Discussion

Chromophobe RCC accounts for 5% of all RCC’s, and patients with Birt-Hogg-Dubé syndrome are at 7 times greater risk than the general population. It is more often incidental than symptomatic, but often large (mean 4.6-8.0 cm) and caught at an early stage. Chromophobe subtype carries a better prognosis than most RCC’s, with a 5-year survival > 90% and 10-year recurrence free survival of 79%. Staging is with the same TNM criteria used for other types of RCC’s, and treated with either nephrectomy or nephron-sparing techniques depending on size. 

Qualitative imaging features of chromophobe RCC’s are not specific. It is typically solid, well-circumscribed with lobulated borders and heterogeneous. It may have calcifications (14-34%) and occasionally has cystic components but rarely contains fat. A central stellate scar is seen in 19-34% of chromophobe RCC’s, which can also be seen in oncocytoma (a benign tumor).  Chromophobe RCC’s reach peak enhancement in the nephrographic phase and generally enhance less than that of the more common clear cell RCC, but more than a papillary RCC. 

Ultrasound findings are quite non-specific and qualitative MRI assessment cannot differentiate from oncocytoma. Retrospective studies on quantitative MRI values have shown that chromophobe can be differentiated from oncocytoma, but this has not been widely validated. Lipid-poor AML’s have high attenuation on non-contrast CT, earlier enhancement in the corticomedullary phase, and consistency low T2 signal intensity, unlike chromophobe RCC which has high T2 signal intensity.

The updated 2019 Bosniak classification cannot be applied to renal lesions with greater than 25% solid component. Recommendations and flowcharts for the workup of incidental solid renal masses are outlined by the CAR Incidental Finding Working Group (2019) and the ACR White Paper (2017).