SFT is an uncommon soft-tissue neoplasm that was first reported as a type of pleural neoplasm in 1931 [3]. SFT derives from fibroblastic or myofibroblastic cells under the mesothelium [4]. It occurs mainly in the thoracic cavity, and rarely, in the retroperitoneal region, as in this case. Simple excision is considered curative for benign SFTs, and is recommended to prevent malignant transformation and metastasis [5]. About 15% of resected SFTs recur, mostly as local metastases. Distant recurrences are rare, but may appear a long time after the primary tumor has been removed [6].
Microscopically, SFTs show multiplying spindle cells with a patternless arrangement, and hemangiopericytoma-like appearance with prominent vascularity [7]. Among immunostains, CD34 is especially useful for differentiating SFT from the other spindle-cell neoplasia [5, 6]. England et al. described high cellularity, high mitotic activity (more than 4/10 HPF), pleomorphism, necrosis, and hemorrhagic changes as criteria for morphological malignancy of SFT [5].
Although SFTs are typically benign, they can become malignant, especially if they grow to a large size or repeatedly recur [1, 8, 9]. According to England’s criteria, the current case was also diagnosed as benign at the initial resection, but the metastases were malignant. This change in malignancy bears concern. Although the primary tumor was considered benign, it potentially harbored highly malignant cells. SFT is often a large tumor, and pathological evaluation of all its cells is difficult; therefore, in an otherwise benign SFT, the possibility of malignant cells, or benign cells that could change morphologically over time, cannot be easily ruled out. Because the tumor was large in this case, we followed up with CT every 6 months for 5 years according to the malignant tumor. Lung metastasis has been pointed out by CT 10 years later coincidentally. Currently, there are no rules regarding follow-up. However, we think that it may be better to follow up for 10 years, because there is a possibility of distant recurrence like this time. However, accumulation of more cases is necessary to better understand this rare tumor.