Papillary Adenoma of the Kidney

Definition

Among renal cell neoplasms, the term adenoma is reserved for those with a papillary or tubular pattern
- No equivalent benign lesion is recognized for clear cell, chromophobe, collecting duct or medullary carcinomas
All of the following criteria must be fulfilled
- Must have only papillary or tubular architecture
  - May not have clear or chromophobe cells
    - All such lesions should be considered carcinomas
- May be no larger than 0.5 cm
  - Any larger lesion should be considered carcinoma
- Must be low grade cytologically
  - Fuhrman/nuclear grade 1 or 2
    - Any higher grade lesion should be considered carcinoma
  - Absent to inconspicuous nucleoli
  - Mitotic figures very rare
- Microscopically localized but not encapsulated
  - May be multiple
    - If numerous, consider adenomatosis
      - Generally >50 in reported cases, but number required is not well defined
      - Significance is not well characterized
  - May have an irregular interface with surrounding tissue
    - No desmoplastic response
- Lining cells usually small with scant pale cytoplasm
  - Similar to papillary carcinoma type 1
  - Occasional cases with abundant eosinophilic cytoplasm
    - Similar to papillary carcinoma type 2
- Psammoma bodies and foamy macrophages may be seen
- Similar changes in lining of Bowman capsule termed "adenomatoid metaplasia of Bowman capsule"

John P Higgins MD
Robert V Rouse MD
Department of Pathology
Stanford University School of Medicine
Stanford CA 94305-5342