With the widespread use of medical imaging (ultrasound, CT, and even MRI or PET-CT), kidney cysts are being detected more and more often. How to deal with these cysts is very disturbing for the patients involved. Although most of the kidney cysts are benign, which ones are malignant? Are those cystic kidney cancers malignant from cysts? This is a frequently asked question, sometimes it is not clear because it will hurt people if it is clear, more often it is not clear because many scientific issues are not clear yet. First of all, can kidney cysts become malignant? Is cystic kidney cancer malignant from cysts? So far, no scientific evidence can be found to answer this question. From my personal knowledge and experience, cyst is cyst and cystic kidney cancer is cystic kidney cancer, there is no relationship between them, if we must say there is a relationship, it is like a sheep and a wolf in sheep’s clothing. If we say so, there will be some people who will say, “So-and-so found a kidney cyst many years ago, and then it became malignant and became cystic kidney cancer, didn’t it?” It’s true. However, after carefully reviewing some of the cases with complete imaging data (patients with plain + enhanced CT scan from the beginning), we found that at the beginning of the discovery of renal cyst, the cyst was actually a cystic kidney cancer, but it was not a cyst at all, but a cystic kidney cancer mistaken as a renal cyst. The cyst was mistaken for a kidney cyst. One patient found “kidney cyst” 5 years ago, by chance, through his friend (also my friend) asked me to look at his CT film, and was shocked at first glance, it is not cystic kidney cancer? And it was so big that there was an enlarged lymph node (i.e. there was probably lymph node metastasis). He was immediately told that the cyst had turned into cystic kidney cancer (a very typical kind) and needed surgery as soon as possible. He accepted the advice and underwent a classical radical kidney cancer surgery, i.e. the whole kidney and its surrounding fatty capsule, perinephric fascia, adrenal glands and regional lymph nodes were removed. Post-operative detailed pathology confirmed a renal clear cell carcinoma with one lymph node metastasis. He is alive today, more than 10 years. What would have been the effect on him if he had been told at that time that cystic kidney cancer had been misdiagnosed for 5 years? What is the impact on the doctors who have been diagnosing him with renal cysts? And what is the impact on the treatment at the moment? I am afraid there is no positive impact except the negative one. Why is cystic kidney cancer misdiagnosed as renal cyst? How to distinguish renal cyst from cystic kidney cancer? This is also a question that is often asked. Actually, it is not clear in three words. Why is cystic kidney cancer misdiagnosed as kidney cyst? To put it simply, some cystic kidney cancers are too similar to kidney cysts, so they are misdiagnosed. How to distinguish kidney cyst from cystic kidney cancer? There are two points that we should pay special attention to, firstly, we should clarify the characteristics of renal cyst and cystic kidney cancer, and secondly, to diagnose and distinguish renal cyst from cystic kidney cancer, we must do plain scan + enhanced thin layer CT scan. According to the characteristics of CT scan, cystic occupying lesions in kidney are divided into simple renal cyst and complex renal cyst, the former has smooth inner wall without separation, while the latter has separation, unsmooth inner wall or nodules, especially if there are nodules with irregular inner wall and enhanced nodules on enhanced CT scan, it should be highly suspected to be cystic renal cancer, in fact, 70-80% of renal cysts with such performance can be cut down for pathological examination. are cystic kidney cancer. What should be done after kidney cyst is found? If the cyst is simple, the cyst is not large and does not affect the kidney function, it can be observed regularly (ultrasound examination once a year). If the cyst is complex, it should be treated with great caution. The safest approach is to remove the cystic lesion in its entirety as in the case of renal cancer, as puncture or debulking surgery may lead to catastrophic consequences. If the cyst is relatively small (e.g., within 2 cm) and cannot be determined with certainty for a while, regular ultrasound examinations and CT scans for close observation may be better than hasty treatment.