RENAL MYXOMA IN A PEDIATRIC TRANSPLANT RECIPIENT
CT: Computed tomography
CTDIvol: Computed tomography dose index volume
DLP: Dose length product
MRI: Magnetic Resonance Imaging
PET: Positron Emission Tomography
POG: Pediatric Oncology group
Ki 67: is a protein that is a marker for cellular proliferation
HMB-45: Human Melanoma Black-45 is a monoclonal antibody
Renal myxoma is a very rare benign neoplasm seen almost exclusively in adults with only 16 reported cases in the literature. All of these cases have been reported in native kidneys with none being reported in a transplant kidney. We report a case of a renal myxoma in a 17-year old boy in a transplant kidney found as an incidental mass on ultrasonography that was further evaluated with a computed tomography and a positron emission tomography scan. Positron emission tomography findings of renal myxoma are reported here for the first time, and a discussion of imaging findings from previous cases are briefly reviewed. This case report highlights the fact that adult-predominant tumors and pathology should always be a consideration in pediatric patients who receive organ transplants from adult donors.
A 17-year-old boy received a living related donor kidney transplant from his mother at age 2 secondary to congenital renal dysplasia. At the age of 5 years, he was treated for post-transplant lymphoproliferative disorder [PTLD - abdominal stage IV Burkitt’s Lymphoma] per Pediatric Oncology Group (POG) 9317 without complication. At the age of 12 years, the patient developed chronic transplant nephropathy and concomitant renal insufficiency. He received a second kidney transplant at age of 15 years secondary to graft failure of the first transplant and this initial transplant kidney was left in place. The second transplanted kidney was removed 1 week later due to renal vein thrombosis. 6 months later, the patient began hemodialysis and continued to be evaluated for another transplant. He underwent an abdominal ultrasound as part of his pre-transplant work-up and a mass was incidentally discovered in the original transplanted kidney. The ultrasound study showed an enlarged heterogeneous transplant kidney measuring 14.2 cm with a 4.2 x 5.4 x 3.0 centimeter nodular area in the midpole [Fig 1]. A subsequent CT scan was done pre- and post-intravenous contrast (CTDIvol- 20.26 mGy and DLP- 921.70 mGy-cm). The images showed an enlarged transplant kidney with heterogeneous enhancement with a hypodense mass [Fig 2]. Hypodense areas in the inferior portion of the kidney appeared to be infiltrating the kidney and extending into the renal pelvis and proximal ureter [Fig 3a, b]. Initial concern was for recurrence of PTLD, but a PET scan done 2 weeks prior to the ultrasound and CT did not show any hypermetabolic areas [Fig 4 a, b] in the mass. The patient underwent a radical right transplant nephrectomy. The kidney was noted to have a neoplasm in the postero-lateral aspect which extended into and was adherent to the retroperitoneal tissues. The specimen consisting the kidney, mass, and attached ureter, measured 13.6 x 6 x 9 cm (craniocaudal x anteroposterior x transverse respectively). The mass had gelatinous, fluffy, nodular soft tissue that filled the hilum and measured 10 x 9 x 4 cm (Fig 5). A myxoid substance was noted to compress the renal medulla and cortex without infiltration and to encase the ureter down to the distal ureteral margin. The tumor was found to be chromosomally normal and analysis showed a 46XX karyotype, suggesting that the tumor was of donor origin. On histological analysis, the tumor had the appearance of a typical myxoma with myxoid stroma and fibroblastic-like spindle cells and stellate cells [Fig 6]. Immunohistochemically, Ki-67 and HMB45 stains were completed, showing a low proliferation rate [2%] on Ki-67 and absence of staining on HMB45.
We report the first pediatric case of renal myxoma, and also the first known case in a renal transplant patient, emphasizing the point that adult-predominant pathology must also be considered in pediatric patients who receive adult organ transplants.
Renal myxomas are exceedingly rare, with only 16 cases previously reported in the literature [1-11]. In general, myxomas occur at various sites including the heart, skeletal muscle, joints, pharynx, tonsils, bones, skin, retroperitoneum, intestines, subcutaneous tissue, aponeurotic tissue, eye, ovary, and in the genitourinary tract [1, 12]. The muscles of the legs and buttocks are the most common sites [2, 12]. The majority of previous cases were discovered incidentally on imaging for unrelated problems; one case presented with a pelvic mass and epigastric pain, and 2 others presented with dull flank pain [1-4, 6-11]. Myxomas rarely present under the age of 20 years . Previously, the youngest age of a patient having a renal myxoma was 27 years . Myxomas occur between the ages of 20 and 89, the majority of which occur between the ages of 40 and 70. Myxomas in patients under 20 years of age are rare. Roughly two-thirds of patients are female . Renal myxomas, however, have not shown a gender preference. The age range for renal myxomas between 27 and 68 years is similar to that of other myxomas.
The origin of renal myxomas is not entirely clear, although different theories have been presented. Some authors have considered myxomas to be degenerative in origin, similar to the changes seen in adipose tissue during brown atrophy of the heart, while others argue that the uniform cellular component seen on ultrastructural analysis suggests that myxomas are neoplastic in origin, arising from a primitive mesenchymal cell with fibroblast-like features [1, 3]. These findings suggest that the renal myxoma can ultimately be defined as a benign fibroblastic tumor that may have differentiated to produce abundant mucopolysaccharide instead of mature collagen [2, 3, 7]. However, even the exclusively benign nature of a myxoma is controversial, and malignant transformations of primary myxomas have been previously reported [13-15].
In our case, the myxoma in the transplant kidney had a karyotype of 46XX. Therefore, the tumor likely originated from donor tissue (the patient’s mother). To the best of our knowledge, there have been no previously reported cases of renal myxoma in a transplant kidney. Even among cardiac myxomas, which are relatively much more common than renal myxomas, there has only been 1 reported case of myxoma of donor origin found in a transplanted heart . Because myxomas are so rare in children, this report highlights the fact that tumors and other pathologies that are adult-predominant must also be considered in children receiving adult transplant organs.
There is a paucity of information on the imaging characteristics of renal myxomas. Murphey et al. reported on the imaging characteristics of 45 intramuscular myxomas  and perhaps some of their findings can be extrapolated to those of myxomas in the kidneys. Overall, the appearance of myxomas were consistently shown to be hypoechoic relative to skeletal muscle on ultrasound, of low attenuation on CT with little to no enhancement post contrast, and low signal intensity on T1-W and high signal intensity on T2-W MRI.
Imaging characteristics of reported cases of renal myxoma [Table 1] are not dissimilar to soft tissue myxomas as they are both solid tumors with significant mucus content. Ultrasound is helpful in characterizing the lesion as solid with heterogeneous echogenicity. CT findings in our case demonstrated the mass to be indistinguishable from surrounding kidney on non-contrasted CT and of relatively low density with poor enhancement post contrast administration. This is similar to the CT findings of others, with some variation in post contrast enhancement [3, 4]. Although we did not perform an MRI, other reports suggest that the renal myxoma is consistently of homogenous low-signal intensity on T1-W imaging and heterogenous high-signal intensity on T2-W imaging [1, 2, 4]. This is consistent with its gross characteristics of a mass with a high water content stemming from its abundance of mucus. There have been no prior reports of the imaging characteristics of renal myxoma on PET. The absence of hypermetabolic activity on FDG- PET steered us away from the diagnosis of recurrent PTLD and is strongly suggestive of a benign lesion.
In summary, although considered a rare benign tumor, renal myxoma should be in the differential diagnosis of a mass in a transplanted kidney regardless of recipient’s age. This neoplasm does not have defining characteristics on CT or MRI, but shows little to no metabolic activity on PET. The combined findings of these modalities make other aggressive neoplastic and infectious processes unlikely.
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