A Case Report: Mucinous Tubular and Spindle Cell Carcinoma of Kidney with Spindle Cell Predominance Mimicking Mesenchymal Tumour

Mucinous tubular and spindle cell carcinoma (MTSCC) of kidney is a rare variant of renal cell carcinoma which was first described in the 2004 World Health Organization classification of tumours of the kidney. Morphologically, MTSCC is composed of tubules merging with bland-appearing spindle cells in a myxoid/mucinous stroma. Diverse morphological patterns have been reported in MTSCC; however, a spindle cell predominant morphology mimicking a mesenchymal tumour is rare and only two cases have been reported so far. We report a case of MTSCC with spindle cell predominance in kidney which was a diagnostic challenge. Though MTSCC usually shows an indolent course, there have been cases showing aggressive behaviour even with bland morphology. Hence, a thorough histopathological evaluation with ancillary studies are required to differentiate spindle cell predominant MTSCC from its mimics. Our case was a 40-year-old female who was incidentally found to have a well-defined hypodense lesion measuring around 2 cm in the upper pole of the right kidney. Right partial nephrectomy was performed which showed a 2.7 × 2.5 × 2 cm well-defined grey tan tumour without necrosis or haemorrhage, limited to kidney. Histopathological examination showed sheets of bland-appearing spindle cells mimicking a mesenchymal tumour. The tumour was extensively sampled, revealing small foci of tubule formation and mucinous stroma. Tumour cells were positive for CK7, AMACR, and PAX8. A final diagnosis of MTSCC was made. Hereby, we discuss ways of differentiating MTSCC from other spindle cell tumours of the kidney.


Introduction
Mucinous tubular and spindle cell carcinoma (MTSCC) of the kidney is a variant of renal cell carcinoma (RCC), comprising less than 1% of all renal cell tumours. MTSCC is grouped under renal cell tumours in the latest 2016 WHO classification (1). Morphologically, MTSCC is composed of tubules merging with bland-appearing spindle cells in a myxoid/mucinous stroma. A review of literature reveals that MTSCC has diverse morphological patterns. MTSCC with spindle cell predominance is rare and we found two cases in literature (2,3). cells. Diagnosis of MTSCC was confirmed. The patient has no recurrence or metastasis at 24 months follow-up.

Discussion
MTSCC is a rare variant of RCC. These tumours were initially called low-grade collecting duct carcinoma or grouped under RCC, unclassified (4). MTSCC was first described in the 2004 World Health Organization classification of tumours of the kidney. The tumour occurs predominantly in adults with age ranging from 17 to 82 years with a predilection for females. It is often an incidental finding but significant number of cases present with flank pain and/

Case history
A 40-year-old female evaluated for polymenorrhoea was incidentally detected to have a right renal mass. She was referred to our institute for further evaluation. She had no urinary complaints and systemic examination was unremarkable. Glomerular filtration rate was reduced on the right side. CT abdomen showed a well-defined hypodense lesion, measuring 2.7 × 2.4 cm in the upper pole of the right kidney with minimal enhancement on contrast. MRI showed T1 hypointense, T2 isointense, to intermediate well-defined lesion with diffusion restriction (Figure 1). The nephrometry score was eight. The patient underwent right partial nephrectomy. No adjuvant chemoradiation was given.
Macroscopic examination showed a well circumscribed, solid, grey tumour measuring 2.7 × 2.5 × 2 cm limited to the kidney. No haemorrhage or necrosis was observed ( Figure 2). Microscopic examination showed a neoplasm composed predominantly of bland-appearing spindle cells arranged in sheets and fascicles ( Figure 3). The cells showed vesicular nuclei with eosinophilic nucleoli visible at 400× magnification. Mitosis was scant. No necrosis or sarcomatoid areas were seen. Patchy lymphoplasmacytic infiltrate and collection of foamy histiocytes were seen. The tumour resembled a pure mesenchymal tumour. The tumour was extensively sampled and embedded entirely which showed small foci of tubular structures lined by cuboidal cells intermingling with spindle cells (Figure 4). Alcian blue stain showed focal mucinous stroma. Provisional diagnosis of MTSCC with WHO/ ISUP nuclear grade 2 was made. The TNM stage was pT1a. Immunohistochemistry showed strong and diffuse positive reaction for pancytokeratin, CK7 ( Figure 5), AMACR ( Figure 6), PAX8, and vimentin, and negative reaction for SMA, S100, and CD34. Ki67 was positive in 10% of tumour   bland-appearing spindle cells. Background shows myxoid/ mucinous stroma. Collection of foamy macrophages and chronic inflammatory cells are often seen. Other than these classical features, mucin-poor stroma, papillary pattern, psammomatous calcification, high-grade nuclear features, sarcomatoid transformation, neuroendocrine differentiation, and heterotopic bone formation can be seen (2, 3, 5, 6). Banyani et al. established that MTSCC originates from embryonal cell rests of impaired differentiation and that explains the tumour's morphological heterogeneity. (7). Mucin-poor stroma and sarcomatoid transformation are associated with poor prognosis (8). Tumours with bland morphology also have shown aggressive behaviour occasionally (9,10).
Our case showed spindle cell predominance with paucity of tubular structures which was a diagnostic challenge. Such tumours can be easily mistaken for mesenchymal tumours with entrapped renal tubules. Hence, complete sampling with extensive search for tubules, papillary structures, and highgrade areas should be done.
Differential diagnosis for MTSCC with spindle cell predominance are as follows: 1. Type I papillary RCC (PRCC) rarely shows overlapping histologic features with MTSCC-like prominent spindle cell stroma and mucin production. Certain subtle histologic features may help in distinguishing MTSCC and PRCC. MTSCC shows isolated short papillae; however,

Conclusions
MTSCC of the kidney shows a wide range of morphological patterns. Our case was a diagnostic challenge showing almost only spindle cells with paucity of tubular structures. Also, other RCCs, like papillary RCC, can show low-grade spindle cell foci. Hence, a thorough histopathological evaluation with ancillary studies are required to differentiate spindle cell predominant MTSCC from its mimics. The MTSCC shows an indolent course, and hence partial or radical nephrectomy would suffice, and it does not require adjuvant chemoradiation (12). However, cases showing aggressive biologic behaviour and metastasis even with bland morphology have been reported (9,10). Hence, close follow-up is warranted (12). Prognostic significance of MTSCC with spindle cell predominance is yet to be studied.