ALK-positive histiocytosis is a rare subtype of histiocytic neoplasm first described in 2008 in three infants with multisystemic disease involving the liver and hematopoietic system. This entity has subsequently been documented in case reports and series to occupy a wider clinicopathologic spectrum with recurrent KIF5B-ALK fusions. The full clinicopathologic and molecular spectra of ALK-positive histiocytosis remain, however, poorly characterized. Here, we describe the largest study of ALK-positive histiocytosis to date, with detailed clinicopathologic data of 39 cases, including 37 cases with confirmed ALK rearrangements. The clinical spectrum comprised distinct clinical phenotypic groups: infants with multisystemic disease with liver and hematopoietic involvement, as originally described (Group 1A: 6/39), other patients with multisystemic disease (Group 1B: 10/39), and patients with single-system disease (Group 2: 23/39). Nineteen patients of the entire cohort (49%) had neurologic involvement (seven and twelve from Groups 1B and 2, respectively). Histology included classic xanthogranuloma features in almost one third of cases, whereas the majority displayed a more densely cellular, monomorphic appearance without lipidized histiocytes but sometimes more spindled or epithelioid morphology. Neoplastic histiocytes were positive for macrophage markers and often conferred strong expression of phosphorylated-ERK, confirming MAPK pathway activation. KIF5B-ALK fusions were detected in 27 patients, while CLTC-ALK, TPM3-ALK, TFG-ALK, EML4-ALK and DCTN1-ALK fusions were identified in single cases. Robust and durable responses were observed in 11/11 patients treated with ALK inhibition, ten with neurologic involvement. This study presents the existing clinicopathologic and molecular landscape of ALK-positive histiocytosis, and provides guidance for the clinical management of this emerging histiocytic entity.

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Adult spinal cord ependymomas are typically low grade and have a relatively favorable clinical course following gross total resection. We report 4 cases of anaplastic spinal cord ependymoma with MYCN amplification, an exceptionally rare finding. All cases occurred in the spinal cord of adolescent and young adult women and had morphological and immunohistochemical features of anaplastic ependymomas (World Health Organization grade III). Chromosomal microarray analysis demonstrated amplification of 2p24 (including MYCN) in all cases. One patient died 6 months after surgery. Another patient recently had removal of metastatic nodules in the thoracic region, following gross total resection and adjuvant radiation therapy of a lumbar ependymoma 1 year previously. One patient responded well after chemotherapy but died after multiple relapses 82 months after diagnosis. We found MYCN amplification reported in 2 other ependymomas, both anaplastic and arising in the spinal cord of adult females (Brain Pathol 2001;11:133–43). One patient had multiple recurrences in the spinal cord and an intracranial metastasis. Although MYCN amplification is rare in ependymomas, the current and previously reported cases suggest that this is associated with higher-grade histology, spinal location, and often unfavorable prognosis. The clinical significance and therapeutic implications of MYCN amplification in ependymomas require further evaluation.

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