Uterine inflammatory myofibroblastic tumor showing an atypical ALK signal pattern by FISH and DES-ALK fusion by RNA sequencing: A case report

Inflammatory myofibroblastic tumors (IMTs) are spindle cell neoplasms of intermediate (borderline) biologic potential with tendency for local recurrence but low risk of metastasis. They affect children more than adults. The most common sites of involvement are the lung, soft tissue, peritoneum, bladder, and less commonly the gynecologic tract. IMTs are characterized by spindle to epithelioid cells with myofibroblastic differentiation, some degree of smooth muscle differentiation, myxoid stroma and usually associated with brisk lymphoplasmacytic infiltrates. In about half of the cases, IMTs are associated with rearrangements of the anaplastic lymphoma kinase (ALK) gene located at chromosome 2p23. The ALK rearrangement can be detected by immunohistochemistry for ALK protein expression (mostly cytoplasmic with or without perinuclear accentuation) or by fluorescent in situ hybridization (FISH) using dual-color break-apart probes for which the typical pattern is seen as split 3′ end (red) and 5′ end (green) probe signals in addition to single normal, unsplit red-green signal pair (yellow). Herein we describe a case of uterine IMT initially misdiagnosed intraoperatively as leiomyoma which showed sparse lymphocytic infiltrates, positive ALK expression by immunohistochemistry, a predominantly atypical FISH signal pattern (1 yellow and 1 red signal only) and few typical signal patterns (1 yellow, 1 red, and 1 green signal) in a smaller population of tumor cells. The RNA sequencing showed a recently described DES-ALK fusion transcript in the tumor cells, suggesting an intrachromosomal inversion and deletion as the likely underlying mechanism for the atypical FISH pattern. Familiarity with the unusual morphology and atypical FISH pattern is crucial given that this tumor has an activating ALK rearrangement and may benefit from targeted tyrosine kinase inhibitors in the future.