BACKGROUND: Up until now, no research has been reported on the association between the clinical growth rate of multilocular cystic renal neoplasm of low malignant potential (MCRNLMP) and computed tomography (CT) imaging characteristics. Our study sought to examine the correlation between them, with the objective of distinguishing unique features of MCRNLMP from renal cysts and exploring effective management strategies.
OBJECTIVE: To investigate optimal management strategies of MCRNLMP.
METHODS: We retrospectively collected and analyzed data from 1520 patients, comprising 1444 with renal cysts and 76 with MCRNLMP, who underwent renal cyst decompression, radical nephrectomy, or nephron-sparing surgery for renal cystic disease between January 2013 and December 2021 at our institution. Detection of MCRNLMP utilized the Bosniak classification for imaging and the 2016 World Health Organization criteria for clinical pathology.
RESULTS: Our meticulous exploration has revealed compelling findings on the occurrence of MCRNLMP. Precisely, it comprises 1.48% of all cases involving simple renal cysts, 5.26% of those with complex renal cysts, and a noteworthy 12.11% of renal tumors coexisting with renal cysts, indicating a statistically significant difference (P = 0.001). Moreover, MCRNLMP constituted a significant 22.37% of the patient population whose cysts demonstrated a rapid growth rate of ≥ 2.0 cm/year, whereas it only represented 0.66% among those with a growth rate below 2.0 cm/year. Of the 76 MCRNLMP cases studied, none of the nine patients who underwent subsequent nephron-sparing surgery or radical nephrectomy following renal cyst decompression experienced recurrence or metastasis. In the remaining 67 patients, who were actively monitored over a 3-year postoperative period, only one showed suspicious recurrence on CT scans.
CONCLUSIONS: MCRNLMP can be tentatively identified and categorized into three types based on CT scanning and growth rate indicators. In treating MCRNLMP, partial nephrectomy is preferred, while radical nephrectomy should be minimized. After surgery, active monitoring is advisable to prevent unnecessary nephrectomy.

OBJECTIVE: TFE3-rearranged renal cell carcinomas (RCCs) harbor gene fusions between TFE3 and 1 of many partner genes. MED15::TFE3 fusion RCC is rare, often cystic, and easily misdiagnosed.
METHODS: This study aimed to characterize 2 cases of MED15::TFE3 fusion RCC with extensive cystic change using fluorescence in situ hybridization and targeted RNA sequencing.
RESULTS: Both patients were young adult women aged 29 and 35 years. Radiologically, both presented with a cystic Bosniak category II renal lesion. The cysts measured 9.3 cm and 4.8 cm in greatest dimension. Both patients underwent cyst enucleation, and neither had tumor recurrence or metastasis at 26 and 6 months of follow-up, respectively. Microscopically, both tumors were entirely cystic, with thick, fibrous cystic walls lined by small clusters of cells with clear to eosinophilic cytoplasm and uniform, round nuclei with inconspicuous nucleoli. There were also small aggregations of similar clear cells within the cystic walls. Foci of basement membrane-like material depositions were noted in 1 case; calcifications were observed in both cases. Both cases demonstrated nuclear positivity for PAX8 and TFE3 and cytoplasmic staining for Melan-A; HMB45, CAIX, and CK7 were negative. Fluorescence in situ hybridization revealed that both tumors were positive for TFE3 rearrangements. RNA sequencing identified MED15::TFE3 gene fusions in both cases.
CONCLUSIONS: The main differential diagnosis of MED15::TFE3 fusion RCC includes multilocular cystic renal neoplasm of low malignant potential and atypical renal cysts. Molecular confirmation of TFE3 fusion is essential for establishing the correct diagnosis.

BACKGROUND: For clear cell renal cell carcinoma (ccRCC) with cystic component similar to multilocular cystic renal neoplasm of low malignant potential (MCRN-LMP) and solid low-grade component simultaneously, we propose the designation \"ccRCC with cystic component similar to MCRN-LMP\" and to study the relationship between MCRN-LMP and it.
METHODS: Twelve cases of MCRN-LMP and 33 cases of ccRCC with cystic component similar to MCRN-LMP were collected from 3,265 consecutive RCCs to compare them in clinicopathological features, immunohistochemical findings (PAX8, CA-IX, CK7, Vimentin, CD10, P504s, TFE3, 34βE12) and prognosis.
RESULTS: There was no significant difference in age, sex ratio, tumor size, treatment, grade and stage between them (P > 0.05). All ccRCCs with cystic component similar to MCRN-LMP coexisted with MCRN-LMP and solid low-grade ccRCCs, and MCRN-LMP component ranged from 20 to 90% (median, 59%). The positive ratio of CK7 and 34βE12 in MCRN-LMPs and ccRCCs\' cystic parts was significantly higher than that in ccRCCs\' solid parts, but the positive ratio of CD10 in MCRN-LMPs and ccRCCs\' cystic parts was significantly lower than that in ccRCCs\' solid parts (P < 0.05). There was no significant difference of all immunohistochemistry profiles between MCRN-LMPs and ccRCCs\' cystic parts (P > 0.05). No patient developed recurrence or metastasis.
CONCLUSIONS: MCRN-LMP and ccRCC with cystic component similar to MCRN-LMP have similarity and homology in clinicopathological features, immunohistochemical findings and prognosis, and form a low-grade spectrum with indolent or low malignant potential behavior. The ccRCC with cystic component similar to MCRN-LMP may be a rare pattern of cyst-dependent progression from MCRN-LMP.