Characterization of indeterminate renal masses with molecular imaging: how do we turn potential into reality?

Background The majority of enhancing renal masses are unable to be effectively characterized as benign or malignant using standard cross-sectional imaging modalities including multi-phase computed tomography (CT) [1] and magnetic resonance imaging (MRI) [2]. This is particularly unfortunate in light of the steady increase in the incidence of these lesions, many of which are discovered incidentally on imaging studies performed for nonurologic indications [3, 4]. Moreover, it has been estimated that upwards of 5600 unnecessary partial and radical nephrectomies are performed each year in the USA for the false presumption of cancer [5]. In light of these data, recently, there has been a growing interest in the use of molecular imaging to characterize the aggressiveness of renal masses [6, 7]. To date, several reports have been published on the ability of Tc-sestamibi planar and single photon emission computed tomography (SPECT)/CT imaging to differentiate mitochondrial-rich benign and indolent renal masses such as oncocytomas and hybrid oncocytic/ chromophobe tumors (HOCTs) from more aggressive renal tumor histologies including the clear cell subtype of renal cell carcinoma (RCC) [8–10]. In this issue of European Journal of Nuclear Medicine and Molecular Imaging Research, Tzortzakakis et al. [11] add to this body of literature with a prospective study evaluating Tc-sestamibi SPECT/CT in 24 patients with 31 clinically localized T1 renal masses planned for biopsy or surgery. The authors reported that 11 of 12 (92%) oncocytomas and 3 of 3 (100%) HOCTs demonstrated Tc-sestamibi uptake, while all other lesions were negative for uptake with the exception of mild uptake in a single papillary RCC. These findings are concordant with earlier studies [9, 10] and dramatically increase the number of Tc-sestamibi positive oncocytomas and HOCTs reported in the literature. As a brief aside, it should be mentioned that there is also a growing interest in the use of renal mass biopsy for determining the histology of renal tumors, particularly as data from two recent systematic reviews have suggested that the rates of complications and nondiagnostic biopsies are tolerable [12, 13]. However, limitations of renal mass biopsy remain, including the intrinsically invasive nature of this procedure, the inaccessibility of some tumors to safe approaches for biopsy, and the heterogeneous population of renal masses that can present as oncocytic neoplasms on biopsy [14, 15]. As such, the non-invasive characterization of renal masses with molecular imaging techniques continues to be of interest and may serve as an adjunct to, or even a replacement of, biopsy in selected cases.