A 49-year-old woman presented to the local hospital with bilateral lower limb oedema for more than a month. The patient did not have abdominal pain, hematuria, or weight loss. She did not report a history of estrogen treatment or significant medical history. Her family history was unremarkable. A physical examination using palpation was completely normal.Ultrasound shows an anechoic area in the right kidney, suspected to be a cyst, and a mixed echogenic nodule, suspected to be a stone. Enhanced computed tomography (CT) shows a Bosniak III cystic mass with a size of 5.0 × 4.0 × 1.3 in the upper pole of the right kidney, accompanied by disseminated kidney stones (Fig. 1A-B). The patient was diagnosed with a benign renal tumor before surgery and subsequently underwent a robot assisted partial nephrectomy on the right side.
Gross examination of the specimen revealed a tumor dimension of 5.0 ×3.5 ×1.3cm and was located at the upper pole of the right kidney. The cut surface of the tumors was mostly solid and firm, with occasional cystic spaces. (Fig. 1.C) Microscopically, the tumor consisted of stroma and epithelium. The partial area of tumor shows multicystic expansion, with the cyst wall covered by urinary tract epithelium or stratified epithelium, accompanied by significant interstitial hyperplasia. (Fig. 1.D). Immunohistochemistry revealed that the stroma cells were positive for estrogen receptor (ER), progesterone receptor (PR),vimentin and Desmin. some epithelial cells revealed the positive expression of CK. The proportion of cells that expressed Ki-67 was 5%. The pathological and immunohistochemical findings were compatible with the diagnosis of MESTK.
The patient recovered well after surgery, and there were no significant abnormalities in both renal color Doppler ultrasound 15 days after surgery. They were discharged for recovery 8 days later. Currently, patients undergo regular CT follow-up every 6–12 months.
In the 2016 WHO classification of renal tumors, adult cystic nephroma was considered a part of the mixed epithelial and stromal tumor (MEST) family. It is a rare and unique renal tumor composed of epithelium and stroma, with solid and cystic structures.3–5 As previously reported, occurring almost exclusively in perimenopausal women. Moreover, most patients have a history of diseases of the female reproductive organs or a history of hormone-dependent treatment.6–10 MESTK was originally described in 1973 by Block et al. as a congenital mesoblastic nephroma.11 In 1998, Michal and Syrucek first proposed the term "mixed epithelial stromal tumor of the kidney", which was recognized by most academic workers.12 The clinical presentation of the disease lacks specificity, and the most common symptoms are urinary symptoms such as low back pain, hematuria, low back mass, and recurrent urinary tract infections. In our case, the patient did not present with urinary-related symptoms, and the only symptom was secondary to bilateral lower extremity edema, which is clinically rare. With the popularity of routine health screening and development in imaging modalities, MESTK is mostly found incidentally during physical examination. As the pathohistology of MESTK is not yet completely clear, imaging is also difficult to distinguish MESTK from other renal tumors, especially cystic renal cell carcinoma, and cystic nephroma.9, 13, 14As a result, there are difficulties in the preoperative diagnosis of MESTK, Histopathological evaluation was usually the means of confirming the diagnosis. To date, most of the MESTK cases reported in China and abroad have been treated with open resection, and only a few cases have been treated with minimally invasive laparoscopic surgery.12, 15–17 However, we believe that MESTK is a benign tumor with a good prognosis and rarely recurs, and renal unit preservation surgery performed in this setting not only preserves part of the renal function, but also reduces postoperative complications and promotes the patient's postoperative recovery to shorten the length of hospital stay. Because the malignant potential of MESTK was recently described by some academics, postoperatively continued follow-up might be advisable to observe the clinical behavior of MESTK in a subset of these patients.18, 19