Mucinous tubular and spindle cell carcinoma of the kidney was initially described in 1998 by He
et al [
7] as an “unclassified renal cell carcinoma with histology mimicking lower-nephron nephrogenesis”. Subsequent reports describing a renal tumor with similar clinical, histological and immunohistochemical features gave different appellations to this group of tumors, such as “low-grade renal cell carcinoma arising from the lower nephron”, “low-grade myxoid renal epithelial neoplasms with distal nephron differentiation”, and “spindle and cuboidal renal cell carcinoma” [
8,
11,
13,
14]. The common findings in the initial 22 reported tumors led to the designation MTSC at the WHO consensus conference on the classification of renal neoplasms in December, 2002 [
4]. The 2004 WHO classification of renal tumors describes MTSC as low-grade tumors with a favorable prognosis, a wide age range (17–82 years) and a female preponderance [
17]. Similar to other renal tumors, it typically presents as an asymptomatic mass, although flank pain and hematuria may occur. Since then these tumors have been well characterized and distinguished from other renal epithelial tumors by immunohistochemical and cytogenetic studies [
2,
5,
14–
16]. Fine
et al [
6] documented the histological spectrum of MTSC, describing several MTSCs with unusual features and absence of the typical abundant extracellular matrix, which they termed a “mucin-poor” variant of MTSC. MTSCs with focal neuroendocrine differentiation have also been reported, adding to the variety of subtypes associated with this entity [
9]. Due to this abundance of histological variations, renal tumors with features of MTSC require careful histological examination and judicious use of immunohistochemical stains to exclude other histological types of renal cell carcinoma.
Sarcomatoid differentiation has been recognized to arise in all types of renal cell carcinoma since the Heidelberg classification of renal cell tumors was published in 1997 [
10]. The presence of SD in renal cell carcinoma represents high-grade transformation that has a deleterious effect on prognosis. Tumors containing SD have a decreased 5-year survival rate from 79% to 22% in stage-matched patient cohorts; tumors containing >50% SD have an even worse prognosis [
3]. Therefore, the presence of SD is a harbinger of poor prognosis and must be reported in any types of renal cell carcinoma.
Our case is an unusual MTSC that presented with symptoms related to bone metastasis. The tumor also has SD on histological examination that leads to a rapidly fatal outcome, a histological finding and clinical outcome, which to our knowledge has not been reported in MTSC. The histological appearance of MTSC is similar to papillary renal cell carcinoma (PRCC), a tumor that can undergo SD, from which MTSC must be distinguished [
12]. The presence of low-grade cuboidal cells in tubules and cord with intimately mixed areas of benign spindle cells is diagnostic in this case for MTSC. Likewise, the immunohistochemical profile of the tumor in our case is consistent with previously reported immunohistochemical characterrization of MTSC and not PRCC [
8,
12,
14,
16]. The areas of SD are markedly different from the spindle cell component of the MTSC portion of the tumor, both histologically and immunohistochemically, confirming that the spindle cell component seen is part of the MTSC and not the SD portion of the tumor. To date, all cases of MTSC were reported to carry a favorable prognosis with one case of local recurrence and two cases of regional lymph node metastasis [
5,
7,
8,
11,
13,
14]. The poor outcome in our case is consistent with previous studies depicting a worse prognosis for renal carcinomas with SD, regardless of histological subtype [
1,
3].
In summary, we report an atypical case of MTSC with extensive sarcomatoid differenttiation, multiple bone and visceral metastases, and a rapidly fatal course in a 64 year old woman who presented with back pain. This is the first reported case of MTSC with sarcomatoid differentiation and a rapidly fatal course. The presence of spindle cells in MTSC may obfuscate the presence of small areas of SD, thereby requiring adequate sampling and careful histological examination of MTSC cases. It is essential that areas of atypical spindle cells, especially when associated with necrosis should be reported and the possibility of SD considered. The presence of SD in MTSC indicates a poor prognosis and requires a close follow up with further clinical and radiological studies to exclude possible metastatic disease.
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