# FAP Expression in Renal Tumors Assessed by [68Ga]Ga-FAPI-46 PET Imaging and FAP Immunohistochemistry: A Case Series of Six Patients from the Prospective Exploratory Trial NCT04147494

## TL;DR

This study explores the use of FAP PET imaging in kidney tumors and finds that FAP expression varies by tumor type, with lower levels in renal cell carcinoma compared to other cancers.

## Contribution

The study provides a novel assessment of FAP expression in different renal tumor subtypes using PET imaging and immunohistochemistry.

## Key findings

- FAP radiotracer uptake was highest in clear cell renal cell carcinoma compared to other renal tumor types.
- FAP PET signal strongly correlated with immunohistochemistry results (r = 0.93).
- FAP expression in renal tumors was lower than in cancers like sarcoma.

## Abstract

Fibroblast activation protein (FAP) has been proposed as a pan-tumor target for PET imaging using FAP-targeted tracers. Here, we explore the potential value of FAP PET in renal tumors. Methods: Six patients with renal tumors (4 with clear cell renal cell carcinoma, 1 with papillary renal cell carcinoma, and 1 with renal oncocytoma) who were included in a prospective imaging study (NCT04147494) underwent [68Ga]Ga-FAPI-46 PET before nephrectomy. FAP PET radiotracer uptake and FAP expression by immunohistochemistry were assessed in the tumors and surrounding renal parenchyma. Results: Tumoral FAP radiotracer uptake was highest in clear cell renal cell carcinoma (median SUVmax, 3.1; range, 2.5–5.3), followed by renal oncocytoma (SUVmax, 1.9) and papillary renal cell carcinoma (SUVmax, 1.1). The FAP PET signal strongly correlated with FAP expression by immunohistochemistry (SUVmax; r = 0.93; P = 0.007). Conclusion: FAP expression in different renal tumors, including renal cell carcinoma, was lower when compared with cancers with known FAP expression, such as sarcoma. Although our data do not favor FAP-based theranostic approaches in renal cell carcinoma, studies in larger cohorts are warranted for conclusive evidence.