Abstract
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Introduction: Incidental detection of solid renal lesions has been increasing with the widespread use of cross-sectional abdominal imaging for other indications. The features of aggressive tumors cannot be reliably differentiated from those of benign and indolent histologies on conventional imaging. 99mTc-Sestamibi preferentially localizes in the mitochondria-rich cells of the oncocytic neoplasms and can differentiate oncocytic tumors from the aggressive renal cell carcinomas (RCC), as the latter have negligible radiotracer uptake. We conducted this study to estimate the diagnostic accuracy of 99mTc-Sestamibi SPECT/CT for characterizing solid renal lesions and comparing it with that of contrast-enhanced CT (CECT).
Methods: Imaging and clinical records of patients who underwent 99mTc-Sestamibi SPECT/CT for work-up of their solid renal lesions, from September 2018 to October 2021 were retrospectively reviewed. Histopathology results obtained either from biopsy or after surgical excision of the lesions formed the reference standard. The histopathologic diagnoses were segregated as malignant/ clinically concerning (RCCs other than chromophobe histology) and benign/ clinically non-concerning (oncocytic renal neoplasms, other benign diagnoses) to calculate the sensitivity and specificity of SPECT/CT and CECT. The clinical reads of the SPECT/CT images were noted for the visual classification of the lesions. Additionally, the images were manually segmented to obtain maximum and mean counts of the lesion and adjacent renal cortex, and lesional maximum and mean HU.
Results: 99mTc-Sestamibi SPECT/CT was performed in 42 patients to evaluate their renal lesions (n = 62). 27 patients (mean age 66.8±10.4 years; 18 men, 9 women), with 36 solid renal lesions had a final histopathologic diagnosis (Biopsy, n = 4; Surgical histopathology, n = 23), and 20 patients had a CECT. The mean lesional size was 3.5±1.9 cm. Overall, the most common diagnosis was of an oncocytic renal neoplasm (15/36; 41.6%), including oncocytoma (n=6), chromophobe RCC (n=2), hybrid oncocytic chromophobe RCC (n=2), RCC with oncocytic features (n=1), and oncocytic renal neoplasms, NOS (n=6). The most commonly identified single histology was clear-cell RCC (13/36; 36.1%). The per-lesion diagnostic test yield for providing a conclusive diagnosis was 84.6% (22/26) for CECT, and 94.4% (35/36) for SPECT/CT. The sensitivity and specificity of SPECT/CT with visual interpretation for distinguishing between clinically concerning and non-concerning renal masses was 66.7% (95% CI: 0.41-0.85) and 89.5% (95% CI: 0.68-0.98), respectively, compared to 10% (95% CI: 0.005-0.40), and 75% (95% CI: 0.47-0.91) for CECT, respectively. Lesion and kidney counts could be calculated for 31 lesions (16 concerning, 15 non-concerning). Lesional HU was available for 35 lesions (19 concerning, 16 non-concerning). The lesion to kidney ratios for maximum and mean counts, mean lesional counts and maximum lesional HU showed significant differences between the two groups. ROC curves drawn with lesion to kidney ratios showed superior performance to visual interpretation (sensitivity and specificity of 86.67% and 87.5% at a cut-off of >0.47). Lesion to kidney mean count ratio (AUC 0.85) performed better than lesion to kidney maximum count ratio (AUC 0.78) for differentiating clinically concerning from non-concerning lesions.
Conclusions: The current literature on the utility of 99mTc-Sestamibi SPECT/CT for characterization of solid renal masses is limited, with a recently published systematic review reporting only four studies. We offer additional evidence on the superior diagnostic accuracy of 99mTc-Sestamibi SPECT/CT over CECT for differentiating the malignant/ aggressive renal tumors from the benign/ indolent ones, therefore potentially avoiding over-treatment and its associated complications in the latter category. Quantitative assessment can further increase the diagnostic accuracy of SPECT/CT and may be used in conjunction with visual interpretation.
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