<i>ALK‐</i> Rearranged Mesenchymal Neoplasm With Hyaline‐Vascular Castleman Disease‐Like Features: A Case Report

Recent reports have revealed various morphological features of ALK-rearranged mesenchymal neoplasms but their characteristics remain to be elucidated. Here, we report a case of ALK-rearranged mesenchymal neoplasm with novel morphological features. A 32-year-old man had an intra-abdominal mass measuring 75 mm in diameter. Histologically, the tumor consisted of many lymphoid follicles with wide fibrosis or hyalinization. The follicles were characterized by regressed germinal centers and expanded mantle zones. Hyalinized blood vessels were distributed in the follicular and interfollicular areas. These histological findings resembled those of hyaline-vascular type unicentric Castleman disease (HV-UCD). However, the fibrous areas exhibited histological findings similar to inflammatory myofibroblastic tumor (IMT), which is characterized by the proliferation of spindle or stellate-shaped fibroblastic cells accompanied by lymphoid, eosinophil, and plasma cell infiltration. Immunohistochemically, ALK-positive spindle cells were observed in the follicular, interfollicular, and fibrous areas. Molecular analysis revealed fusion between ATIC (exon 7) and ALK (exon 20). Immunofluorescence staining suggested that ALK-positive tumor cells in follicular areas were derived from CD23-positive follicular dendritic cells, and those in interfollicular areas were derived from α-SMA-positive fibroblastic reticular cells or myofibroblasts. Therefore, we report an extremely rare case of ALK-rearranged mesenchymal neoplasm with features of both HV-UCD and IMT.