Multilocular cystic nephroma (MCN) is a rare benign cystic neoplasm of the kidney. It exhibits a bimodal age distribution — peaking in boys <4 years and women aged 40-60 years. It presents as a well-circumscribed, encapsulated, entirely cystic multilocular mass. Thin to moderately thick septa are present between cyst cavities but NO solid nodular component — this is the most critical distinguishing feature. The lesion characteristically may herniate into the renal pelvis (herniation sign). Histologically, cysts are lined by hobnail epithelium and ovarian-type stroma may be seen (especially in adult women). In the WHO 2022 classification, it is categorized within the 'mixed epithelial and stromal tumor' (MEST) family. Treatment is generally partial or total nephrectomy because preoperative definitive distinction from cystic renal cell carcinoma (especially Bosniak III-IV) is not possible. Prognosis is excellent — no recurrence expected after complete resection.
Age Range
1-65
Peak Age
-
Gender
Female predominant
Prevalence
Rare
The exact etiology of multilocular cystic nephroma is not fully elucidated, but in the WHO 2022 classification it is categorized within the mixed epithelial and stromal tumor (MEST) family. The adult female form suggests a hormonal association — the presence of ovarian-type stroma correlates with estrogen and progesterone receptor positivity, which may explain the marked female predominance. The pediatric form may arise from nephrogenic rests (nephroblastic remnants) — hence the older terminology 'cystic partially differentiated nephroblastoma'. Histopathologically, the lesion consists of numerous cyst cavities of variable size within a fibrous capsule. Cysts are lined by flat or hobnail epithelium (cuboidal, columnar, or attenuated). The septal stroma contains variable amounts of fibroblasts, smooth muscle cells, and in adult women, characteristic ovarian-type stroma. Critically, blastemal or embryonal elements are ABSENT — this feature distinguishes it from Wilms tumor. Capillaries and small vessels within the septa form the physiological basis for septal enhancement on contrast-enhanced imaging. Cyst fluid may be serous or mildly hemorrhagic. Herniation of the lesion into the renal pelvis results from the lesion displacing sinus fat and growing toward the collecting system — this characteristic finding helps distinguish MCN from other cystic masses.
Herniation of multilocular cystic nephroma into the renal pelvis — the lesion displaces renal sinus fat and extends toward the collecting system, protruding into the pelvic lumen. Best seen as a filling defect within contrast-filled pelvis on excretory phase. This finding is highly characteristic of MCN (rarely seen in other cystic renal neoplasms) and serves as an important diagnostic clue. The herniation sign reflects the lesion's tendency to grow into the renal sinus — resulting from MCN's close anatomical relationship with the renal pelvis.
Well-circumscribed, encapsulated, multilocular cystic mass on non-contrast CT. Cyst cavities are near water density (0-20 HU). Thin septa may be visible between cavities but are not prominent on non-contrast imaging. Calcification is rare and may appear as thin curvilinear intraseptal calcification. The mass is clearly demarcated from renal parenchyma and surrounded by a fibrous capsule. Size is variable, generally ranging from 5-15 cm. If hemorrhagic cyst components are present, density may increase to 20-40 HU.
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A well-circumscribed, encapsulated, multilocular cystic mass is seen in the kidney; cyst cavities are near water density without solid component.
On portal venous phase, septa show thin, smooth enhancement — this is the most important CT finding of MCN. Septal thickness is generally <3 mm with smooth and uniform enhancement. NO enhancing solid nodule or irregular thick septa — absence of this finding distinguishes from malignant cystic neoplasm. Cyst cavities do not enhance (remain at water density). The capsule may show thin, smooth enhancement. Herniation sign may be observed at the interface with renal pelvis — protrusion of the lesion toward the collecting system. This combination of findings corresponds to Bosniak II-F or III classification.
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On portal venous phase, septa show thin smooth enhancement without enhancing solid nodule or thick irregular septa; consistent with multilocular cystic nephroma.
On delayed phase (especially excretory phase), herniation of the lesion into the renal pelvis (herniation sign / pelvic protrusion) is best evaluated. The lesion displaces renal sinus fat and extends toward the collecting system, protruding into the pelvic lumen. On excretory phase, it may be seen as a filling defect within the opacified collecting system. This finding is highly characteristic of MCN and rarely seen in other cystic renal masses. Septal enhancement continues or slightly decreases on delayed phase — does not show progressive washout. Cyst cavities do not enhance on delayed phase either.
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On delayed phase, herniation of the cystic mass into the renal pelvis is observed; this finding is characteristic of multilocular cystic nephroma.
Multilocular cystic structure is best evaluated on T2-weighted sequences. Cyst cavities show homogeneous high signal intensity (consistent with simple fluid). Septa appear as thin low signal intensity structures on T2 — reflecting fibrous tissue content. Variable-sized cyst cavities may create a 'cluster of grapes' appearance. Septal thickness is uniform and smooth without nodular thickening. Signal intensity of cyst cavities may vary slightly depending on protein content — T2 signal may decrease in hemorrhagic components. Capsule appears as a thin low signal intensity rim on T2. MRI superiority over CT lies in much more detailed evaluation of septal architecture and cyst content.
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On T2-weighted sequences, a multilocular cystic structure with high signal cavities and thin smooth low-signal septa is observed; no solid nodular component is identified.
On T1-weighted sequences, cyst cavities generally show low signal intensity (consistent with simple fluid). However, cavities with high protein content or hemorrhagic component may show high T1 signal — this creates signal heterogeneity between cyst cavities and is typical for MCN. Septa appear as thin low-to-intermediate signal intensity structures on T1. Capsule is of low signal intensity. Pre-contrast T1 images form the basis for post-contrast subtraction analysis — comparison with pre-contrast T1 is necessary for accurate assessment of septal enhancement.
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Variable signal intensity is observed in cyst cavities on T1-weighted sequences, with high T1 signal in some cavities consistent with proteinaceous or hemorrhagic content.
On diffusion-weighted imaging (DWI), cyst cavities show no signal or minimal T2 shine-through effect at high b-value (b=800-1000). On ADC map, cyst fluid shows high ADC values (>2.0 × 10⁻³ mm²/s) — reflecting free water diffusion. Septa do not show significant diffusion restriction on DWI. This finding is important because malignant cystic tumors show low ADC values (<1.2 × 10⁻³ mm²/s) as diffusion restriction in septa or solid components, reflecting high cellularity. Absence of diffusion restriction in MCN supports benign nature and may help differentiate from cystic RCC.
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No significant diffusion restriction is detected in cyst cavities and septa on diffusion-weighted imaging with high ADC values; consistent with benign nature.
On B-mode ultrasound, a well-circumscribed multilocular cystic mass is seen. Cyst cavities appear anechoic (completely fluid-filled) and show posterior acoustic enhancement. Thin echogenic septa are present between cavities. Septa are smooth and of uniform thickness — NO focal nodular thickening or solid echogenic component. Low-level internal echoes (internal debris) may be seen in some cavities — consistent with proteinaceous or hemorrhagic content. Capsule may be visible as a thin, smooth, echogenic rim. US is the first-line evaluation modality in children and involves no radiation. Lesion size and septal architecture are well assessed with US but contrast-enhanced CT or MRI is needed for definitive characterization.
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On ultrasound, a well-circumscribed, multilocular, anechoic cystic mass is seen in the kidney with thin echogenic septa between cavities; no solid component is identified.
On color Doppler ultrasound, minimal or low flow is detected within septa. If intraseptal vascularity is present, it shows a low-resistance arterial flow pattern — reflecting benign vascular architecture. NO solid enhancing nodule or hypervascularity is observed. Cyst cavities are avascular (no Doppler flow detected). Power Doppler is more sensitive than color Doppler for demonstrating low-flow septal vascularity. Absence of or minimal intraseptal flow supports Bosniak II-F classification. In the presence of significant septal vascularity or nodular vascularity, a higher Bosniak category should be considered and further imaging recommended.
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Minimal vascularity is observed within septa on color Doppler without solid nodular vascularity or hypervascularity.
Criteria
Occurs in boys under 4 years of age. Male predominance is marked (M:F ≈ 2-3:1). Mean age at diagnosis is 2 years. Creates differential diagnostic difficulty due to age overlap with Wilms tumor. Previously termed 'cystic partially differentiated nephroblastoma' (CPDN) in older terminology.
Distinct Features
In pediatric form, stroma does not show ovarian-type features — fibroblastic stroma predominates. Cysts may contain immature tubular structures. Blastemal elements are ABSENT (differentiating criterion from Wilms tumor). Generally unilateral. Size is generally smaller (mean 5-8 cm). Preoperative imaging distinction from cystic Wilms tumor is very difficult — definitive diagnosis is generally made by postoperative pathological examination.
Criteria
Peaks in women aged 40-60 years. Marked female predominance (F:M ≈ 8-10:1). Presence of ovarian-type stroma defines this form. Estrogen and progesterone receptor positivity supports hormonal association. Classified within mixed epithelial and stromal tumor (MEST) family in WHO 2022.
Distinct Features
In adult female form, ovarian-type stroma (ER/PR positive) is characteristic — not present in pediatric form. Hormone sensitivity is associated with the hormonal milieu in premenopausal women. Size is generally larger (mean 8-15 cm). Herniation into renal pelvis is more prominent in this form. Cyst fluid may be serous or mildly hemorrhagic. This is the form most commonly confused with cystic RCC — preoperative distinction is difficult. Some authors position this form at the cystic end of the MEST spectrum.
Criteria
In WHO 2022 classification, MCN is categorized within the MEST family. The cystic-dominant variant of MEST overlaps with MCN. MEST generally occurs in adult women and may contain both cystic and solid components. Purely cystic forms are classified as MCN, those with significant solid component as MEST.
Distinct Features
In MEST-associated form, unlike purely cystic MCN, minimal solid stromal component may be observed — this component is smooth-bordered, thin, and shows mild enhancement after contrast. Ovarian-type stroma is more prominent. Association with hormonal therapy (HRT) use has been reported. Immunohistochemically, ER, PR, SMA, and desmin positivity is typical. Malignant transformation is extremely rare but has been reported — particularly if solid component is prominent, careful pathological evaluation is necessary. Prognosis is excellent after complete resection.
Distinguishing Feature
Cystic RCC has solid enhancing nodular component, thick irregular septa (>3 mm), or irregular wall thickening — these are ABSENT in MCN. In cystic RCC, septal enhancement is irregular and nodular while in MCN it is smooth and thin. Cystic RCC is generally classified as Bosniak III-IV. However, preoperative distinction can be very difficult and definitive diagnosis is usually made by pathological examination. On DWI, diffusion restriction may be seen in solid components of cystic RCC (low ADC), not seen in MCN.
Distinguishing Feature
Bosniak III cysts are a heterogeneous category encompassing various pathologies including MCN. Bosniak III has thick irregular septa (≥3 mm) and/or measurable septal/wall enhancement. MCN is typically classified as Bosniak II-F or low III because septal enhancement is present but smooth and thin. Classic Bosniak III lesions carry 40-60% malignancy risk while MCN is benign. Key difference: MCN NEVER has solid enhancing nodules, septa are smooth and uniform. However, this distinction may not be definitively made by preoperative imaging.
Distinguishing Feature
Cystic/necrotic variant of clear cell RCC can mimic MCN, but distinct differences exist: (1) Clear cell RCC has solid component with avid arterial enhancement — absent in MCN; (2) RCC shows irregular thick septa and nodular enhancement pattern; (3) Diffusion restriction (low ADC) is detected in solid components of RCC on DWI; (4) Washout pattern seen in RCC — avid arterial enhancement with decrease in portal venous phase; (5) RCC is generally heterogeneous with irregular-bordered necrotic areas. MCN is entirely cystic, well-circumscribed, with smooth thin septal enhancement.
Distinguishing Feature
Simple renal cyst is unilocular (single cavity), contains NO septa, has very thin smooth wall. Classified as Bosniak I. On US, shows completely anechoic content, thin smooth wall, and prominent posterior acoustic enhancement. MCN is multilocular (numerous cavities), contains thin septa, and is classified as Bosniak II-F or III. On contrast-enhanced CT/MRI, simple cyst shows no enhancement at all, while MCN shows septal enhancement. Clinically, simple cyst is very common (present in 50% of population >50 years) and requires no follow-up or treatment, while MCN is rare and requires surgical evaluation.
Urgency
routineManagement
surgicalBiopsy
Not NeededFollow-up
specialist-referralMultilocular cystic nephroma is a benign neoplasm and prognosis is excellent. However, management strategy is surgically oriented because definitive preoperative distinction from cystic RCC by imaging is not possible. Partial nephrectomy (nephron-sparing surgery) is the preferred approach — especially if lesion localization and size permit. Total nephrectomy may be necessary for large or central lesions. Percutaneous biopsy is generally not recommended because samples from cystic lesions may be non-diagnostic and biopsy carries complication risk (hemorrhage, tumor seeding — if malignancy cannot be excluded). Definitive diagnosis is generally made by postoperative pathological examination. No recurrence is expected after complete resection and long-term follow-up is not routinely required. In pediatric patients, surgery is always recommended to exclude Wilms tumor. In adults, for small lesions classified as Bosniak II-F, active surveillance (6-12 month CT/MRI follow-up) may be considered as an alternative, but patient education and shared decision-making are important.
While multilocular cystic nephroma is benign, surgical excision (partial nephrectomy) is generally recommended because definitive differentiation from cystic RCC is not possible radiologically. Recurrence is rare and prognosis is excellent.