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25 July 2009 Dear Editor, A LYTIC RIB LESION IN A NEONATE Sinus histiocytosis with massive lymphadenopathy, also known as Rosai-Dorfman disease (RDD), is a rare histiocytic proliferative disorder that is characterised by massive lymphadenopathy as a result of accumulation of lymphocytes and histiocytes distending the lymph node sinuses.1 This disorder generally presents within the first two decades of life and has a self-limiting course. Because of its rarity and mostly subclinical manifestations during the infantile and neonatal period,2 exclusively extranodal involvements without cardinal lymphadenopathy would create a diagnostic challenge to paediatricians.3 Here we describe the first case to our knowledge of a neonate with congenital RDD presenting with solitary osseous involvement of the rib without association with characteristic lymphadenopathy or other systemic manifestations. A full-term Taiwanese male neonate was born vaginally at 39 weeks of gestation with a birth weight of 3038 g (50 percentile), head circumference 34.5 cm (50–75 percentile) to non-consanguineous parents. He was transferred to our sick baby room soon after birth because of tachypnoea associated with a history of meconium-stained liquor. The antenatal fetal anomaly scan at 36–38 weeks of gestation was normal. Blood cultures were taken and intravenous antibiotics were commenced. A postnatal chest X-ray revealed no evidence of meconium aspiration. An eccentrically lytic defect at his right 7th rib was found coincidentally (Fig. 1a). His physical examination was reassuring. He had a normal full-blood picture and blood film and the erythrocyte sedimentation rate was within normal limits. The rib lesion was imaged further with a computerised tomography scan of the chest (Fig. 1b). Excision biopsy of the lesion was undertaken on day 6 of life. Histopathological studies of the lesion revealed multiple histiocytes containing a few intact lymphocytes. This is a known feature of emperipolesis. Further immunohistochemical studies on those histiocytes were positive for S-100 protein and CD68, but negative for CD1a. Those pathological features were consistent with the diagnosis of RDD. Postoperative Gallium whole-body scan revealed no other regions suggestive of disease. The baby was discharged uneventfully at 14 days of age. Repeat radiographs during regular clinic follow-up revealed only the healed curettage defect of the right rib without evidence of recurrence. (a) chest X-ray on day-1-old displayed a scalloping osteolytic lesion at upper margin of the right 7th rib (arrowheads). (b) Three-dimension reconstructed CT revealed an eccentric cortical defect at the same location. RDD, first described in 1969 by Rosai and Dorfman,1 is a rare histocytic disorder that tends to occur in children and young adults with a slight predilection for males and individuals of African descent. The aetiology of RDD remains unclear, yet a disorder as a result of an inappropriate immune response to an unknown infectious agent has been suggested.4 Massive painless cervical lymphadenopathy accompanied by fever is the hallmark of this histopathological entity, whereas an isolated extranodal involvement is found in about 23% of RDD patients, mainly of the head and neck.5 Conversely, an exclusively osseous involvement without associated lymphadenopathy only accounts for 2% of RDD.3, 4 Of these, the skull and ribs are the mostly affected regions.4, 6 On radiography, lesions are typically lytic with primarily medullary involvement and fuzzy non-sclerotic margins.5, 6 The differential diagnoses of lytic bone lesions in newborns include infantile myofibromatosis, congenital syphilis, neonatal osteomyelitis, neonatal neuroblastoma, congenital pseudoarthrosis and infantile systemic hyalinosis. Although congenital RDD is known to occur rarely in neonates,2 a primary osteolytic lesion of congenital RDD has never been reported previously. As a rare entity, RDD might be often underdiagnosed especially among neonates because of a low index of suspicion by paediatricians or radiologists..5 From our experience of this unique presentation of RDD, we suggest that a congenital costal defect detected in a neonatal roentgenogram should alert paediatricians to the possibility of hematopoietic disorders, such as RDD. Although benign course is expected in most case, fatal outcomes have been documented in some cases.3 We suggest early diagnosis through recognition of the characteristic radiological and histopathological features is essential.