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HISTORY

An eight-year-old neutered male Bouvier des Flandres presented to his primary care veterinarian for head shaking and mild hyporexia. Two months ago he experienced an acute episode of severe hind limb weakness that resolved without medical intervention. Abdominal palpation revealed a firm swelling in the cranial mid-abdomen that prompted abdominal radiographs. A mid-abdominal mass effect was identified causing decreased serosal detail and displacement of the small intestines cranially and colon dorsally.

The dog was referred for an abdominal ultrasound which further revealed a large cavitated central abdominal mass with difficulty in identifying an organ of origin. Prior to surgical resection, whole body computed tomography was performed. On the day of computed tomography, the patient presented with a moderately regenerative anemia. No significant abnormalities were identified on a biochemical panel and clotting times revealed a mildly elevated aPTT with normal cit-PT.

Computed tomography was performed with the patient sedated and spontaneously breathing.

 

CT of the trunk in soft tissue algorithm pre- and post-contrast.

Lung CT, lung algorithm

In the central abdomen, there is a large heterogeneously contrast-enhancing mass with a cavitated appearance, with hypoattenuating regions representing areas of hemorrhage or necrosis, and amorphous areas of mineralization. This mass measures approximately 26.5 x 18 x 25 cm. The mass is lobulated with retroperitoneal components. Dorsal portions of the mass become confluent with the caudal pole of the right kidney. It contributes to a severe mass effect with dorsal and slight caudal displacement of the right kidney and dorsal and cranial displacement of the left kidney. The left kidney is otherwise unremarkable.

The mass displaces the portal vein dorsally and to the left. It displaces the caudal vena cava and aorta dorsally with variable compression of the caudal vena cava. The renal vasculature is well visualized but dorsoventrally compressed. The ureters are difficult to entirely trace partially related to lack of contrast within their lumen due to timing of acquisition.

Within the peritoneal cavity, multifocal pockets of effusion are seen with variable mottling of the abdominal mesentery supporting a component of mesenteric edema/inflammation.

The visible portions of the left adrenal gland are unremarkable. The right adrenal gland is difficult to confidently identify.

The urinary bladder is moderately distended. No definitive luminal or mural abnormalities are definitively identified. The prostate is unremarkable.

The medial iliac lymph nodes are mildly prominent in size bilaterally. The inguinal soft tissues are diffusely mottled the inguinal lymph nodes are prominent-mildly enlarged and heterogeneous, displaying a subtle heterogeneous pattern of contrast enhancement.

The spleen is normal.
In the liver, a few curvilinear hypoattenuating areas are seen which are most supportive of a small volume of effusion between liver lobes. No hepatic masses and no convincing nodules are appreciated within the liver.
The gallbladder is mildly distended. with a mild quantity of amorphous, mildly mineralized sludge and outlining some of the margins of the gallbladder wall.

The stomach is displaced cranially, dorsally and slightly more to the left of midline by the abdominal mass but is otherwise normal.
The colon and small intestines are variably displaced into the left and caudal abdomen without apparent abnormalities.

Visualized portions of the pancreas are unremarkable.

At the lumbosacral junction, the intervertebral disc space is narrowed with mild endplate sclerosis. Ventral-right-sided spondylosis deformans is identified with areas of vacuum phenomenon.  At the lumbosacral junction, intervertebral disc protrusion is identified with attenuation of the ventral epidural fat. It contributes to dorsal deviation of the nerve roots of the cauda equina.

Throughout the pulmonary parenchyma, there are several soft tissue attenuating nodules. The largest is identified within the right caudal lung lobe measuring ~ 7 mm in diameter. Patchy groundglass attenuation is also present within the pulmonary parenchyma, greater caudodorsally supporting the presence of coexisting hypoinflation/atelectasis.-moderate sternal lymphadenopathy is identified. These lymph nodes exhibit a mild heterogeneous pattern of contrast enhancement. The cranial mediastinal lymph nodes are also prominent in size. The tracheobronchial lymph nodes are unremarkable.  No definitive cardiovascular abnormalities are seen.

Imaging Diagnosis

  • Large, cavitated, abdominal mass with areas of mineralization, hemorrhage and /or necrosis with a retroperitoneal component. Portions of the mass are confluent with the caudal pole of the right kidney.Primary differential is a neoplastic mass of right renal origin (for example carcinoma, potentially extraskeletal osteosarcoma given the degree of mineralization present). Mesenteric infiltration and/or mass of mesenteric origin (for example mesenchymal neoplasia) with right renal involvement cannot be excluded.
  • Pulmonary metastatic neoplasia with areas of hypoinflation/atelectasis.
  • Sternal and cranial mediastinal lymphadenopathy.
    Primary differential is metastatic lymphadenopathy.
  • Peritoneal effusion (e.g. hemorrhage, ascites from congestion, paraneoplastic effusion) and mesenteric edema are present.
    Additional changes supporting edema are seen along the inguinal soft tissues, presumed secondary to congestion, with mild medial iliac and inguinal lymphadenopathy reactive versus metastatic.
  • Degenerative lumbosacral stenosis

A right ureteronephrectomy was performed. A large mass, approximately 25 cm, was occupying the entire mid-abdomen with omental adhesions. Adhesions were transected until the right kidney, with the mass arising from the caudal pole, was identified and isolated. Once free from all attachments, the right kidney and mass were removed from the abdomen along with the right ureter after ligation at the bladder’s trigone. After anesthetic recovery, the patient developed no significant complications and was discharged from hospital three days post-op.

Histology
Expanding and extensively effacing renal parenchyma is a variably infiltrative neoplasm composed of spindle to polygonal cells arranged in streams, bundles, and solid areas. These cells have oval to fusiform nuclei, stippled chromatin, small nucleoli, and moderate amounts of eosinophilic cytoplasm. There is moderate to marked anisocytosis and anisokaryosis, with >40 mitoses per 10 high power fields. There are scattered multinucleated cells. Cells frequently surround and are embedded in amorphous to mineralized, eosinophilic material (osteoid). There are occasional entrapped glomeruli and tubules. There is multifocal necrosis and hemorrhage. Neoplastic cells occasionally form aggregates within dilated cystic spaces, which could potentially be tumor-embolised vessels. Neoplastic cells appear to extend to margins of the specimens. Given the imaging, surgical and histologic features of this mass, a primary renal osteosarcoma is prioritized, as opposed to primary mesenteric root osteosarcoma.

DISCUSSION

Primary neoplasms may arise from mesenchymal tissues in the kidney. The most common tumors are undifferentiated sarcoma, fibroma/fibrosarcoma and hemangioma/hemangiosarcoma. Cases of leiomyoma/leiomyosarcoma and rare examples of lipomas, osteoma, chondroma, or their malignant counterparts are reported.

Renal osteosarcoma is a rare mesenchymal tumor characterized by osteoid formation and is classified as an extraskeletal form of osteosarcoma. This type of neoplasm is rare in dogs and has been reported in different organs and tissues including liver, esophagus, lung, spleen, eye, mammary gland, adrenal glands and kidneys1,2. Extraskeletal osteosarcoma most commonly occurs in geriatric dogs without breed or sex predilection1,3. It is a highly malignant neoplasm with high rates of distant metastasis, most commonly to the lungs, soft tissues of the liver and kidney1. This tumor type has been described in humans and exhibits similar behavior to the dog2. In this case, mineralization noted in the computed tomography findings and histology is consistent with osteoid formation in cases diagnosed histologically with extraskeletal osteosarcoma2. Given the high rate of distant metastasis, CT confirmed pulmonary metastatic neoplasia in this dog which is similar to previously reported metastatic sites1. Sternal and cranial mediastinal lymphadenopathy is considered most likely metastatic from the primary renal neoplasm due to where these nodes receive lymphatic drainage.

Median survival time in dogs with extraskeletal osteosarcoma varies from 23 to 74 days. Despite a poor prognosis, publication in both humans and dogs shows surgical resection increases survival time. Surgery in addition with chemotherapy has been reported to substantially increase median survival times (146 days as compared to 33 days) and should be considered for dogs when possible1,2.

References

  1. Duffy D, et al. Outcome following treatment of soft tissue and visceral extraskeletal osteosarcoma in 33 dogs: 2008-2013. J Vet and Comp Oncol 2015;15:46-54.
  2. Munday J, et al. Renal osteosarcoma in a dog. J Small Anim Prac 2004;45:618-622.
  3. Bryan J, et al. Primary renal neoplasia of dogs. J Vet Intern Med2006;20:1155-1160.