Mucinous tubular and spindle cell carcinoma of the kidney

Mucinous tubular and spindle cell carcinoma (MTSCC) has recently been classified in the 2004 World Health Organization classification of renal tumors [1]. It is a relatively low-grade carcinoma and is associated with a good prognosis [2]. The clinical characteristics of MTSCC are similar to those of papillary renal cell carcinoma (RCC) with sarcomatoid transformation. Hence, a precise differential diagnosis between both of these diseases is important because they have different biological behaviors [3]. Herein, we report a case of MTSCC and review the relevant literature.

MTSCC is characterized by tightly packed tubules separated by pale mucinous stroma and a spindle cell component [1]. Renal tumors with these histologic features were previously categorized as a variant of solid papillary RCC with compressed and elongated papillae, metanephric adenomas, or unclassified or sarcomatoid carcinomas [4]. Sarcomatoid transformation is the most important feature for the differential diagnosis of MTSCC from papillary RCC [3]. Papillary RCC has an overall 5-year survival rate of approximately 86 % [5]. Sarcomatoid or spindle cell differentiation is reported to occur in 3–5 % of papillary RCC cases and is considered as a strong indicator for adverse prognostic outcome with 1- and 5-year survival rates of 59 and 22 %, respectively [6]. In addition, unlike MTSCCs that rarely metastasize, papillary RCCs, the second most common RCC subtype (occurrence 11–18 %), have been shown to metastasize to the lymph nodes in 7 % of the cases [5]. Thus, accurately distinguishing MTSCC from papillary RCC with sarcomatoid transformation is absolutely essential [5].

MTSCCs have been reported to have no characteristics on CT and magnetic resonance imaging (MRI) [7, 8]. Thus, the preoperative differential diagnosis of MTSCC from papillary RCC may be difficult on the basis of either CT or MRI.

MTSCCs are polymorphic neoplasms showing a wide range of morphologic variability [2]. Histologically, they are generally characterized by the presence of cuboidal and spindle cells [9]. However, there are several morphological variants of MTSCC that are difficult to diagnose. Some cases might show predominance of various components: some MTSCCs may show spindle cell predominance and mimic sarcomatoid carcinomas, and others may show tubular cell predominance and mimic the solid variant of papillary RCC [2]. Other characteristic features include tumors with poor mucin content, areas with extracellular hyaline matrix and blue-tinged mucin within tubules, and solid areas with epithelioid cells [2].

MTSCCs are known to originate from the collecting duct epithelium or cells of the loop of Henle [8]. Tumor cells of MTSCCs have a complex and variable immunophenotype [10]. Many MTSCC tumors are positive for AMACR, epithelial membrane antigen, and cytokeratin AE1/3 [11]. CD10 immunoreactivity is noted in 93–100 % of papillary RCCs [9] and in only 15 % of MTSCCs [10].

Papillary RCC is characterized by chromosomal gains, particularly chromosomes 7 and 17, and the loss of chromosome Y [12]. On the other hand, MTSCCs are characterized by the combination of losses of chromosomes 1, 4, 6, 8, 13, and 14 and gains of chromosomes 7, 11, 16, and 17 [1]. These chromosomal abnormalities are potentially useful for the early and accurate distinction of the different histotypes of RCCs.

MTSCCs and papillary RCCs have a similar immunohistochemical staining pattern as well as histological characteristics; this may hinder the differential diagnosis. In the absence of any significant cell abnormalities, pathologists and urologists should be aware of the histological spectrum of MTSCCs to ensure an accurate diagnosis.