the diagnosis and follow-up of RDD, which presents as nonspecific lesions. To the best of our knowledge, this case report is the first to describe the presence of fat in lymphadenopathy in RDD, either in the mediastinal or other nodal groups. Our report contributes to the clinical and imaging data on this disease, to increase awareness among clinicians, as early diagnosis of extranodal RDD is imperative for the management of the disease and improvement of prognosis.