Leiomyosarcoma (LMS) is the most common sarcoma in adults. Rarely, LMS dedifferentiates into an undifferentiated sarcoma. Very few cases of LMS with heterologous osteosarcomatous differentiation (OS) have been reported. The purpose of this study was to evaluate the clinicopathologic features of LMS with OS. Of 5570 LMS cases diagnosed from 2006 to 2022, 15 cases (0.2%) of LMS with OS were identified, affecting 13 females and 2 males; ages ranged from 32 to 66 years (median: 53 y). Ten tumors arose in the uterus, 2 in the retroperitoneum, and 1 each in the mesentery, mediastinum, and rectum. Primary tumors ranged from 7 to 20 cm (mean: 16 cm). The LMS components showed conventional spindle cell morphology in most cases; 3 cases showed marked pleomorphism; 3 cases contained an epithelioid component; and 1 case showed myxoid features. In 5 cases OS was identified in the primary tumor, whereas in 10 cases OS was first detected in metastases. One metastatic and 2 primary LMS showed both OS and chondrosarcomatous differentiation. Prominent osteoclastic giant cells were seen in the OS components in 11 cases. Mitotic activity ranged from 17 to 61/10 HPF with tumor necrosis in 10 cases. Twelve patients developed metastases; sites included lungs, diaphragm, kidney, adrenal glands, colon, small intestine, liver, bone, and pancreas. At last follow-up, 8 patients had died of disease, and 4 patients were alive with metastases. The interval between OS and death ranged from 3 weeks to 18 months (median: 6.5 mo). Development of OS in LMS is exceptionally rare. This form of heterologous differentiation may occur in both primary tumors and metastases. LMS with OS is highly aggressive with poor outcomes. Awareness of this phenomenon is important to avoid misdiagnosis as osteosarcoma.

平滑肌肉瘤 (LMS) 是成人最常见的肉瘤。LMS 很少分化为未分化肉瘤。已报道的具有异源性骨肉瘤分化 (OS) 的 LMS 病例很少。本研究的目的是评估 LMS 与 OS 的临床病理学特征。在 2006 年至 2022 年诊断出的 5570 例 LMS 病例中，发现了 15 例（0.2%）伴有 OS 的 LMS，其中女性 13 例，男性 2 例；年龄范围为 32 至 66 岁（中位数：53 岁）。子宫内出现 10 个肿瘤，腹膜后 2 个，肠系膜、纵隔和直肠各 1 个。原发性肿瘤的范围为 7 至 20 厘米（平均：16 厘米）。在大多数情况下，LMS 组件显示出常规的梭形细胞形态；3例表现出明显的多形性；3例含有上皮样成分；1例表现为粘液样特征。在 5 例中，在原发肿瘤中发现了 OS，而在 10 例中，首先在转移瘤中发现了 OS。1 个转移性 LMS 和 2 个原发性 LMS 显示 OS 和软骨肉瘤分化。11例OS成分中可见明显的破骨巨细胞。有丝分裂活性范围为17至61/10 HPF，10例肿瘤坏死。12 名患者发生转移；部位包括肺、隔膜、肾、肾上腺、结肠、小肠、肝、骨和胰腺。末次随访时，8例患者因病死亡，4例转移灶存活。OS 与死亡之间的时间间隔为 3 周至 18 个月（中位数：6.5 个月）。在 LMS 中开发 OS 非常罕见。这种形式的异源分化可能发生在原发性肿瘤和转移灶中。带有 OS 的 LMS 具有很强的攻击性，但结果不佳。意识到这种现象对于避免误诊为骨肉瘤很重要。