SFT was first described in 1931 by Klemperer and Rabin as a distinct mesothelial tumor arising from the pleura [1]. Thereafter, several synonyms, such as localized mesothelioma, fibrous localized mesothelioma, benign fibrous mesothelioma, and pleural fibroma, have been used because of its heterogenetic theory of origin. Currently, SFTs are widely recognized as mesenchymal neoplasms based on immunohistochemical and ultrastructural findings. In the 2020 World Health Organization (WHO) classification of soft tissue tumors, the terminology has been unified under SFT only [2]. Thus far, SFTs have been reported in other numerous sites, such as the peritoneum, parotid gland, paranasal sinuses, orbit, skin, and intracranial areas [3], thereby, supporting a mesenchymal rather than a mesothelial origin. Although SFT can occur at nearly all anatomic locations, intrapulmonary SFTs are relatively rare. The development of intrapulmonary SFT may be attributed to (i) the direct continuity between the subpleural mesenchyme and interlobular septa or (ii) the presence of lung fibroblasts in the submesothelial areas of normal pulmonary parenchyma [4].

Histologically, spindle-shaped cells may be seen in fibrosarcoma, leiomyosarcoma, schwannoma, and others. Therefore, the diagnosis of SFTs may not be confirmed without immunohistochemical staining. The most valuable immunohistochemical markers in the diagnosis of SFTs are CD34 and STAT6, which were diffusely and strongly positive in the present case. On a molecular level, SFTs have been shown to be pathogenetically linked to a gene fusion secondary to a paracentric inversion on chromosome 12q13 and involves NAB2 and STAT6 [5], which are highly sensitive and specific markers for SFT. In the present case, the NAB2-STAT6 gene fusions detected by reverse transcription-polymerase chain reaction and direct sequence led to the definitive diagnosis of SFT. According to the WHO classification [2], the prediction of metastatic risk in SFTs are follows: (1) patient age in years (≥ 55); (2) mitoses per 10 high-power fields; (3) tumor size in cm, and (4) tumor necrosis. The tumor in the present case was considered as low risk of metastasis.

To date, 50 cases of intrapulmonary SFT have been reported in the English language literature [4, 6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25]. We summarized a total of 51 cases, including the present case, with a literature review of intrapulmonary SFT cases (Table 1). All patients, except one, were treated by surgical resection, which encompassed wedge resection to pneumonectomy. Histologically, five tumors showed intermediate or malignant features. Although multiple factors, such as age and tumor size, have been associated with survival [26,27,28], higher histologic grade has been considered to have the strongest correlation with prognosis. Of these five tumors diagnosed as intermediate or malignant SFT, two (40%) had recurrence.

Resection with free margins is considered the treatment for intrapulmonary SFT. Adequate wedge resection, anatomic segmentectomy, and lobectomy, depending on the location of the mass, are the common procedures for surgical resection of intrapulmonary tumors. Outcomes have been favorable in a majority of patients histologically diagnosed as benign SFTs. However, tumor behavior does not always correlate with histologic findings, because a few histologically benign SFTs reportedly recurred [19, 21]. Rao et al. [19] reported metastases in 10% of benign cases of intrapulmonary SFT. Inoue et al. [21] documented local recurrence of SFT 2 years after complete wedge resection of the left upper lobe and emphasized the importance of longer-term postsurgical follow-up, even for benign-appearing tumors. Judging the tumor as benign or malignant solely by histologic findings is considered to be difficult. Therefore, we should be careful to follow the postoperative course of patients, even in cases histologically diagnosed as benign SFT.