In many institutions around the world, including our own department, ultrasound is considered first choice imaging modality when a renal lesion is suspected clinically. Due to the high diagnostic quality of today's state of the art ultrasound equipment a constantly increasing number of both benign and malign focal renal lesions of smaller and smaller size is detected. Thus, it is of the utmost importance that the ultrasound diagnostician has in-dept knowledge of the most common focal renal lesions and the variance of their presentation on an ultrasound scan as well as the differential diagnoses one has to consider. Many algorithms have been proposed as classification system for focal renal lesions. In daily routine work the most useful may be the widely accepted approach to do an initial differentiation between solid and cystic lesions. In case malignancy is suspected based on the ultrasound appearance further diagnostic work-up with CT or MRI should be recommended. On the other hand, typical benign lesions specifically should be reported as such to avoid further diagnostic work-up.
The reverse sequence of imaging studies i.e. an equivocal lesion diagnosed on CT or MRI is referred for further evaluation by ultrasound may occur in an institution where high level ultrasound expertise is available or if percutaneous ablation is considered. In some cases contrast enhanced ultrasound (CEUS) may be helpful, but CEUS does rarely at its present stage play a decisive role. In such cases US-Guided biopsy may be considered the ultimate solution to reach a final diagnosis.