A 55-year-old man presented with a large cystic mass of the left kidney without any subjective discomfort. Fifteen years before admission, he was diagnosed with a left renal cyst and subsequently underwent three paracenteses combined with anhydrous alcohol sclerotherapy. Thereafter, he received regular color Doppler ultrasound or computed tomography (CT) examinations. The objective data showed the cystic lesion measuring 6.4 x 5.5 cm in 2013, 9.5 x 7.6 cm in 2017, and 10.0 x 8.2 cm in 2019 with partial cyst wall calcification ( Figures 1A-C ). He had a medical history of blood transfusion due to gastric perforation 31 years earlier and a 13-year history of multiple hepatic cysts. Physical examination and routine blood tests revealed no abnormalities.
An admission dynamic contrast-enhanced CT revealed a mixed-density mass dominated by low density in the left kidney, measuring 10.3 x 8.5 cm in cross-section, with multiple high-density patches at the rim ( Figures 1D-F ,  eFigure 1 ). No distinct change in mass density was found during the phase of enhancement, and the imaging impression was a complex cyst (Bosniak classification grade IIF). Laparoscopic partial nephrectomy (PN) of the left kidney was then performed. Intraoperative inspection revealed the cystic mass occupying the upper pole of the left kidney, with an uneven surface and adhesion to surrounding fat. When a minor incision of about 0.5 cm was made, a large quantity of jelly-like mucus and bean-curd-like necrotic tissue was found filling the lesion. On gross specimen examination, the removed cystoid tissue was approximately 9.2 x 6.2 x 1.8 cm in size and 0.2 x 0.9 cm in wall thickness, with a smooth outer wall and a rough inner wall. Microscopically, extensive mucus was found in the fibrocyst wall, and the tumor cells were scattered or glandular in arrangement ( Figure 2A ). A pathological diagnosis of mucinous adenocarcinoma was presented. Immunohistochemistry (IHC) demonstrated that the tumor cells were positive for PCK, Villin, CDX2, SATB2 (some cells), Syn (a few cells), and Ki67 (50%) but negative for CgA, Napsin A, TTF-1, GATA-3, CK7, PAX-8, CA9, RCC, and P63. ( Figures 2C-H ,  eFigure 2 ).
The patient underwent further general examination. Painless gastroscopy and colonoscopy revealed no abnormal lesions, while the carbohydrate antigen 19-9 (CA19-9), carbohydrate antigen 72-4 (CA72-4), and carcinoembryonic antigen (CEA) indexes were elevated (79.20 U/ml, 50.10 U/ml, 5.24 ng/ml, respectively). A CT scan of the head and chest showed no evidence of metastasis, and a repeated abdominal CT scan revealed a localized encapsulation of gas and effusion ( Figures 1G-I ).
Subsequently, laparoscopic radical nephrectomy (RN) of the left kidney was performed on June 19, 2020, during which dense adhesions with adjacent organs were observed. In gross specimen examination, a cystic mass of approximately 6.0 x 5.0 x 5.0 cm was seen immediately within the renal capsule with no visible invasion of the collecting system ( Figure 3 ). The cut surface of the tumor was gray, with hemorrhage and necrotic tissue invading the capsule. Histopathological examination demonstrated fibrous cystic tissue in the renal parenchyma ( Figure 2B ). A few mucinous adenocarcinoma cells with multiple mucinous foci and scattered calcifications were found in the cystic wall. No tumor involvement was observed in the ureteral stump. Based on the above examinations, intraoperative and pathological findings, the final diagnosis of mucinous adenocarcinoma derived from the left renal parenchyma was proposed.
One month after the radical surgery, a dynamic contrast-enhanced CT scan revealed a cystic lesion in the original left kidney area measuring 8.0 x 5.7 cm in cross-section ( Figures 4A, B ). Multidisciplinary consultation suggested that the patient should be given salvage therapy. From July 2020 to May 2021, he received eight cycles of chemotherapy with the SOX (oxaliplatin plus S-1) regimen in the first cycle and then the XELOX (oxaliplatin plus capecitabine) regimen in cycles 2-8 due to a general rash during oral S-1. After completion of chemotherapy, he was then subjected to 28 local radiotherapies at the original left renal area delivered by volumetric modulated arc therapy (VMAT) with a total dose of 5600 cGy in 200 cGy per fraction.
Periodic dynamic contrast-enhanced CT scans showed a gradual reduction in the cystic lesion in the left renal region, and a re-examination on August 2, 2021, showed that the lesions had disappeared ( Figures 4C, D ). Until this report, the patient remained stable with outpatient follow-up for 30 months and no evidence of recurrence ( Figures 4E, F ). The levels of the tumor markers gradually decreased after subsequent treatments during follow-up ( eFigure 3 ). The timeline of interventions and outcomes for the patient is presented in  eFigure 4 .