-To became confident with the more and less common cystic renal masses and with their Imaging behaviour -To discuss the additional contribution of the "new" US technique (CEUS, elastography when possible).

The classification of renal epithelial neoplasms significantly changed during the last 3 decades because of advantages in immunoistochemistry, cytogenetics and molecluar pathology. After the WHO 2004 classification, other tumor entities were identified, a fact that led to the more recent ISUP Vancouver classification of renal neoplasms. When considering cystic masses, besides the more common variants of solid neoplasms, we can find uncommon mesenchymal tumors and 5 new histotypes ( Table 1 Table 2 ). Their characterization is now a new challenge for Imaging.

A) COMMON LESIONS In this group we can find the cystic variants of classical solid lesions. 1) Relatively uncommon (2-3% of all cases of RCC), it shows the best prognosis of all cases of RCC, it was forst described in 1928, multilocular cystic clear cell carcinoma (Fig 1, Fig 2) was included in the WHO 2004 classification. They are characterized by a malignant cell lining and small collections of cells without formation of a malignant nodule (Fig. 1 ). The patologic criteria for the diagnosis...

The correct interpretation of complex cystic lesions - besides Bosniak classification - is very hard to be obtained, but it should be helpful, differentiating aggressive lesions from more indolent one, all presenting with Bosniak III or IV appearence. Although CT and MRI are still the gold standard, an important role can be played by ultrasoud with its "new" applications. CEUS infact is able to exactly depict the distribution and the timing of the vascularization. ARFI can be useful in the evaluation of the solid components,...

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