Microscopic description:

The tumor is un-encapsulated, well-circumscribed and shows low-grade polymorphic tumor cells with tubular growth pattern spindle cells and minimal extracellular basophilic mucinous material. Clusters of foamy macrophages are seen scattered throughout the tumor.

The differential diagnosis includes Papillary renal cell carcinoma (PRCC), Collecting duct carcinoma, Sarcomatoid renal cell carcinoma, Metanephric adenoma and Renal carcinoid.

On immunohistochemistry (IHC), tumor cells are positive for CK7, AMACR, PAX8 and CD10 and negative for Alcian blue.

PRCC commonly shows diffuse CD10 expression (patchy positive to negative in MTSCC) and has trisomy 7/17 genomic signature (absent in MTSCC). Sarcomatoid renal cell carcinoma and collecting duct carcinoma are infiltrative and tumor cells show higher degree of nuclear pleomorphism. Metanephric adenoma shows tubular architecture but lacks spindle cells and mucinous stroma and is negative for CK7 and AMACR and positive for WT1 and CD57. Renal carcinoid shows stippled chromatin and trabecular growth pattern and lacks mucinous stroma.

Based on the histopathological features and IHC, the tumor is consistent with a diagnosis of Mucinous tubular and spindle cell carcinoma (mucin-poor).

Discussion:

Mucinous tubular and spindle cell carcinoma (MTSCC), a typically low-grade epithelial neoplasm accounting for <1% of all renal tumors

Seen in patients 13-82 years of age (3-4:1 female-to-male ratio).

Most MTSCCs present as incidentaloma on abdominal imaging (primarily arising in the renal cortex, rarely within the renal medulla).  Some present with palpable abdominal mass, flank pain and/or hematuria. Clinical course is typically indolent, though in some cases may involve local recurrence or metastasis.

MTSCC tumors have an average diameter of 2-4cm and may exceed 18 cm.

Grossly tumors are well circumscribed, partially encapsulated tan, soft-yellow, or muted pink solid masses typically without hemorrhagic or necrotic foci.

Histologically, MTSCC consists of tubular and spindle cell components demarcated by mucinous stroma. Extracellular mucin is predominantly basophilic, though sometimes eosinophilic, with blue-gray bubble-like appearance occasionally appearing as numerous small vacuoles resembling clear cells.“Classic” and “mucin-poor” variants of MTSCC are delineated upon extracellular mucin content and tubules/spindle cells present after sampling. Tubules are typically organized in tightly packed, parallel arrangements of round, ovoid, or elongated structures anastomosing with a collapsed central lumen, occasionally merging into cordlike structures or forming a solid growth pattern. Focally clear cells, oncocytic change, and vacuolations may be seen within tubule cytoplasm.

Microscopic findings of round nuclei, evenly dispersed chromatin, and occasional grade 2 nucleoli (WHO/ISUP) are common.  High-grade atypical nuclei and sarcomatoid changes are rarely identified.  Many morphologic variations mimic other renal carcinomas. Findings reported primarily in mucin-poor tumors include foamy histiocyte aggregates, papillations, microscopic tumor necrosis, cuffed lymphoplasmacytic infiltrate surrounding tumor cells nests, psammomatous calcifications, and heterotopic bone formation.

Relative to the gamut of malignant renal tumors, the prognosis of MTSCC is favorable after surgical removal. Partial or complete nephrectomy is curative in most cases.

A small subset of patients may present with aggressive clinical phenotypes of MTSCC predisposing to poor outcome requiring follow-up.  No consensus exists regarding application of systemic treatment to metastatic MTSCC tumors.

References:

Nathany S, Monappa V. Mucinous Tubular and Spindle Cell Carcinoma: A Review of Histopathology and Clinical and Prognostic Implications. Arch Pathol Lab Med. Jan 2020;144(1):115-118. doi:10.5858/arpa.2017-0506-RS

Zhao M, He XL, Teng XD. Mucinous tubular and spindle cell renal cell carcinoma: a review of clinicopathologic aspects. Diagn Pathol. Sep 17 2015;10:168. doi:10.1186/s13000-015-0402-1

Gaafar A, Valenti C, Echevarria C, Laforga JB, Lopez JI. Renal mucinous and tubular spindle cell carcinoma: a clinicopathological study of 4 cases. Ann Saudi Med. Nov-Dec 2006;26(6):466-70. doi:10.5144/0256-4947.2006.466

Sun N, Fu Y, Wang Y, Tian T, An W, Yuan T. Mucinous tubular and spindle cell carcinoma of the kidney: A case report and review of the literature. Oncol Lett. Mar 2014;7(3):811-814. doi:10.3892/ol.2014.1783

Andeen NK, Tretiakova M. Mucinous tubular and spindle cell carcinoma. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/kidneytumormalignantmucinoustubular.html.

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