A MEST up classification? Review of the re-classification of mixed epithelial and stromal tumor and adult cystic nephroma for the abdominal radiologist

Differentiating mixed epithelial and stromal tumor family from predominantly cystic renal cell carcinoma using magnetic resonance imaging-based Bosniak classification system version 2019

PURPOSE

To differentiate mixed epithelial and stromal tumor family (MESTF) of the kidney from predominantly cystic renal cell carcinoma (RCC) using the magnetic resonance imaging (MRI)-based Bosniak classification system version 2019 (v2019).

MATERIALS AND METHODS

The study included 36 consecutive patients with MESTF and 77 with predominantly cystic RCC who underwent preoperative renal MRI. One radiologist evaluated and documented the clinical and MRI characteristics (age, sex, laterality, R.E.N.A.L. Nephrometry Score [RNS], surgical approach, the signal intensity on T2-weighted imaging, restricted diffusion and enhancement features in corticomedullary phase). Blinded to clinical and pathological information, another two radiologists independently evaluated Bosniak category of all masses. Interobserver agreement based on Bosniak classification system v2019 was measured by the weighted Cohen/Conger's Kappa coefficient. Furthermore, predominantly cystic RCCs and MESTFs were divided into low (categories I, II, and IIF) and high-class (categories III, and IV) tumors. The independent sample t test (Mann-Whitney U test) or Pearson Chi-square test (Fisher's exact probability test) was utilized to compare clinical and imaging characteristics between MESTFs and predominantly cystic RCCs. The performance of the Bosniak classification system v2019 in distinguishing MESTF from predominantly cystic RCC was investigated via receiver operating characteristic curve analysis.

RESULTS

MESTF and predominantly cystic RCC groups significantly differed in terms of age, lesion size, RNS, restricted diffusion, and obvious enhancement in corticomedullary phase, but not sex, laterality, surgical approach, and the signal intensity on T2WI. Interobserver agreement was substantially based on the Bosniak classification system v2019. There were 24 low-class tumors and 12 high-class tumors in the MESTF group. Meanwhile, 13 low-class tumors and 64 high-class tumors were observed in the predominantly cystic RCC group. The distribution of low- or high-class tumors significantly differed between the MESTF and predominantly cystic RCC groups. Bosniak classification system v2019 had excellent discrimination (cutoff value = category III), and an area under curve value was 0.81; accuracy, 80.5%; sensitivity, 87.0%; and specificity, 66.7%.

CONCLUSION

The MRI-based Bosniak classification system v2019 can effectively distinguish MESTF from predominantly cystic RCC if category III was used as a cutoff reference.

Mixed epithelial and stromal tumor of the kidney: A retrospective clinicopathological evaluation

OBJECTIVE

Mixed epithelial and stromal tumor of the kidney (MESTK) is a rare renal tumor for which there is limited clinical case experience. The aim of this study was to investigate the clinicopathological features, imaging manifestations, immunohistochemical phenotype, diagnosis, and differential diagnosis of MESTK.

METHODS

We systematically evaluated the clinicopathological characteristics of 13 patients diagnosed with MESTK from 2016 to 2022.

RESULTS

There were 12 female cases and 1 male case, and their age ranged from 24 to 62 (mean age 47 years). The tumor was located in the left kidney in 11 patients. Seven of these patients were located in the upper pole of the left kidney. The mean diameter of the tumor was 4.6 cm (range 1.3-8.6 cm), and the main body of the tumor was mainly located in the medulla or medulla. In 7 cases, the tumors were clearly outlined, 4 cases had clear but irregular borders, and in 3 cases, the masses broke through the renal capsule but did not invade the surrounding organs. All tumors had varying proportions of cystic and solid components. In most tumors, the epithelial cell component was predominant. Among the epithelial components, we observed a predominance of large cysts. Small cysts and tubular structures were also seen; the least common were papillary structures. The most common types of lining epithelium were flat and cuboidal. The cell cytoplasm was predominantly eosinophilic with hyaline degeneration. The tumor cells were composed of different proportions of sparse hypocellular areas and dense hypercellular areas between these cells. In most cases, the cellular stroma had an ovarian-like appearance. It is characterized histologically by mature adipocytes, thick-walled blood vessels, and chronic inflammatory cells. Calcification and collagenization were seen in a few cases. In all female patients, estrogen and progesterone receptors in the interstitial component were almost always positively expressed. In male patients, all interstitial components showed locally positive expression of androgens.

CONCLUSION

MESTK had unique characteristics with complex and variable lesion shapes. There was a high degree of overlap with cystic kidney cancer, and the rate of missed and misdiagnosis was extremely high. The diagnosis could not be confirmed by preoperative imaging. The final diagnosis depended on pathomorphology.

Renal Neoplasms in Young Adults

Renal cell carcinoma (RCC) is usually diagnosed in older adults (the median age of diagnosis is 64 years). Although less common in patients younger than 45 years, RCCs in young adults differ in clinical manifestation, pathologic diagnosis, and prognosis. RCCs in young adults are typically smaller, are more organ confined, and manifest at lower stages of disease. The proportion of clear cell RCC is lower in young adults, while the prevalence of familial renal neoplastic syndromes is much higher, and genetic testing is routinely recommended. In such syndromic manifestations, benign-appearing renal cysts can harbor malignancy. Radiologists need to be familiar with the differences of RCCs in young adults and apply an altered approach to diagnosis, treatment, and surveillance. For sporadic renal neoplasms, biopsy and active surveillance are less often used in young adults than in older adults. RCCs in young adults are overall associated with better disease-specific survival after surgical treatment, and minimally invasive nephron-sparing treatment options are preferred. However, surveillance schedules, need for biopsy, decision for an initial period of active surveillance, type of surgery (enucleation or wide-margin partial nephrectomy), and utilization of ablative therapy depend on the presence and type of underlying familial renal neoplastic syndrome. In this pictorial review, syndromic, nonsyndromic, and newer RCC entities that are common in young adults are presented. Their associated unique epidemiology, characteristic imaging and pathologic traits, and key aspects of surveillance and management of renal neoplasms in young adults are discussed. The vital role of the informed radiologist in the multidisciplinary management of RCCs in young adults is highlighted. Online supplemental material is available for this article. ^©RSNA, 2022.