18F-FDG PET/CT and 68Ga-FAPI-4 PET/CT Findings of Bilateral Knee Osteoarthritis in a Patient With Uveal Malignant Melanoma

Abstract

We present the findings of 68 Ga-FAPI-4 PET/CT and 18 F-FDG PET/CT of a metastatic malignant melanoma patient with osteoarthritis. A 65-year-old woman with a history of metastatic uveal malignant melanoma was referred to 18 F-FDG PET/CT for restaging after enucleation and chemotherapy. 18 F-FDG PET/CT imaging showed high radiotracer uptake in liver metastases; additionally mild uptake due to osteoarthritis was observed in both knees. However, although 68 Ga-FAPI-4 showed lower uptake in liver lesions, it showed a more prominent uptake in both knee joints compared with 18 F-FDG.

Emerging Role of FAPI PET Imaging for the Assessment of Benign Bone and Joint Diseases

“…In this setting, fibroblast activation protein (FAP) inhibitors (FAPI), labelled with both 18 F or 68 Ga, are emerging as promising radiopharmaceuticals. FAPI are molecules able to bind to FAP, a membrane serine protease that is highly expressed on activated fibroblasts present in a wide range of pathophysiological conditions, such as wound-healing, inflammation and cancer [ 10 , 11 , 12 ]. In this scenario, the role of FAPI PET/CT for the evaluation of cancer has been evaluated in several studies and the preliminary results are promising, as it also seems to allow the evaluation of tumors with low [ 18 F]FDG uptake.…”

“…The rationale behind the use of FAPI PET/CT for the assessment of inflammatory bone and joint diseases is that FAP is overexpressed in tissue remodeling sites associated with arthritis and fibrosis, such as chondrocytes [ 10 , 11 ]. However, on the other side, some authors hypothesized a different mechanism of uptake, related to the increased vascularity and capillary permeability due to inflammatory response, resulting in high perfusion and blood-pool effects [ 20 ].…”

“…Several case reports have reported that radiolabeled FAPI uptake is associated with a high number of benign conditions that affect the bones and the joints, such as fibrous dysplasia [ 22 ], tuberculosis [ 23 ], osteoarthritis, enthesopathies, myositis ossificans [ 24 ], Erdheim–Chester disease [ 25 ], ankylosing spondylitis [ 26 ], fractures [ 20 ], synovitis, osteitis and rheumatoid arthritis (RA) [ 12 ]. In particular, arthritis is generally underlined as an incidental finding at radiolabeled FAPI PET/CT and the most common sites of uptake are facet, shoulder, sternoclavicular joints and knees [ 11 , 20 , 27 , 28 , 29 ].…”

FAPI-PET/CT in Cancer Imaging: A Potential Novel Molecule of the Century

“…In a case report of liver metastasis from malignant melanoma, the lesion showed moderate 68 Ga-FAPI-04 uptake (SUVmax of 6.5) but was not compared with the current PET standard tracer 18 F-FDG ( 58 ). In another retrospective analysis of FAPI PET/CT imaging of rare malignancies, patients with melanoma (one case) also showed more favorable imaging results ( 59 ).…”

Skin Cancer Pathobiology at a Glance: A Focus on Imaging Techniques and Their Potential for Improved Diagnosis and Surveillance in Clinical Cohorts

“…Most of the molecular imaging studies in skin cancer focus on melanoma since it is the most metastatic and deadliest disease among all the other cutaneous subtypes. The biochemical processes and molecular targets that are currently under investigation for melanoma imaging are associated with the immune response (e.g., PD-1/PD-L1 [ 266 ], indoleamine 2,3-dioxygenase (IDO) [ 267 ]), TME dynamics (e.g., fibroblast activation protein (FAP) [ 268 ]), melanomagenesis (e.g., melanin [ 269 ]), invasion (e.g., melanocortin receptor subtype 1 (MC1R) [ 270 ]), or angiogenesis (e.g., integrins [ 271 ]) and hence many other novel antibodies and angiogenesis-related molecules continue to reside in the arena of research and development. Here, we will focus on the most well characterized targets that showed promise for use in SPECT/PET imaging.…”