異所性脾組織：限本性検体および術中コンサルテーションにおける診断上の課題

The histologic diagnosis of ectopic splenic tissue can be challenging in limited biopsy and intraoperative samples. Accurate recognition is crucial to avoid misdiagnosis and guide appropriate patient management. This multi-institutional study of 41 lesions describes the clinicopathologic features of ectopic splenic tissue, with emphasis on diagnostic pitfalls. Fourteen patients (36%) had a history of malignancy-10 solid tumors, 3 hematologic neoplasms, and 1 with both. Notably, 1 patient demonstrated a superimposed low-grade B-cell lymphoma arising within ectopic splenic tissue. Thirteen had prior splenectomy. Among 35 specimens with available imaging, all demonstrated mass lesions, yet ectopic splenic tissue was suspected in only 17%. Initial radiologic impressions often favored neoplastic processes, including neuroendocrine tumor (3 patients), metastasis (1), and hepatocellular adenoma or carcinoma (3). Histologically, ectopic splenic tissue exhibited a broad morphologic spectrum, often mimicking inflammatory or neoplastic conditions. Fourteen specimens showed white pulp-predominant patterns resembling lymphoid tissue. Seven demonstrated mixed neutrophilic and lymphohistiocytic infiltrates suggestive of abscess or other inflammatory processes. Five specimens with red pulp predominance closely resembled hemorrhagic or vascular tumors. Eight exhibited solid or nested growth of monotonous epithelioid cells, mimicking well-differentiated neuroendocrine tumors. One showed mixed inflammatory infiltrates with fibroblastic proliferation, resembling an inflammatory myofibroblastic tumor. CD8 immunohistochemical staining consistently highlighted sinusoidal endothelial cells, providing key diagnostic confirmation. In conclusion, ectopic splenic tissue presents a wide range of morphologic appearances that can pose diagnostic challenges, especially in limited samples. Recognition of these patterns and use of CD8 immunostaining are essential to avoid misdiagnosis and unnecessary interventions.