Detection of ¹⁸F-FDG PET/CT Occult Lesions with ¹⁸F-DCFPyL PET/CT in a Patient with Metastatic Renal Cell Carcinoma

Abstract

Renal cell carcinoma (RCC) is common with more than 60,000 new cases in the United States yearly. No curative therapies are available for metastatic RCC. Improved methods of imaging metastatic RCC would be of value in identifying sites of occult disease and potentially for judging response to therapy. A 58-year-old male with known metastatic clear cell RCC was imaged with both ¹⁸F-FDG and ¹⁸F-DCFPyL PET/CT. ¹⁸F-DCFPyL is a small molecule inhibitor of the prostate-specific membrane antigen (PSMA), a target known to be highly expressed on solid tumor neovasculature. Relative to ¹⁸F-FDG, ¹⁸F-DCFPyL identified more lesions and demonstrated higher tumor radiotracer uptake.

Figure 1.

A 58-year-old male patient with known clear cell RCC metastases to the skeleton was imaged contemporaneously with two PET/CT examinations, one with ¹⁸F-FDG and a second with ¹⁸F-DCFPyL, a small molecule inhibitor of PSMA. The maximum intensity projection images from the two examinations (¹⁸F-FDG, A, and ¹⁸F-DCFPyL, B) demonstrate concordance of multiple radiotracer-avid lesions including the proximal right humerus and both iliac bones (red arrowheads). However, additional subtle sites of ¹⁸F-DCFPyL uptake are noted that do not have corresponding ¹⁸F-FDG uptake (blue arrows). These sites include subtle endosteal scalloping of the left posterior ninth rib and the left iliac bone without accompanying ¹⁸F-FDG uptake (C and E, blue arrows). In contrast, the axial ¹⁸F-DCFPyL PET/CT images demonstrated moderate radiotracer uptake at these sites (D, SUV_max (lean body mass corrected) 3.2, blue arrow and F, ¹⁸F-DCFPyL SUV_max 2.7, blue arrow).

Figure 2.

Large lytic lesions were present in the posterior aspects of both iliac bones in this patient, and both sites were found to have intense uptake of both radiotracers (¹⁸F-FDG in A and B and ¹⁸F-DCFPyL in C and D, red arrowheads). However, ¹⁸F-DCFPyL uptake was both visually and quantitatively higher than ¹⁸F-FDG. For the right iliac lesion, ¹⁸F-FDG uptake yielded SUV_max of 3.3, while for the same lesion ¹⁸F-DCFPyL uptake generated SUV_max of 16.6. For the left iliac, ¹⁸F-FDG SUV_max was 4.0 while ¹⁸F-DCFPyL SUV_max was 13.9. In aggregate, our findings in this patient with metastatic clear cell RCC are suggestive that ¹⁸F-DCFPyL may able to identify more lesions and has higher tumor uptake than ¹⁸F-FDG. Although a significant body of work has examined the role of ¹⁸F-DCFPyL and other small molecule inhibitors targeted against PSMA in the detection of prostate cancer [1-4], the use of such radiotracers for non-prostate applications has been limited to date [5, 6]. This is despite the fact that PSMA is highly expressed on the tumor neovasculature of many solid tumors, including RCC [7, 8]. Indeed, a previous case report has demonstrated the ability of a ⁶⁸Ga-labeled small molecule inhibitor of PSMA (⁶⁸Ga-PSMA) to identify sites of disease in a patient with metastatic clear cell RCC [5]. In that report, the authors noted concordance between ¹⁸F-FDG and ⁶⁸Ga-PSMA uptake for all described lesions, though the ⁶⁸Ga-PSMA PET acquisition was notable for improved lesion conspicuity. In combination with the earlier findings utilizing ⁶⁸Ga-PSMA, this report confirms that further study with PSMA radiotracers in metastatic RCC is warranted.