Multilocular cystic renal neoplasm of low malignant potential
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Multilocular cystic renal neoplasm of low malignant potential (MCRNLMP) are low-grade adult renal tumors composed entirely of numerous cysts. The entity was previously known as multilocular cystic renal cell carcinoma, which usually had clear cell morphology, but was redefined in the 2016 WHO classification of tumors of the kidney due to no recurrence or metastasis in patients with these tumors 1.
The reported incidence of MCRNLMPs is 1-2% of all renal tumors 2,3. They are typically encountered in middle-aged adults 5. There is a slight male predilection 5.
As these tumors are slow-growing and have an indolent natural history they are usually asymptomatic and are found incidentally when imaging of the kidneys is carried out for other indications 5. Occasionally, if the tumor is large it can present with mass effect, hematuria or flank pain 5.
MCRNLMPs have histopathological features and genetics that are similar to that of clear cell renal cell carcinoma 5. These malignant cells are found in small groups within the septae of adjacent cysts without expansive growth 5.
The epithelial cells lining the cysts and the clusters of clear tumor cells show positivity for epithelium markers like epithelial membrane antigen (EMA), cytokeratin 7 (CK7), and carbonic anhydrase IX (CA-IX), which can be useful in making a more precise diagnosis and discriminating it from other cystic renal lesions 2.
MCRNLMP is a multicystic lesion without nodular solid component. The cysts have thin walls and are filled with simple or heterogenous fluid 5. Contrast enhanced ultrasound has been shown to be as effective as CT and MRI for lesion characterization 5.
MCRNLMPs appear as entirely cystic lesions with a number of intervening septae.
The Bosniak classification system of renal cystic lesions is used to classify such renal lesions to anticipate the underlying malignant potential. These are usually classified as Bosniak type IIF (minimally complex cystic) or III (indeterminate cystic) lesions 2,5.
Although MCRNLMPs cannot be accurately differentiated from other cystic renal cell carcinomas clinically or radiologically, it has been suggested that the density measurement in the cyst septae during the corticomedullary phase can be helpful. Other cystic renal cell carcinomas have a significantly higher value during this phase of the scan and a cut-off value of ≥38 HU has a sensitivity of 83% and specificity of 80% 2,4.
MRI can be helpful in further assessing indeterminate lesions of CT or for patients who cannot have CT contrast 5.
Treatment and prognosis
The prognosis of these tumors is excellent. There are no reported cases of metastatic spread or local recurrence/growth 5.