Oligometastatic Renal Cell Carcinoma With Sarcomatoid Differentiation Demonstrating Variable Imaging Phenotypes on 68Ga-PSMA and 18F-FDG PET/CT: A Case Report and Review of the Literature

Sarcomatoid differentiation is observed in approximately 5% of renal cell carcinomas (RCCs) and is an adverse prognostic factor. The molecular imaging characteristics of sarcomatoid RCC remain largely unknown, with limited case reports describing intense uptake on F-18-fluorodeoxyglucose (FDG) positron emission tomography (PET). We present a patient with oligometastatic clear cell RCC with sarcomatoid differentiation, who had 2 subpopulations of disease with different imaging phenotypes on Ga-68-prostate-specific membrane antigen (PSMA) and F-18-FDG PET/computed tomography A slowly growing site of local disease recurrence showed intense Ga-68-PSMA avidity and low-intensity F-18-FDG uptake. Conversely, a rapidly growing pulmonary metastasis with confirmed sarcomatoid differentiation was intensely F-18-FDG avid, but showed near absent Ga-68-PSMA uptake with corresponding high GLUT-1 and absent PSMA staining on immunohistochemistry. The case suggests that sarcomatoid differentiation is associated with a loss of PSMA expression from the neovasculature, with an opposing upregulation of GLUT-1 transporter expression. This 'flip-flop' phenomenon, in which the molecular imaging phenotype shifts as de-differentiation occurs, highlights the utility of PET to characterize heterogeneity between individual sites of metastatic disease in vivo.