Expression of fibroblast activation protein in intrathoracic solitary fibrous tumor

Fibroblast activation protein (FAP) is a serine integral membrane protease, the expression of which has been confirmed in various cancer types. Solitary fibrous tumors of the pleura (SFTP) are rare mesenchymal fibroblastic neoplasms. We present a case of ¹⁸F-labeled FAP inhibitor ([¹⁸F]FAPI-74) PET imaging and its correlation with histological FAP expression and review an SFTP series at our institution in relation to the extent of FAP expression.

This retrospective study included 13 patients who underwent surgery between March 2011 and December 2022 at our institute. One of the patients also underwent [¹⁸F]FAPI-74 PET imaging. We semi-quantitatively evaluated FAP expression in SFTPs using immunohistochemical staining and H-scores.

Nine of the 13 patients were male, with a median age of 64 years (range, 28-79 years). The median tumor size was 6.6 cm (1.1, 16 cm). In the pathological findings, expression levels of Ki67 were 1-5% in 12 of 13 cases. Furthermore, FAP expression was observed in all patients, and the median H-score was 160 (range, 10-280). The H-score of FAP expression in two of the 13 patients was low (10 in both), and that in two of the 13 patients was high (240 and 280). The SUVmax value of [¹⁸F]FAPI-74 PET was 3.57 in a patient in whom the H-score of FAP expression was 180.

SFTPs expressed FAP to varying degrees in different patients and the [¹⁸F]FAPI-74 PET results in one patient reflected FAP expression in the tumor tissue.