Mixed epithelial and stromal tumor of the kidney (MESTK) is a rare neoplasm. The first case was described in 1998 by Michal and Syrucek.1 Histopathologically, it is composed of epithelial and stromal components, thereby appearing as a cystic renal mass with varying proportions of solid components on imaging. Until 2010, approximately 150 cases had been reported,2 although most reports focused on the pathologic and radiographic features.3–5 However, preoperative diagnosis of MESTK is problematic, since most of these tumors are Bosniak Category IV or solid lesions. Therefore most cases are treated surgically.4 This is a retrospective analysis of six cases of MESTK with detailed imaging profiles, treatment decision-making, and clinical outcomes.
Between March 2004 and November 2013, there were six cases of MESTK diagnosed histopathologically in a single institution. We followed the provisions of the Declaration of Helsinki and obtained informed consent from all six patients. Patients’ demographic data (i.e., age, sex, and menopausal status), laterality of the mass, hormone replacement, clinical symptoms, imaging findings, and peri-operative outcomes were reviewed.
Noncontrast and contrast-enhanced computed tomography (CT) before surgery were performed. The CT imaging included the largest diameter of the renal tumor, proportion of cystic component, calcification status, contrast enhancement, sinus invagination, septum, Bosniak category,6 and RENAL nephrometry score.7
The peri-operative outcome variables assessed were surgical technique, estimated blood loss, operative time, ischemia time for partial nephrectomy, complications, and recurrence. Serum creatinine was checked preoperatively and at 3-months postoperatively. Measurement of effective renal plasma flow (ERPF) using scintigraphy with I-131-orthoiodohippurate was also done preoperatively and at 3–6-months postoperatively. The estimated glomerular filtration rate (eGFR) was calculated with the Modification of Diet in Renal Disease equation. The split renal function for the five patients who underwent partial nephrectomy was calculated as the percentage of ERPF of the diseased kidney divided by the total ERPF. Differences in renal function preoperatively and postoperatively were evaluated using Student t test for continuous variables.
Based on the demographic parameters, all of the patients were female, with a mean age of 50.5 years (range, 44–68 years). Half of them were menopausal, but no patient received hormone replacement therapy. Two patients presented with ipsilateral flank pain, while the rest were incidental findings.
The CT imaging findings revealed that all of the patients had a contrast-enhanced renal tumor without satellite lesions (Fig. 1). The mean tumor diameter was 4.4 cm (range, 2.2–9.0 cm). Five tumors contained cystic components, but the proportion varied (10–80%); the other one presented as a solid renal tumor. Only one tumor had calcification (Fig. 2). Half of the tumors showed sinus invagination (Fig. 3). Based on the Bosniak classification, four tumors were Bosniak Category IV and one was Bosniak Category III (Table 1). The mean RENAL nephrometry score was 9.3 (range, 8–11). The clinico-radiological features of our series and other studies are summarized in Table 1.4,12,14
The peri-operative and functional outcomes are reported in Table 2. One patient with a tumor diameter of 9 cm underwent open radical nephrectomy. Another underwent laparoscopic partial nephrectomy, while four received open partial nephrectomy. The mean operative time was 262.5 minutes (range, 190–300 minutes). The mean estimated blood loss was 118.3 mL (range, 50–200 mL), and no patient required a blood transfusion. The mean ischemia time for the five patients who underwent partial nephrectomy was 43 minutes (range, 23–80 min). There were neither peri-operative nor postoperative complications. The average postoperative follow-up without tumor recurrence was 18 months (range, 4–55 months).
All of the six tumors were benign MESTK that presented with a variable mixture of spindle and epithelial cell components. The spindle component varied from ovarian stroma to scar-like fibrous tissue (Fig. 4A). The epithelial component showed tubular, cystic, and glandular structures (Fig. 4B).
Based on the functional outcomes (Table 3), there was no difference in the preoperative and postoperative eGFR. Similarly, there was no difference in the preoperative and postoperative split renal function in the five patients who underwent partial nephrectomy.
The main feature of MESTK is it's pathologically biphasic structure: the epithelial and matrix components. The epithelial component exhibits a cystic structure lined with flat, hobnail, and cuboidal epithelium. The matrix part surrounding the cyst is composed of spindle cells. According to different cell distributions ranging from loose to dense, the matrix part manifests as hyalinizing stroma to ovarian-like stroma.3
The pathogenesis of MESTK remains unknown. Most cases are benign, but malignant transformation has been reported.8 Also, most cases occur in middle-aged, peri-menopausal or postmenopausal women.3 Some patients receive estrogen as hormone replacement treatment,9,10 suggesting that estrogen and the pathogenesis of MESTK may be related. Nonetheless, some male cases of MESTK have also been reported, including one who received estrogen therapy for underlying prostate cancer.10 In this series, all six women were middle-aged, but none of them had undergone estrogen therapy. Thus, the hormone etiology hypothesis cannot explain all cases of MESTK.
The typical clinical symptomatology of MESTK includes hematuria, lower back pain, and a palpable mass.11 However, recent literature and this study reveal an increasing number of asymptomatic cases.12 The possible reason may be advances in imaging modalities and the prevalence of health examinations.
Radiographically, diagnosis of MESTK reported in literature often relies on CT as a diagnostic imaging tool.12,13 All patients in Table 1 had received CT preoperatively. On CT, MESTK appears as a multilocular cystic mass with variable cystic components. In this series, the range of difference was 0–80%, and calcification was noted occasionally. The cases belong to Bosniak Categories III to IV or pure solid on imaging. Common differential diagnoses of MESTK on CT include cystic renal cell carcinoma and cystic nephroma. Preoperative radiologic diagnosis of MESTK is problematic. According to literature on the management of cystic renal mass, Bosniak Category III lesions have 30–100% chance of malignancy and surgery is recommended, whereas Bosniak Category IV is deemed malignant until proven otherwise, and surgery is strongly indicated.13,14 Also, the role of needle core biopsy is not recommended for cystic renal masses, unless areas with a solid pattern are present.14 Therefore, surgery is strongly indicated in most MESTK cases.
Common differential diagnoses of MESTK on CT include cystic renal cell carcinoma and cystic nephroma. Half of the MESTK cases in this series revealed renal sinus invagination, which made nephron-sparing surgery more difficult. The anatomic features of renal tumors are evaluated using RENAL nephrometry. The mean RENAL score in the six patients was 9.3 (range, 8–11), which means moderate-to-high complexity of the lesions.7
Although the vast majority of MESTK cases are benign and prognosis is good, management should follow the renal cell carcinoma protocol.10,15 More than half of patients with MESTK undergo radical nephrectomy.2,8,10,12 Nephron-sparing surgery cannot be done in cases with a very large tumor size, sinus invagination, or endophytic growth. In this series, five of the six MESTK cases had moderate-to-high complexity and underwent partial nephrectomy. Multi-planar reformation of the CT image improved the identification of tumor margins and clarified the interface between the tumor and the collecting system (Fig. 3), and was performed on four patients to facilitate the nephron-sparing surgery. Five patients had no peri-operative complications and no statistical difference in preoperative and postoperative split renal function (SRF) and eGFR. There was also no tumor recurrence noted.
MESTK should be considered in middle-aged women who may have hormone replacement history with contrast-enhanced cystic renal tumors. Twenty percent to 60% of our MESTK cases were managed with nephon-sparing surgery (Table 1). In our series, only one underwent radical nephrectomy for a large tumor and high RENAL score (11 points). However, the remaining five who received partial nephrectomy still had tumors with moderate-to-high complexity (RENAL score 8–10 points).7 Based on the current findings, partial nephrectomy is suitable as a treatment for MESTK, even for difficult cases. Kamel et al16 report partial nephrectomy for very large MESTK.
Limitations of this study are the small case number, retrospective design, and scarcity of MESTK cases. With advances in imaging modalities and the rising popularity of health examination, diagnosis of asymptomatic and small MESTK will increase. With the benign nature of MESTK and the development of minimally invasive techniques, nephron-sparing surgery may be the treatment of choice in the future.
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