Solitary Fibrous Tumor of the Prostate Shown on FAPI PET/CT

Abstract

Solitary fibrous tumors are fibroblast tumors that occur mainly in the peritoneum, extremities, and pleura. Here, we report the MRI, FDG PET/CT, and FAPI PET/CT findings of a rare prostate solitary fibrous tumor. A 57-year-old man was pathologically diagnosed with a solitary fibrous tumor. To detect any systemic metastases or other primary lesions, the patient underwent FDG PET/CT and FAPI PET/CT examination sequentially. Mild FDG uptake was observed in the primary prostatic lesion, but there was a significant uptake of FAPI in the prostate. This case highlighted that FAPI PET/CT may outperform FDG PET/CT in identifying solitary fibrous tumors.

FIGURE 1: A 57-year-old man was admitted to our hospital because of increased frequency of micturition and urinary urgency. Ultrasonographic examination revealed a cystic solid nodule in the prostate. We then performed MRI to determine whether the nodule was prostatic cancer. T2-weighted imaging showed a cystic solid nodule in the right side of the transition zone of the prostate (A, arrow). The solid nodule had a slightly high signal intensity on diffusion-weighted imaging (B, arrow), a reduced apparent diffusion coefficient value (C, arrow), and abnormal moderate enhancement on an enhanced scan (D, arrow). According to the Prostate Imaging Reporting and Data System (PI-RADS) Version 2.1,¹ the lesion had a PI-RADS score of 5. A prostate biopsy revealed diffuse infiltration of small round cells in the prostate tissue (E, hematoxylin-eosin stain, ×200). Combined with the immunohistochemical results of CD34 positivity (F, ×200) and STAT6 positivity (G, ×400), the patient was diagnosed with a solitary fibrous tumor.^2,3 The Ki67 expression of the lesion was approximately 1% (H, ×200).

FIGURE 2: We performed FDG PET/CT to detect any systemic metastases or other primary lesions. Surprisingly, the primary prostatic lesion showed low FDG uptake on the MIP image (A, arrow). Similarly, on the axial PET (B, arrow) and corresponding PET/CT images (C, arrow), the lesion showed mild FDG uptake with an SUV_max of 2.804. The patient was enrolled in an ongoing clinical trial (YX-2021-113-01), which was approved by the institutional review board of our hospital. This clinical trial was a prospective study of FAPI PET/CT in baseline assessment and efficacy evaluation of various tumors, chronic inflammation, and myocardial infarction. After obtaining informed consent from the patient, we performed a FAPI PET/CT examination 2 days later. There was significant uptake of FAPI in the prostate on the MIP image (F, arrow), without other abnormal activity elsewhere. Axial PET (D, arrow) and corresponding PET/CT fusion images (E, arrow) showed high FAPI accumulation with an SUV_max of 24.423 in the prostatic lesion. Solitary fibrous tumors are morphologically diverse fibroblast tumors associated with high risks of metastasis and death.^4,5 Solitary fibrous tumors can show aggressive behavior, even in the absence of any morphologic evidence of malignancy at onset.⁶ Furthermore, solitary fibrous tumors that initially have benign histopathologic features can occasionally metastasize.^7,8 FAPI PET/CT might have the potential to be used as a complementary imaging modality to identify solitary fibrous tumors.