Our study suggests that the frequent description of small (1.5 cm or less in diameter) hypoattenuating renal lesions on portal venous phase contrast enhanced CT as “too small to accurately characterize” or “indeterminate” is not entirely correct, because both subjective impression and CT attenuation can help distinguish small cysts from renal cell carcinomas. In this study, lesions that appeared possibly to definitely solid on visual inspection were likely to be renal cell carcinomas, with a sensitivity of 100% and a positive predictive value of 67–100%, although this sign was somewhat subjective, with paired kappa values of 0.33 to 0.69 for comparisons between the three readers. Subjective criteria may be better suited for evaluating some renal masses than objective attenuation measurements. For example, a recent study documented the utility of assessing lesion heterogeneity as a marker of malignancy in the distinction of high-density cysts from renal cell carcinoma on portal venous phase contrast-enhanced CT (27), so it is not surprising that this finding is also valid in the evaluation of small hypoattenuating lesions. In addition, we found small lesions with a CT attenuation of 50 HU or more were likely to be renal cell carcinomas, with a sensitivity of 100% and a positive predictive value of 43–55%. This sign demonstrated greater interobserver agreement, with paired intraclass correlation coefficients of 0.90–0.93. The threshold attenuation of 50 HU for the identification of small renal cysts is higher than the normal upper limit of simple fluid attenuation, usually considered to be 20 HU (10). This higher threshold is presumably due to the effects of partial volume averaging and pseudoenhancement, which are known to result in higher computer generated attenuation measurements in small lesions [2
Currently, the recommendation for indeterminate small renal masses is follow-up imaging studies [4
] because it is thought that most of these lesions are cysts, and that even if the lesions are malignant, there is little risk of advancement of tumor stage [24
]. However, it is also known that small renal malignancies can metastasize if left untreated [25
], and earlier treatment may improve prognosis [26
], since the tumor stage at the start of treatment is the most important prognostic factor for primary renal malignancy [27
]. Our findings should not be interpreted as replacing these recommendations or indicating that definitive characterization of small hypoattenuating renal lesions is possible; rather, we are suggesting that our findings may influence the strength of recommendations for additional evaluation of these lesions, whether by ultrasound or MR imaging. Incidental small hypoattenuating renal lesions are encountered every day in routine practice, and aggressive follow-up of all such lesions is not practical, feasible, or commonly performed. Our findings may help stratify patients with small hypoattenuating renal lesions and identify the subgroup most needing additional imaging for possible renal cell carcinoma. Such a selective approach to these lesions may also improve the likelihood that a referring physician will request the additional imaging studies suggested. This could, in turn, lead to an earlier diagnosis of renal cell carcinoma and potentially contribute to improved survival.
Our study has a number of limitations. The sample size (n
=20) was small, which was partially a reflection of the limited number of surgically resected renal cell carcinomas under 1.5 cm at our institution. We identified patients with renal cell carcinoma by review of histopathological findings. This is a possible source of selection bias, since lesions that are more obviously malignant on imaging may be more likely to undergo surgery. Unfortunately, there is no method that allows us to identify cases of clinically unrecognized renal cell carcinoma. Subjective impression of small renal lesions is not amenable to objective definition and demonstrated greater interobserver variability than CT attenuation measurement. CT attenuation measurements may be dependant on CT scanner model and CT technique [7
]. Notwithstanding this caveat, lesions measuring less than 50 HU in our series were all benign cysts rather than renal cell carcinomas. Finally, our study could be criticized for not including small high-density cysts. However, given that CT density measurement is critical to the diagnosis of high-density cysts and that CT density measurements are unreliable for small lesions, it would be difficult to construct a methodology that would identify such cysts. It is even possible that our study did include such lesions, since we cannot reliably make the distinction of small simple cysts from small high-density cysts.
In conclusion, small hypoattenuating renal masses can be characterized at contrast-enhanced CT with reasonable accuracy by subjective impression and CT attenuation; lesions that appear possibly to definitely solid on visual inspection or have an attenuation value of 50 HU or more are likely to be renal cell carcinomas.