CT findings and outcome after nephrectomy as sole therapy for renal nephroblastoma in a cat

Introduction

Renal nephroblastoma, or Wilms tumor, is a pediatric malignancy that has been primarily described in young cats. In a retrospective study of 233 cats aged under 1 year diagnosed with neoplasia, only a single case of renal nephroblastoma was reported. ¹ Similarly, in a series of 19 feline non-lymphomatous renal tumors, only one renal nephroblastoma was reported. ² One report described a 3-year-old male cat with renal nephroblastoma who underwent localized radiotherapy resulting in partial remission, followed by surgical excision of the tumor and mitoxantrone chemotherapy. The cat remained disease-free for more than 3 years after multimodal therapy. ³

Nephroblastoma arises from metanephric blastema and contains blastemal, epithelial and mesenchymal components. ⁴ Nephrectomy is the mainstay of treatment, sometimes combined with chemotherapy, although outcome data remain limited. ⁵ In dogs and cats, metastasis to the lungs, liver, mesentery and lymph nodes has been described. ⁶ However, the rate of metastasis has not been consistently reported. In one canine study, metastatic disease was detected at diagnosis or death in 3/4 dogs affected by nephroblastoma. ⁵

On contrast-enhanced imaging in humans, tumors appear heterogeneous owing to necrosis and hemorrhage, with areas of hypoenhancement. ⁷ On ultrasonography, renal masses alter the renal parenchyma, creating a characteristic ‘claw sign’ where normal parenchyma appears to wrap around the mass. ⁷ However, this feature is non-specific.

Baseline imaging findings for both feline and canine renal nephroblastoma have been minimally reported. One publication described CT findings of a feline renal nephroblastoma originating from the caudal pole of the right kidney, displacing the caudal vena cava and obscuring the right adrenal gland. ⁸ Contrast excretion from the remaining right renal parenchyma was preserved, and the left kidney demonstrated normal enhancement with opacification of the renal pelvis and ureter. ⁸ A study describing CT findings in dogs and cats described one dog with a nephroblastoma. CT revealed a renal mass with irregular margins and heterogeneous internal architecture. The lesion was hypoattenuating on pre-contrast images and demonstrated moderate, relatively uniform contrast enhancement across post-contrast phases, consistent with a plateau-type enhancement pattern. Attenuation values increased after contrast administration, peaking during the nephrographic phase. ⁹

The current report describes a young cat with a renal nephroblastoma treated with nephrectomy as the sole therapy, experiencing long-term survival, highlighting imaging findings and the potential for less aggressive behavior of renal nephroblastoma in some cats.

Case description

A 1-year-old, neutered male domestic shorthair cat presented with lethargy and anorexia. Examination revealed dull mentation, bradycardia with a heart rate of 120 beats/min and weak femoral pulses. Abdominal palpation identified a mass effect in the mid-abdomen. The remainder of the examination was normal. The bradycardia detected at presentation was suspected to be a vagal response secondary to pain. The cat was stabilized with intravenous fluid therapy and methadone (0.1 mg/kg IV), and the bradycardia and weak femoral pulses resolved.

Initial diagnostics included complete blood count, serum chemistry profile and urinalysis. Chemistry results showed an increased blood urea nitrogen (BUN) (52 mg/dl, reference interval [RI] 16–36). The creatinine was normal (1.3 mg/dl, RI 0.8–2.4). There were no additional abnormalities. Urinalysis revealed urine specific gravity (USG) above 1.050, proteinuria (30 mg/dl) and hematuria (10 red blood cells per high-power field). Glucosuria was noted, which resolved on subsequent follow-up. Prothrombin time, activated partial thromboplastin time and thromboelastography were normal.

Abdominal radiographs revealed unilateral renomegaly, with the right kidney measuring more than five times the length of the L2 vertebral body (normal is two to three times). The right kidney maintained smooth margins; however, decreased serosal detail was present because of retroperitoneal effusion. No additional abnormalities were noted. Three-view thoracic radiographs were normal. Abdominal ultrasound revealed a 6.4 × 5.4 × 4.7 cm heterogeneously hyperechoic mass-like lesion associated with the right kidney (Figure 1). The mass-like lesion resulted in compression of the lateral renal margin, but underlying renal architecture and blood flow remained normal. An echogenic retroperitoneal effusion was present, and sampling revealed grossly hemorrhagic fluid, which was not assessed cytologically. An ultrasound-guided fine-needle aspirate of the renal mass-like lesion was performed without complication. Cytology revealed atypical mesenchymal cells, neutrophilic inflammation and necrosis.

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Figure 1

(a) Transverse and (b) longitudinal ultrasound images of the right kidney. The right kidney (outlined by white arrowheads) is compressed secondary to a large, heteroechoic mass (white arrows) along its lateral margin. (a) A small amount of slightly echogenic retroperitoneal effusion is present adjacent to the kidney and mass (white asterisk). (b) Power Doppler application shows persistent blood flow within the renal vasculature

Three-phase contrast-enhanced abdominal CT was performed for staging and surgical planning with the cat in sternal recumbency using a 64-detector scanner (GE Revolution EVO; GE Healthcare). The cat was sedated with dexmedetomidine (4 µg/kg IV), methadone (0.2 mg/kg IV) and alfaxalone (0.5 mg/kg IV). During the scan, 9.5 ml of iohexol 300 (1.6 ml/kg IV) was administered as a bolus.

CT of the right kidney revealed a round, smoothly marginated, heterogeneous, soft tissue attenuating mass-like lesion in close contact with the right kidney and compressing the lateral renal margin. The lesion was heterogeneously soft tissue attenuating on pre-contrast images, containing a central lobular hyperattenuating region (45 Hounsfield units (HU)) surrounded by more hypoattenuating soft tissue peripherally (~40 HU) (Figure 2), as well as a poorly defined, partially contrast-enhancing capsule. Scant amorphous mineral was present along the dorsal margin of the lesion. At its caudal pole, a small area of wispy contrast enhancement extended from the renal cortex, associated with a concave defect in the adjacent cortical margin (Figure 3). The remainder of the lesion was non-contrast-enhancing. The renal architecture was compressed but parenchymal contrast enhancement remained normal, and the right ureter was patent and inserted normally at the urinary bladder trigone. Small pockets of non-contrast-enhancing retroperitoneal and peritoneal soft tissue were present (up to 11 HU).

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Figure 2

(a) Transverse pre-contrast and (b) venous post-contrast soft tissue images centered on the right kidney. The patient’s right side is to the left of the images. The perirenal mass is outlined by white arrows, resulting in compression of the lateral margin of the right kidney (black asterisk), only evident on the post-contrast image. Scant amorphous mineral is present along the dorsal margin of the mass (white arrowhead)

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Figure 3

(a,b) Dorsal and (c) transverse venous post-contrast soft tissue images centered on the right kidney (window width 160, window level 69). The patient’s right side is to the left of all images and cranial is at the top for panels (a,b). The slice in panel (a) is more dorsally located, whereas panel (b) is along the ventral extent of the mass. The right kidney (black asterisks) is compressed by the mass, with focal indentation along the caudal pole (white arrowhead) and amorphous regional contrast enhancement (white arrows). A thin capsule (white brackets) separates the mass from non-contrast-enhancing retroperitoneal soft tissue (white asterisks)

The cat was taken for exploratory celiotomy (day 0) with the plan to perform a right ureteronephrectomy. Premedication included methadone (0.2 mg/kg IV) and midazolam (0.2 mg/kg IV). Anesthesia was induced with alfaxalone (2 mg/kg IV) and maintained with isoflurane in oxygen. The surgical duration was estimated to be over 90 mins and therefore warranted perioperative antibiotics. ¹⁰ At induction, cefazolin (22 mg/kg IV) was administered and repeated every 90 mins until completion of the procedure.

A ventral midline approach was performed; upon inspection of the abdomen, the capsule of the right kidney was found to be markedly distended. A blood clot was present in the retroperitoneal space. Once removed, the irregularly shaped right kidney with a perirenal mass was identified. This was suspected to be the source of the previous hemorrhage. The right kidney, mass and a portion of the right adrenal gland were isolated from the retroperitoneal space. The renal artery and vein were sealed and transected using a bipolar sealing device. The right ureter was sealed and transected close to the kidney. The kidney and mass were removed, and the retroperitoneal space was assessed for hemorrhage. No other abnormalities were detected during the celiotomy. Intraoperatively, the cat received fentanyl (5 µg/kg/h IV) for additional analgesia. Fluid therapy intraoperatively consisted of lactated Ringer’s solution (5 ml/kg/h IV). Bupivacaine liposome injectable suspension (0.4 ml/kg) was instilled into the incision line at closing. Abdominal palpation during recovery elicited pain, so an additional dose of methadone (0.2 mg/kg IV) was administered. The cat was hospitalized for 1 additional day. Gabapentin (10 mg/kg PO q8–12h) and buprenorphine (0.015 mg/kg transmucosal q8h) were dispensed for continued care at the time of discharge. The cat did not eat while hospitalized postoperatively, which was suspected to be due to postoperative pain, post-anesthetic nausea or secondary to being hospitalized. Postoperative analgesia was provided, and the cat was otherwise stable. With this, the cat was discharged with ondansetron (0.5 mg/kg PO q8h as needed). On day 1, BUN (25 mg/dl, RI 22–33) and creatinine were within normal limits (0.77 mg/dl, RI 0.7–1.9).

Histopathology revealed an infiltrative, poorly defined and encapsulated neoplasm composed of a combination of epithelial, blastemal and mesenchymal components. The neoplastic cells exhibited indistinct borders, abundant eosinophilic fibrillar cytoplasm and oval to elongated nuclei with coarsely stippled chromatin and indistinct nucleoli. No mitotic figures were observed in 10 high-power (400×) fields. Neoplastic cells extended to within 1 mm of the histologic margin. These findings are consistent with renal nephroblastoma.

Given the limited information regarding the benefit of postoperative chemotherapy, active surveillance was recommended. The cat was re-evaluated on postoperative days 14, 77, 171, 251, 336, 420, 511, 685, 864 and 1247 and was doing well. Re-evaluations included a serum chemistry profile, urinalysis, three-view thoracic radiographs and abdominal ultrasound. The subsequent imaging showed no evidence of local recurrence, metastasis or other abnormalities. The serum creatinine remained normal, in the range of 0.77–1.9 mg/dl, with a USG of at least 1.024 at each assessment.

More than 3 years after surgery, the cat remains free of recurrence or metastasis and continues to do well.

Discussion

In humans, favorable histology tumors, lacking anaplasia, are often cured with surgery alone, whereas high-risk or anaplastic tumors require adjuvant chemotherapy or radiotherapy.^7,11 The tumor in this case demonstrated a triphasic structure similar to human nephroblastoma and lacked mitotic figures or anaplasia, suggesting a less aggressive phenotype. Assessment and staging indicated a renal mass without metastasis, supporting that surgical resection alone was reasonable without a definitive preoperative diagnosis. Complete nephrectomy without adjunctive therapy resulted in long-term survival exceeding 3 years, supporting the notion that feline renal nephroblastoma confined to the kidney and perirenal space has the potential to follow a more indolent course than previously reported cases, which often progressed rapidly or metastasized without multimodal therapy.^2,3,8

Imaging characteristics in this cat paralleled human findings. On ultrasound and contrast-enhanced CT, the mass-like lesion was heterogeneous with central hyperattenuation, consistent with hemorrhage and necrosis; ⁷ however, these features are non-specific for neoplasia, and a subcapsular hematoma was initially included as a differential. Interpretation was complicated by minimal contrast enhancement throughout much of the lesion, a finding that may occur in non-neoplastic and neoplastic conditions. In contrast to a previously reported canine nephroblastoma, which demonstrated relatively uniform post-contrast enhancement and peaking during the nephrographic phase, ⁹ the lesion in this cat showed little enhancement despite similar internal heterogeneity.

Clinically, the cat presented with non-specific signs, including lethargy and anorexia, whereas human patients often present with asymptomatic abdominal masses. ¹² Serial imaging over more than 3 years after nephrectomy showed no recurrence or metastasis, in contrast to other feline cases where metastasis was reported at the time of or shortly after the diagnosis.^2,8 These findings highlight that, similar to low-risk human cases, surgery alone may provide durable disease control in select feline cases.

Conclusions

This case suggests that less aggressive variants of feline renal nephroblastoma may exist, based on the clinical course observed in this cat. However, this conclusion must be interpreted cautiously, as it reflects a single case that was likely diagnosed at an early stage because of proactive diagnostic investigation. Although complete nephrectomy without adjunctive therapy resulted in survival beyond 3 years, exceeding outcomes previously reported, the majority of available literature continues to support a predominance of more aggressive behavior in feline nephroblastoma. Diagnostic imaging played a key role in surgical planning and demonstrated features consistent with those described in human nephroblastoma.