Ga68-FAPI Imaging and Lu177-FAPI Therapy in a Case of Metastatic Solitary Fibrous Tumor

Abstract

Solitary fibrous tumor (SFT) of the central nervous system (previously called hemangiopericytoma) is a rare mesenchymal tumor. Malignant SFT has a tendency to recur after surgery and can metastasize to distant organs. Treatment options for metastatic disease are limited. This case demonstrated high expression of FAP (fibroblast activating protein) in all metastatic sites with Ga-FAPI positron emission tomography-computed tomography imaging. Subsequently, the patient was treated with Lu177-FAPI-targeted radionuclide therapy. There was significant clinical response. There was mild partial morphological response seen on follow-up imaging.

Keywords: Fibroblast activation protein; Ga68-FAPI; Lu177-FAPI; hemangiopericytoma; solitary fibrous tumor; targeted radionuclide therapy.

Figure 1:

Ga68-DOTATATE PET-CT whole-body MIP (maximum intensity projection) image (a). There was moderate-to-high SSTR expression (Krenning's score 3) in an osteolytic lesion in the sacrum involving the body with associated soft-tissue component (b), with extension of lesion to the spinal canal and along presacral soft tissue alongside the sacral nerve roots. However, the multiple metastatic liver lesions (c) showed no significant SSTR expression (Krenning's score 0). There were also multiple metastatic lung nodules and masses seen (d and e) – few of these showed low-grade SSTR expression (Krenning's score 2) while some showed no significant SSTR expression (Krenning's score 0). PET-CT: Positron emission tomography–computed tomography, SSTR: Somatostatin receptor

Figure 2:

FDG PET-CT whole-body mip image (a) showed low-grade FDG uptake in the sacral lesion (b), there was mild FDG uptake in the liver lesions (c) – SUVmax ranging from 3.1 to 4.0, and there was variable low-to-high FDG avidity in the lung masses (d and e) – SUVmax ranging from 1.5 to 8. FDG: Fluorodeoxyglucose, PET-CT: Positron emission tomography–computed tomography

Figure 3:

Ga68-FAPI (04) PET-CT revealed intensely increased tracer uptake in all the metastatic lesions (a) including sacral (b), variable moderate-to-high FAPI uptake in lung lesions (c) and intense uptake in multiple liver metastases (d and e). In addition, few other smaller bone metastases were also seen. PET-CT: Positron emission tomography–computed tomography

Figure 4:

Whole-body images of Lu-177-FAPI acquired on gamma camera with medium energy collimator 5 days postintravenous injection of 200 mCi Lu177-FAPI (09) showed high retention of radiopharmaceutical in the multiple metastatic lesions similar to that seen on the imaging PET-CT. PET-CT: Positron emission tomography–computed tomography

Figure 5:

Ga68-FAPI PET-CT scan mip images of the initial scan done in July 2022, 8 weeks after the first cycle of Lu177-FAPI therapy (October 2022), and 8 weeks after the second cycle of Lu177-FAPI therapy (December 2022). There was a partial reduction in FAP expression seen in the sacral lesion. There was persistent high FAP expression in liver lesions though they showed reduction in number. Persistent moderate-to-high FAP expression seen in lung lesions. No new lesions were seen. PET-CT: Positron emission tomography–computed tomography

Figure 6:

Ga68-FAPI PET-CT scan – cross-sectional CT and fused PET-CT images of the Sacrum of the initial scan done in July 2022, 8 weeks after the first cycle of Lu177-FAPI therapy (October 2022), and 8 weeks after the second cycle of Lu177-FAPI therapy (December 2022). There was a partial reduction in FAP expression seen in the sacral lesion. There were mild reduction seen in the extent of the soft-tissue component and reduction in the extent of the soft-tissue thickening along the exiting sacral nerve roots. PET-CT: Positron emission tomography–computed tomography, CT: Computed tomography

Figure 7:

Ga68-FAPI PET-CT scan – cross-sectional CT and fused PET-CT images of the liver of the initial scan done in July 2022, 8 weeks after the first cycle of Lu177-FAPI therapy (October 2022), and 8 weeks after the second cycle of Lu177-FAPI therapy (December 2022). There was a mild reduction in size of the largest liver lesion with persistent high FAP expression. The other multiple liver lesions also showed a partial reduction in size. PET-CT: Positron emission tomography–computed tomography, CT: Computed tomography