Renal MRTs was originally reported to be a rhabdomyosarcomatoid variant of Wilms tumor because of cells’ resemblance to rhabdomyoblasts in Beckwith’s First National Wilms Tumor Study [14].MRTs of the kidney have only recently been distinguished as a separate malignant renal neoplasm from Wilms tumors after subsequent studies failed to confirm myogenic differentiation [10]. The development of a concise treatment regimen for Wilms tumors demonstrates the advancements made in pediatric oncology, and now has a survival rate of 70% for stage IV disease [1].

Since the identification of MRTs, the tumor has been reported in a number of extrarenal sites including the central nervous system, liver, soft tissues, lungs, skin, heart, orbit, mediastinum, retroperitoneum, pancreas, gastrointestinal tract, and the urogenital system [3,10]. The common presentation among pediatric MRT cases includes a palpable mass, hematuria, abdominal or flank pain [3]. In adults, the presenting symptoms are flank pain and hematuria according to a study by Han et al in 2022 of eight other adult cases of MRTs [5]. Metastatic symptoms are common at diagnosis and mainly affect the lungs, liver, and brain [3]. Other symptoms included constitutional symptoms such as fatigue and weight loss [5]. Confirmation of MRTs can be from nephrectomy, core biopsy, or autopsy specimen [3]. Initial evaluation of suspected MRTs typically rely on clinical features and are necessary for preoperative diagnosis early in disease course. These features include very young age, fast growth, and typically, due to the aggressive nature of the tumor, early metastasis [1,6].

MRTs commonly have similar radiographic findings as that of other renal tumors like Wilms tumors [6]. Additional noted features among MRTs, primarily in pediatric cases, include subcapsular fluid and curvilinear calcifications but these findings are not pathognomonic and can be found in many different malignancies [1]. The only consistent finding across imaging is irregular borders and lymph node metastasis [1]. Radiographically, this case had those findings, including a significant burden on the retroperitoneal and mesenteric lymph nodes as well as the irregular borders of the primary renal tumor. The ambiguity of the radiographic findings can even mislead the diagnosis, such as this case, when the obvious and severe lymph node disease appeared to be consistent with lymphoma.

As stated, pure MRTs are rarely found in the adult population, however, it has been noted in the literature that some renal cell carcinomas can have rhabdoid features. These studies have shown that divergent differentiation of rhabdoid tumors may arise from any subtype of other renal cell carcinoma, often clear cell; however within these studies, histologic features of the originating malignancy were found adjacent to the rhabdoid tumor masses [4,15,16]. Still, the question is raised of whether the presenting case truly had a de novo MRT or if the rhabdoid tumor was a clonal component of a different underlying carcinoma. The two core biopsies from the right kidney were 1.0 cm or less per gross description, thus limiting the true extent of the tumor, and further obscuring the possibility for an adjacent tumor of different features. Renal medullary carcinoma (RMC) was also considered due RMC’s common presentation in young patients of African descent with sickle cell trait, similar to the patient’s presentation. RMC is one of few other SMARCB1-deficient tumors as signified by loss of INI also including MRT, and atypical teratoid rhabdoid tumor, narrowing the differential substantially [17]. MRT was considered the final diagnosis due to the immunohistochemical staining, which notably was negative for PAX8 and CK7, both of which are commonly positive in RMCs. Despite this and the diffusely rhabdoid morphology, including eosinophilic cytoplasm with prominent nucleoli, there was hesitance in the diagnosis because of the patient’s age, representing this unusual presentation.

Some have described MRTs as the most aggressive primary renal neoplasm, due to the practically uniformly fatality despite treatment [1,18]. Typical treatment of pediatric MRTs consists of resection of the primary tumor, chemotherapy and radiation. Chemotherapy regimens often include cyclophosphamide, actinomycin, vincristine, doxorubicin, methotrexate, among others [19] Tomlinson et al showed that in the National Wilms’ Tumor Study, there was no difference in survival rate of patients with MRTs who did or did not receive doxorubicin [20]. Chemotherapy regimen for MRTs is still quite variable, but tends to follow Wilms tumor, or even soft tissue sarcomas, chemotherapy regimens [21,22]. For adults, the treatment is much less consistent due to the rarity of disease. From the few previous adult cases in the review by Han et al, treatment included a chemotherapy regimen similar to those above with the addition of interleukin-2 (IL2), which is commonly added to regimens for metastatic renal cell carcinomas [5]. Another case cited using IL-2 along with interferon-alpha (IFN-alpha), used for a comparable purpose as IL-2. Similarly, one of the cases used atixinib, which is typically used for advanced renal cell carcinomas as well.

In the present case, treatment was unable to be completed due to severity of disease burden on the liver. As this further demonstrates that MRTs are not solely pediatric tumors, but those that can also occur in adults, established treatment regimens and early diagnosis are necessary to prolong the course of inevitable mortality. If the tumor can be identified before severe metastatic disease limits surgical intervention, surgery with adjuvant chemotherapy provides the best likely outcome. Addition of IL-2 and IFN-alpha to the chemotherapy regimen in adults has not shown to be beneficial in the review by Han et al as one patient who received this treatment lived over a year after initial diagnosis, while the other died within 5 months [5].

MRT prognosis is abysmal for both pediatric and adult cases as most reported cases of MRTs have short survival times after initial diagnosis along with early-onset local and distant metastasis [3]. The Qureshi group reported that of the patients (n = 14, median age = 11 months) treated with curative intent, 70% of MRT patients developed systemic recurrence and died of progressive disease at a median of 7 months since diagnosis [Qureshi 2020]. In adults, there was a report of a 38-year-old white woman with MRT who died five months after radical nephrectomy and metastasis [10]. Another case report showed that a 65-year-old man with a longstanding history of renal calculi had a month-long history of right flank pain and passed away a few days after MRT diagnosis was established and before any therapeutic decision was made [3].

The difference in regimens likely stems from the age difference. The pediatric Wilms tumor chemotherapy regimens are well studied, and although treatments are usually unsuccessful for MRTs, these therapies are familiar to pediatric oncologists, despite the rarity of the rhabdoid tumor. In children, Tomlinson et al found that age can be considered to be a prognostic factor. Pediatric cases in patients less than one year old have more dismal outcome with early central nervous system metastasis compared to greater than one year old patients with better prognosis [20]. As for adults, the oncologists are faced with an even rarer form of a rare disease. Due to the patient’s young age of 21 years-old in this presented case, discussing with pediatric oncology and choosing a regimen common for other pediatric renal malignancies was an appropriate choice, albeit limited by advancing disease. It is unclear whether other regimens used by previous works, including IL-2 or IFN-alpha change prognosis, as one case that used it died over 18 months from diagnosis while the other died within 5 months [5].

The SMARCB1 protein is an abbreviation of its full name: SWI/SNF Related, Matrix Associated, Actin Dependent Regulator of Chromatin, Subfamily B, Member 1, alteration of which disrupts chromatin-remodeling and alters global transcription patterns [17]. Deletion of this IMI-1 gene, a tumor suppressor gene and part of the SWI/SNF chromatin remodeling complex, in knockout mice led to the formation of rhabdoid tumors at an early age, offering an explanation of why these tumors are often lethal in childhood and rarely diagnosed in adulthood [23]. The question still arises, then, why do these cases of late-onset MRTs exist and through what changes do they develop? The possibility still stands that these tumors arise de novo, particularly in those reported in a literature review by Han et al where treatment often included radical nephrectomy, thus eliminating the obscurity of a small sample, as encountered in this case report [5]. Although, slow progression from a different malignancy in the form of rhabdoid transformation is still possible.