Thoracic SMARCA4-deficient undifferentiated tumor (SMARCA4-UT) is a recently recognized tumor characterized by inactivation of SMARCA4, a SWItch/Sucrose NonFermentable chromatin remodeler, detectable by immunohistochemistry. SMARCA4-UT shows undifferentiated or rhabdoid morphology with claudin-4 negativity. However, thoracic undifferentiated tumors with the same histologic features as SMARCA4-UTs but a preserved SMARCA4 expression have so far been underrecognized. We herein report 3 cases of thoracic undifferentiated tumors with isolated loss of SMARCA2 but retained expression of SMARCA4 and SMARCB1. The present tumors were found in 2 men and 1 woman, 40 to 50 years old. All patients were heavy smokers (≥20 pack-years). The tumors were generally large masses located in the mediastinum, lung>chest wall, or lung and composed of relatively monotonous, round to epithelioid cells with variably rhabdoid cells. Immunohistochemically, the tumors showed claudin-4 negativity with variable expression of cytokeratin. All cases showed highly aggressive clinical behavior with overall survival of 2 to 10 months. These SMARCA2-deficient tumors with preserved SMARCA4 expression appeared to be clinicopathologically indistinguishable from SMARCA4-UTs, except for in their SMARCA4 status. This variant may expand the spectrum of SWItch/Sucrose NonFermentable–deficient undifferentiated tumors of the thoracic region beyond SMARCA4-UT.

胸部 SMARCA4 缺陷型未分化肿瘤 (SMARCA4-UT) 是最近发现的一种肿瘤，其特征是 SMARCA4 失活，SMARCA4 是一种 SWITCH/蔗糖不可发酵的染色质重塑剂，可通过免疫组织化学检测到。SMARCA4-UT 显示未分化或横纹肌样形态，claudin-4 阴性。然而，与 SMARCA4-UT 具有相同组织学特征但保留 SMARCA4 表达的胸部未分化肿瘤迄今为止尚未得到充分认识。我们在此报告了 3 例胸部未分化肿瘤，其中 SMARCA2 单独缺失，但保留了 SMARCA4 和 SMARCB1 的表达。目前发现的肿瘤为2男1女，年龄40至50岁。所有患者均为重度吸烟者（≥20包年）。肿瘤通常是位于纵隔、肺>胸壁或肺的大肿块，由相对单调的圆形至上皮样细胞和不同的横纹肌样细胞组成。免疫组织化学显示，肿瘤显示claudin-4阴性，细胞角蛋白表达不同。所有病例均表现出高度侵袭性的临床行为，总生存期为 2 至 10 个月。这些保留 SMARCA4 表达的 SMARCA2 缺陷型肿瘤在临床病理学上似乎与 SMARCA4-UT 无法区分，除了它们的 SMARCA4 状态。该变体可能会将 SWItch/蔗糖不可发酵缺陷型未分化肿瘤的范围扩大到 SMARCA4-UT 之外。