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.

The gold standard for diagnosis of central nervous system lymphomas still regards a stereotactic brain biopsy, with the risk of major complications for the patient. As tumor cells can be detected in cerebrospinal fluid (CSF), CSF analysis can be used as an alternative. In this respect, mutation analysis in CSF can be of added value to other diagnostic parameters such a cytomorphology and clonality analysis. A well-known example of targeted mutation analysis entails MYD88 p.(L265P) detection, which is present in the majority of Bing Neel syndrome and primary central nervous system lymphoma (PCNSL) patients. Unfortunately, tumor yield in CSF can be very low. Therefore, use of the highly sensitive droplet digital PCR (ddPCR) might be a suitable analysis strategy for targeted mutation detection. We analyzed 26 formalin fixed paraffin embedded (FFPE) samples (8 positive and 18 negative for MYD88 p.(L265P) mutation) by ddPCR, of which the results were compared with next generation sequencing (NGS). Subsequently, 32 CSF samples were analyzed by ddPCR. ddPCR and NGS results on FFPE material showed 100% concordance. Among the 32 CSF samples, 9 belonged to patients with lymphoplasmacytic lymphoma (LPL) and clinical suspicion of Bing Neel syndrome, and 3 belonged to patients with PCNSL. Nine of these samples tested positive for MYD88 p.(L265P) (8 LPL and 1 PCNSL). This study shows that sensitive MYD88 mutation analysis by ddPCR in CSF is highly reliable and can be applied even when DNA input is low. Therefore, ddPCR is of added value to current diagnostic parameters, especially when the available amount of DNA is limited.

IMPORTANCEThe diagnostic workup of patients suspected of having vitreoretinal lymphoma (VRL) is primarily based on vitreous fluid analysis, including the recently emerging myeloid differentiation primary response gene 88 (MYD88) mutation analysis. Aqueous humor paracentesis is a relatively less invasive and safer procedure than taking vitreous fluid specimens, and aqueous humor-based MYD88 mutation analysis would provide an additional liquid biopsy tool to diagnose and monitor patients with VRL. OBJECTIVE To investigate whether the detection of MYD88 L265P by highly sensitive droplet digital polymerase chain reaction (ddPCR) is feasible in the vitreous fluid and aqueous humor of patients with VRL. DESIGN, SETTING, AND PARTICIPANTSThis cohort study includes aqueous humor and vitreous fluid samples from patients with VRL who were treated at the University Medical Center Utrecht, in Utrecht, the Netherlands, from August 2005 to August 2017. Ocular fluids were randomized and masked before MYD88 L265P analysis, which was performed using an in-house validated ddPCR platform. Patients with uveitis were included as a comparison group. MAIN OUTCOMES AND MEASURESThe presence of MYD88 L265P mutation detected by ddPCR in AH and VF. RESULTSThe study included 96 samples from 63 individuals, including 23 patients with VRL (of whom 10 were female and 13 male, with a mean [SD] age of 72 [7.3] years) and 40 individuals with uveitis (of whom 23 were female and 17 male, with a mean [SD] age of 58 [20.9] years). In 17 of 23 patients with VRL (74%), MYD88 L265P was detected; it was not detected in any of the patients with uveitis. It was detectable in both vitreous fluid and aqueous humor samples. In the paired samples, the mutation was detected in 8 of 9 aqueous humor samples (89%) of the MYD88 L265P-positive vitreous fluid samples. In vitreous fluid, the MYD88 ddPCR test showed a sensitivity of 75% (95% CI, 50%-92%) and a positive predictive value of 100%; in aqueous humor, sensitivity was 67% (95% CI, 42%-92%), and positive predictive value was 100%. Specificity was 100% in both fluids. After treatment, the mutation was no longer detectable in any ocular fluids. CONCLUSIONS AND RELEVANCEThe high concordance between aqueous humor and vitreous fluid samples suggests that use of the easily accessible aqueous humor is nearly as informative as vitreous fluid in the identification of key somatic mutations in patients with VRL. This approach may provide an additional minimally invasive tool for accurate diagnosis, detection of recurrence, and monitoring of treatment.

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Nearly 20 years ago, the first description of a translocation involving chromosome 17 on which USP6 resides was described. Since then, not only the culprit gene but also many fusion partners, leading to transcriptional activation of USP6, have been detected. The first neoplasm known to harbor USP6 rearrangements was aneurysmal bone cyst. Since then, other entities like nodular fasciitis, myositis ossificans, fibro-osseous pseudotumor of digits, and a subgroup of fibromas of tendon sheath, probably representing tenosynovial nodular fasciitis, have been added to the list of USP6-rearranged lesions. Remarkably, all of them share clinical as well as morphological characteristics, and authors have suggested that these entities actually belong to the same spectrum. This review summarizes the current knowledge regarding USP6-rearranged lesions and further elaborates on how these neoplasms relate to one another. We propose to call these lesions UAN ( Usp6-associated neoplasm).