The incidence of renal cysts is rising due to increased abdominal imaging. Renal complicated cysts have been traditionally classified according to the Bosniak classification, which distinguishes cystic masses by specific features of walls and septa. The categories I and II are benign and class IIF most probably benign but needs a short radiological follow-up. Categories III and IV have been traditionally operated due to the increased risk of renal cell carcinoma. However, recently published studies show that approximately 50% of the operated Bosniak III cystic masses are benign, which means that half of the cases are overtreated by surgery. It has also been shown that surgical pathology of stable Bosniak IIF cysts is malignant in less than 1%, while the cysts, which are upgraded to higher Bosniak classes will show malignant surgical pathology in 85%. So far, there is lack of prospective data on active surveillance in Bosniak III cystic masses.

The aim of the study is to compare active surveillance and surgery in patients with Bosniak III renal cystic masses. Patients will be randomized in active surveillance or immediate surgical excision of a cystic mass. In the active surveillance group, patients are followed according to the study protocol for 10 years and treated with delayed surgery if the cystic mass upgrades into Bosniak IV/solid, becomes symptomatic or grows over a preclassified threshold. The primary objective is to compare surgical pathology between patients treated with immediate surgery versus delayed surgery.

According to recent retrospective data, active surveillance of Bosniak III cystic masses is reasonable and oncologically safe. Therefore a prospective randomized controlled trial is needed to get high level evidence to support a change in the treatment strategy. The study may significantly reduce unnecessary operations performed in patients with Bosniak III cystic masses.

Inclusion Criteria:

• A sporadic single CM of Bosniak 3 according to the Bosniak Classification v.2019 is diagnosed with CT or MRI
• Maximal diameter of CM 10-70 mm
• Age ≥50 years
• ECOG performance status <2
• Life expectancy ≥5 years
• Patient is fit to undergo surgery and AS.
• Patient understands a national language or English
• Signed informed consent

Exclusion Criteria:

• Genetic syndromes associated with RCC
• Previously or simultaneously diagnosed and pathologically verified RCC
• Previously or simultaneously radiographically identified solid mass or CM of Bosniak 3/4 with diameter ≥10mm
• The target CM of Bosniak 3 has progressed in sequential imaging from Bosniak 1-2F
• Presence of radiographic findings which are suspect for nodal or distant metastatic disease
• Symptomatic CM
• Kidney insufficiency (GFR<55 ml/min/1,73m2)
• Patients who have contraindications for both CT and MRI imaging. . Anatomically solitary kidney

All IPD that underlie results in a publication.

Starting 6 months after publication.

Requests are reviewed by the steering committee.