Inflammatory Myofibroblastic Tumor of Adults ≥40 Years: A Clinicopathologic Study of 34 Cases.

Inflammatory myofibroblastic tumor (IMT) is a rare mesenchymal neoplasm with a predilection for children and adolescents. Data regarding IMTs in adulthood is limited, and evidence suggests that ALK expression/rearrangement rate decreases with age. We sought to better characterize IMT in patients ≥40 years. IMT cases in this age group were retrieved and re-reviewed. Various histomorphologic data were reported, and ALK status was documented. A total of 34 tumors were identified (21 females, 13 males; 40 to 77 y; median age 54 y), and tumor size ranged from 0.7 to 10 cm (median 2.5 cm). Predominant disease sites included the lung (12), followed by the urinary bladder (9), the uterus (4), and the head and neck (4). Morphologically, tumors exhibited loose fascicles of spindled fibroblasts with inflammatory infiltrate, with the majority being myxoid (25). Mild cytologic atypia was appreciated in 12 cases, and 6 cases showed focal necrosis. ALK expression was identified in 91% of cases through immunohistochemistry (28) and/or molecularly (24). Most common ALK fusion partners, identified by next-generation sequencing, included FN1 , TIMP3 , and EML4 . Follow-up data on 28 patients (3 to 165 mo; median 42) revealed mostly indolent behavior, but one ALK -negative patient had lung metastasis, and another ALK -positive patient had a recurrence. IMTs may arise in adulthood and mostly manifest in visceral sites. Despite earlier reports, ALK is frequently expressed/rearranged in tumors in this age group.