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Preoperative differentiation of oncocytomas from renal cell carcinoma (RCC) is often challenging. 99m Tc-MIBI imaging could play a potential role in differentiating oncocytoma from RCC, which in turn could guide surgical decision-making. We present the use of 99m Tc-MIBI SPECT/CT to characterize a renal mass in a 66-year-old man with a complex medical history, including history of bilateral oncocytomas. 99m Tc-MIBI SPECT/CT showed features suspicious of a malignant tumor, which was confirmed postnephrectomy as a chromophobe and papillary RCC collision tumor. This case supports 99m Tc-MIBI imaging for preoperative differentiation of benign versus malignant renal tumors.

697 Background: Creatinine and cystatin C are routinely used to assess renal function. Given creatinine originates from skeletal muscle and cystatin C is produced by nucleated cells, a creatinine to cystatin C ratio (Cr/Cys-C) may positively correlate with muscle mass. Low Cr/Cys-C has also been associated with decreased overall survival (OS) in cancer, including in a combined cohort of genitourinary malignancies. Furthermore, elevated cystatin c has been associated with shorter OS and recurrence free survival (RFS) in renal cell carcinoma (RCC). Cr/Cys-C may be a simple and affordable tool to assist with patient-specific risk stratification. We assess the ability of Cr/Cys-C to predict OS and RFS in patients with RCC. Methods: Retrospective review of a prospectively maintained database identified patients that underwent partial or radical nephrectomy for RCC from 2018-2021. Included patients had preoperative creatinine and cystatin C and 1+ year of follow up. Cr/Cys-C associations with patient/tumor characteristics were determined by generalized chi-square or Fisher’s exact tests for categorical variables and Wilcoxon rank-sum test for continuous variables. Cr/Cys-C ability to predict OS and RFS was analyzed with Kaplan-Meier method and Cox hazards models. Statistical tests were two-sided with type I error set at 0.05. Results: 219 patients were identified. Median age was 64, with most being male (67%). 62% and 29% of patients were white and black, respectively. Median eGFR was 72mL/min/1.73m2. Median (IQR) Cr/Cys-C was 1 (0.8-1.2). 55% were stage T3-T4, with 12% N1 and 16% M1 at time of surgery. 72% had clear cell histology. Low Cr/Cys-C was significantly associated with older age, males, Eastern Cooperative Oncology Group score ≥ 1, radical nephrectomy, T3-T4 stage, and metastasis. Kaplan-Meier curves showed low Cr/Cys-C association with decreased OS (p=0.0003) and RFS (p=0.0094). Cox regression analysis revealed lower Cr/Cys-C as independent predictor of decreased OS (binary HR=3.66, 95% CI 1.2-11.3, p=0.02; continuous HR=0.05, 95% 0.0-0.8, p=0.03) and RFS (binary HR=4.8, 95% CI 1.6-14.6, p=0.006; continuous HR=0.02, 95% 0.0-0.3, p=0.006;Table). Conclusions: Lower Cr/Cys-C may be associated with decreased OS and RFS in patients with RCC. [Table: see text]

616 Background: Accurate prediction of future glomerular function rate (fGFR) following nephrectomy in renal cell carcinoma (RCC) patients is of interest. A postoperative estimated glomerular filtration (eGFR) rate ≥ 45 mL/min/1.73m2 is considered favorable, with guidelines reflecting these findings. Though nephron sparing surgery is preferable, radical nephrectomy is often the preferred oncologic surgery. Various models to predict postoperative renal function have been proposed, with a new 2021 equation by Palacios et al. exhibiting strong performance in predicting fGFR. We aim to validate this fGFR equation in a large institutional cohort using both the Chronic Kidney Disease Epidemiology (CKD-EPI) 2009 equation with race, used in the creation of the formula, as well as the CKD-EPI 2021 equation without race, which is currently the recommended CKD-EPI creatinine equation. Methods: Using an institutional database, patients that underwent partial or radical nephrectomy for RCC from 2005-2021 were identified. Patients with creatinine values preoperatively and 3-12 months postoperatively were included, with end stage renal disease serving as exclusion criteria. Correlation/bias/accuracy/precision of the fGFR equation (fGFR=35+ [preoperative eGFR x 0.65]- 18 [if radical] - [age x 0.25]+ 3 [if tumor >7cm]-2 [if diabetes]) with observed postoperative eGFR was determined by both the CKD-EPI-2021 and CKD-EPI 2009 equations. Results: 1,443 patients were included in our analysis. 71% (1,024) and 22.9% (331) patients were white and black, respectively. A majority of patients underwent radical nephrectomy (60.3%). 40% of patients had T3-T4 RCC, with 14.8% of patients having M1 disease. Median observed vs predicted fGFR was 58.0 vs 58.7 ml/min/1.73m2 for CKD-EPI 2021 and 56.0 vs 57.5 for CKD-EPI 2009. The correlation/bias/accuracy/precision of the fGFR equation was 0.805/-0.5/81.7/7.9-9.0 for CKD-EPI 2021 and 0.809/-0.8/81.3/-8.1-8 for CKD-EPI 2009 equations (Table). Conclusions: The fGFR equation accurately predicted renal function in our large and diverse institutional cohort using both the CKD-EPI 2009 including race and CKD-EPI 2021 excluding race equations. [Table: see text]