Discussion

Renal mucinous tubular and spindle cell carcinoma (MTSCC) is an epithelial tumour that accounts for less than 1% of all neoplasms of the kidney [1, 2, 5-6]. It affects primarily adult patients aged between 13 and 81 years old (mean age: 58 years old) and shows a female predominance like our patient [1, 2, 5-6]. MTSCC is usually incidentally discovered on abdominal imaging as our case [1-2]. However, it can be revealed with some non-specific signs as hematuria, flank pain, nephrolithiasis and a palpable abdominal mass [1-2]. Unlike our patient, this tumor is found usually in the renal cortex and rarely in the medulla [1, 2-5].
On imaging, MTSCC is presented as a well-limited, exophytic and ovoid renal mass with an expansive growth pattern [1-2]. On magnetic resonance imaging (MRI), MTSCC has a low-grade appearance and a slow enhancement [7].
Histologically, the tumour is characterized by a mixture of tubular and spindle cell components, separated by variable amounts of mucinous stroma. The tubules are round, ovoid, with a collapsed central lumen. They are often tightly packed and arranged in parallel and sometimes merge into cord-like structures or even form a solid growth pattern [1-2]. The transition between the elongated tubules and the spindle cells are commonly obvious [1-2]. The stroma shows extracellular abundant mucin with a bubbly appearance [1-2]. A mucin-poor variant has recently been described where the mucinous component may be absent or in small amounts [1-2].
MTSCC express distal nephron markers (EMA, CK19, CK7, E-Cadherin) and proximal tubule markers (RCC Ma, AMACR, CD15) [5-6].
The main differential diagnosis of MTSCC is papillary renal cell carcinoma (RCC) in its compact variant [1, 2, 3, 5-6]. Therefore, MTSCC may be distinguishable from papillary RCC by evaluating the presence of a trisomy for chromosomes 7 and 17 [1, 2,3-5]. Sarcomatoid carcinoma is a diagnosis to be made when the spindle cell component dominates [1,2-5]. It behaves aggressively with neoplastic spindle-shaped cells with large, hyperchromatic nuclei, mitosis and necrosis [1, 3-5]. However, MTSCC can undergo sarcomatoid transformation [5]. MTSCC can also be confused with mesenchymal tumours such as leiomyoma, leiomyosarcoma, inflammatory myofibroblastic tumour, angiomyolipoma and juxtaglomerular tumour [1-2].
MTSCC with classic morphology has an excellent prognosis [1, 2, 5, 6-8]. It is a low-grade carcinoma and the treatment consists on partial or radical nephrectomy [1, 2, 5-6]. Patients with sarcomatoid change have the worst prognosis and can suffer from recurrence, loco-regional and distant metastasis and death [1, 2-8].