Kidney tumor

Benign / borderline adult tumors

Mixed epithelial and stromal tumor

Editorial Board Member: Maria Tretiakova, M.D., Ph.D.
Editor-in-Chief: Debra L. Zynger, M.D.
Bonnie Choy, M.D.

Last author update: 8 September 2020
Last staff update: 7 February 2024

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PubMed Search: Mixed epithelial and stromal tumor

Bonnie Choy, M.D.
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Cite this page: Choy B. Mixed epithelial and stromal tumor. website. Accessed May 19th, 2024.
Definition / general
  • Uncommon neoplasm composed of variable cystic and solid components with morphologically diverse epithelial and stromal elements
  • Classified as part of MEST family, with adult cystic nephroma at opposite end of spectrum, by the 2016 World Health Organization (WHO) classification
Essential features
  • Typically occur in perimenopausal women, often with long term hormonal treatment
  • Solitary, well circumscribed tumors, with variable solid and cystic components
    • Versus adult cystic nephroma - predominantly cystic tumors with no solid components
  • Stroma and epithelium with wide variety of histologic features
    • Versus adult cystic nephroma - noncommunicating epithelial-lined cysts separated by fibrous septae
  • Mostly benign but rare malignant transformation of epithelial or stromal component
  • Stromal cells typically positive for ER / PR
ICD coding
  • ICD-O: 8959/0 - Mixed epithelial and stromal tumor
  • Little is known
  • Epithelial and stromal components appear to arise from common cell of origin; same pattern of nonrandom inactivation of X chromosome seen in both components (Am J Surg Pathol 2011;35:1114)
Clinical features
  • Diagnosis by histologic examination of tissue
Radiology description
Radiology images

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Prognostic factors
Case reports
  • Nephron sparing surgery (partial nephrectomy), whenever feasible; radical nephrectomy if not possible
Gross description
Gross images

Contributed by Bonnie Choy, M.D.

Solitary, well circumscribed

Mixed solid and cystic

Microscopic (histologic) description
Microscopic (histologic) images

Contributed by Bonnie Choy, M.D. and @katcollmd on Twitter

Predominantly cystic area

Predominantly solid area

Hypocellular stroma

Hypercellular stroma

Cuboidal epithelium

Hobnailed epithelium

Mixed epithelial and stromal tumor Mixed epithelial and stromal tumor Mixed epithelial and stromal tumor Mixed epithelial and stromal tumor

Mixed epithelial and stromal tumor





Cytology description
Positive stains
Negative stains
Electron microscopy description
  • Spindle cells in stromal component:
    • Nonspecific features of fibrocystic cells (Am J Surg Pathol 2000;24:958)
    • Myogenic features: cytoplasmic fibrils consistent with myofilaments, subplasmalemmal aggregates of thin filaments
    • Characteristics of smooth muscle differentiation: number of pinocytic vesicles and dense plaques, near continuous external lamina and relative paucity of rough endoplasmic reticulum
  • Intercellular spaces in stromal component:
    • Filled with collagen fibrils
Molecular / cytogenetics description
  • Similar mRNA expression profile between MEST and adult cystic nephroma may represent opposite ends of same spectrum of tumors (Am J Surg Pathol 2009;33:72)
    • Highest differentially expressed gene: insulin-like growth factor 2
    • Lowest differentially expressed gene: carbonic anhydrase II
  • Lack abnormalities in chromosomes 8, 11 or 17 seen in congenital cellular mesoblastic nephroma (Hum Pathol 2001;32:513)
  • CDC73 germ line mutation, associated with hyperparathyroidism jaw tumor syndrome, in family with MESTs (Urology 2019;124:91)
Sample pathology report
  • Right kidney, mass, partial nephrectomy:
    • Mixed epithelial and stromal tumor, measuring 2.5 cm in greatest dimension (see comment)
    • Surgical margins, negative for tumor
    • Comment: The sections show a well circumscribed tumor composed of occasional glands and cysts embedded in a variably cellular stroma. Immunohistochemical stains for ER and PR are positive in the stromal component of the tumor. Overall, the morphologic and immunohistochemical findings support the diagnosis of mixed epithelial and stromal tumor.
Differential diagnosis
Board review style question #1

A 60 year old woman with hematuria was found to have a 3.5 cm variably cystic and solid renal mass on CT imaging. Partial nephrectomy showed a well circumscribed tumor with the above morphology. The stromal component is positive for ER and PR. What is the likely diagnosis?

  1. Angiomyolipoma
  2. Cystic partially differentiated nephroblastoma
  3. Metanephric adenofibroma
  4. Mixed epithelial and stromal tumor
  5. Multilocular cystic neoplasm of low malignant potential
Board review style answer #1
D. Mixed epithelial and stromal tumor

Mixed epithelial and stromal tumor is a well circumscribed, variably solid and cystic mass. Cystic partially differentiated nephroblastoma and metanephric adenofibroma typically occur in children. Multilocular cystic neoplasm of low malignant potential does not have a cellular stroma. None of the above choices, except for mixed epithelial and stromal tumor, demonstrate immunoreactivity for ER and PR.

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Reference: Mixed epithelial and stromal tumor
Board review style question #2
Which of the following is true about mixed epithelial and stromal tumor of the kidney?

  1. Commonly bilateral and multiple
  2. Harbor DICER1 gene mutations
  3. Most behave in benign fashion
  4. Typically occur in male patients
Board review style answer #2
C. Most behave in benign fashion

Mixed epithelial and stromal tumor is typically seen in perimenopausal women. These tumors are solitary, with very rare exceptions. Most of these tumors have benign behavior following surgical resection. Unlike pediatric cystic nephroma, DICER1 gene mutations are not found in mixed epithelial and stromal tumor.

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Reference: Mixed epithelial and stromal tumor
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