ALK-positive anaplastic large cell lymphoma with a monomorphic small-cell pattern masquerading as inflammatory gastric lesions.

INTRODUCTION: Anaplastic lymphoma kinase-positive (ALK+) anaplastic large cell lymphoma (ALCL) with a non-common pattern can be diagnostic challenging. Pathologists can be unavoidably and unintentionally blind to non-descript tumor cells in a lymphohistiocytic- (LH) or small-cell (SC)-pattern. We report a case of primary systemic ALK+ ALCL with a SC pattern that presented as secondary gastric lesions with a mixed LH and SC pattern that was masqueraded as inflammatory lesions. CASE REPORT: A 34-year-old woman with intractable epigastric pain was referred to have repeated endoscopy with biopsy. She was found to multiple gastric erosions and nodules that were diagnosed as inflammatory lesions both endoscopically and histologically. Meanwhile, she developed an acute onset of severe back pain associated with a pathologic compression fracture in the T3 thoracic vertebral body. Imaging studies disclosed a disseminated systemic disease involving abdominopelvic lymph nodes and cervical and thoracic vertebral bodies. The needle biopsy of the pelvic lymph node disclosed diffuse proliferation of monomorphic small round cells that were diffusely positive for CD30 and ALK. A diagnosis of ALK+ ALCL with a monomorphic SC pattern was rendered. DISCUSSION: A retrospective review of the gastric biopsies with the aid of immunohistochemistry enabled us to recognise the presence of lymphomatous infiltrates with a mixed LH and SC pattern in every piece of gastric biopsies that were repeatedly misdiagnosed as inflammatory lesions. This case illustrates a significant diagnostic pitfall of the LH- and SC-patterns in ALK+ ALCL, in which the tumour cells featuring lymphoid, plasmacytoid or histiocytoid appearance can be masqueraded as inflammatory cells.