Malakoplakia is a characteristic inflammatory condition, which is usually seen in the urogenital tract, and less frequently in the gastrointestinal tract. We present a case of colonic malakoplakia in an immunocompromised patient. A 55-year-old female visited the outpatient clinic for routine cancer surveillance. Her past medical history was significant for kidney transplantation 11 years ago, and she had been taking immunosuppressants. A colonoscopy revealed several depressed flat lesions and elevated polyps, which were 0.3 to 0.4 cm in size and accompanied by whitish exudates. A biopsy revealed an infiltration of histiocytes with ample granular eosinophilic cytoplasm, with some lymphocytes and plasma cells. Many histiocytes had the characteristic morphology, described as Michaelis-Gutmann bodies: one or several round basophilic structures of approximately 1 to 10 µm in size with some being laminated, some appearing homogeneous, and others having a dense central core with a targetoid appearance. These Michaelis-Gutmann bodies were positively stained on von Kossa stain, and were diagnostic for malakoplakia.

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Metaplastic thymoma (MT), accepted in the World Health Organization 2004 scheme, is a circumscribed tumor of the thymus exhibiting biphasic morphology. We herein describe the clinicopathologic features of four MTs and the differential diagnoses of this unusual tumor. There were three women and one man with mean age of 49.5 years. The patients were found to have mediastinal masses, and underwent surgical excision. One exhibited symptoms of myasthenia gravis, and the serum titer for anti-acetylcholine receptor antibody was positive. Grossly, the tumors were encapsulated, and showed vaguely multinodular, solid, tan-white to yellow cut surfaces. Histologically, they comprised epithelial islands intertwining with bundles of delicate spindle cells. The patients remained well after surgical excision at 5-55 months. Because of the distinctive histological appearance and benign clinical course, MT should be distinguished from other more aggressive mediastinal neoplasms displaying biphasic feature.

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