Discussion of case

The main differential diagnoses in this case includes metanephric adenoma, the solid variant of papillary RCC, and adult Wilms tumor. An intriguing contradiction lies in the fact that while papillary RCC, a malignant tumor, is often encapsulated, metanephric adenoma, considered a benign tumor, typically lacks encapsulation. Wilms tumor typically exhibits two or three components—epithelial, blastema, and stromal elements—although monophasic cases are rare. Its cellular morphology appears less mature compared to metanephric adenoma, and it usually displays a higher mitotic rate.

Immunohistochemistry might prove helpful in distinguishing between metanephric adenoma and papillary RCC. Papillary RCC is positive for CK7, AMACR, and EMA, whereas metanephric adenoma usually does not show these markers. Metanephric adenoma may exhibit positivity for WT1, CD56, and CD57, while papillary RCC is usually negative for these markers. However, these immunomarkers are not useful in distinguishing metanephric adenoma from adult Wilms tumor, as they often share the same staining patterns.

Key differential diagnosis

Papillary renal cell carcinoma, particularly solid variant:
Less tightly packed tubules and less basophilic, usually has foamy macrophages
Vimentin+, PAX8+, CK7+, AMACR+, WT1-, CD57-, BRAF-

Nephroblastoma:
Difficult to differentiate if tumor is monophasic, (blastema component only)
PAX8+, WT1+, vimentin-, CD57-, BRAF-

Metanephric-like ALK rearranged renal cell carcinoma:
Similar morphology to metanephric adenoma
Vimentin+, PAX8+, CK7+, ALK+, AMACR+, BRAF-, CD57-, WT1-

Metastatic thyroid carcinoma:
Tumor cells with nuclear grooves and pseudonuclear inclusions
PAX8+, TTF1+, BRAF+, WT1-

References

1. Kinney SN, et al. Metanephric adenoma: the utility of immunohistochemical and cytogenetic analyses in differential diagnosis, including solid variant papillary renal cell carcinoma and epithelial-predominant nephroblastoma. Mod Pathol. 2015;28(9):1236-48. PMID: 26248896
2. Pinto A, Signoretti S, Hirsch MS, Barletta JA. Immunohistochemical staining for BRAF V600E supports the diagnosis of metanephric adenoma. Histopathology. 2015;66(6):901-4. PMID: 25130952.
3. Udager AM, et.al. Molecular and immunohistochemical characterization reveals novel BRAF mutations in metanephric adenoma. Am J Surg Pathol. 2015;39(4):549-57. PMID: 25602792
4. Kuroda N, et al. ALK rearranged renal cell carcinoma (ALK-RCC): a multi-institutional study of twelve cases with identification of novel partner genes CLIP1, KIF5B and KIAA1217. Mod Pathol. 2020;33(12):2564-2579. PMID: 32467651.

Summary of clinical history

53-year-old female with well-circumscribed right renal mass, partial nephrectomy was performed

Gross findings

4.7 cm tan, well-circumscribed and focally hemorrhagic mass, confined to kidney

Microscopic findings

The tumor is well-circumscribed and composed of tightly packed tubules. Tumor nuclei are round or oval with occasional grooves and no obvious nucleoli are present and the cell cytoplasm is scant. In some areas, curvilinear tubules imparting papillary appearance are seen. Psammomatous calcifications are abundant.

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Immunohistochemical findings

Positive: WT1, CD57, BRAF, PAX8
Negative: CK7, AMACR