Diagnostic Utility of Abdominal Ultrasonography for Routine Staging at Diagnosis of Skeletal OSA in Dogs

Introduction

Osteosarcoma (OSA) accounts for up to 85% of all malignancies originating in the bone, making it the most common primary bone neoplasm in the canine patient.^1–3 OSA is known not only to be locally invasive but also aggressively metastatic.^4,5 Common staging modalities after initial diagnosis of OSA include regional lymph node palpation with or without cytology, three-view thoracic radiographs, complete blood count, serum biochemical profile, nuclear bone scintigraphy, and, in some cases, thoracic computed tomography and abdominal ultrasonography.³ Many studies have been performed to determine the initial metastatic rate to various organ systems. Pulmonary metastasis is reported to be the most common as 5–10% of dogs have radiographically detectable metastases at the time of diagnosis.^3,6 Between 1.4% and 28% of dogs have evidence of osseous metastasis, and 4.4% of dogs have evidence of metastasis to regional lymph nodes at the time of diagnosis.^7–11

To the authors’ knowledge, the rate of abdominal organ metastases from canine skeletal OSA is not known. In addition, no published information exists regarding the diagnostic utility of abdominal ultrasonography in initial staging for OSA. The purpose of this study was to determine whether abdominal ultrasonography should be performed routinely at the initial diagnosis of skeletal OSA in all dogs or if certain abnormal physical examination/diagnostic findings would provide greater indication to perform this diagnostic procedure. The authors of this report hypothesized that the rate of skeletal OSA metastases to the abdominal organs at diagnosis would be very low, and that abdominal ultrasonography would not be a useful routine staging diagnostic for canine patients with OSA.

Materials and Methods

Records from canine OSA cases seen at the Colorado State University College of Veterinary Medicine Veterinary Teaching Hospital from 1996 to 2010 were retrospectively reviewed. Dogs were selected for inclusion in the study if they had a diagnosis of skeletal OSA (axial or appendicular) based on histopathology, cytology, or radiography and had a complete abdominal ultrasonogram performed at diagnosis. Abdominal ultrasonography reports and case records were reviewed to obtain information of interest, including the reason for performing abdominal ultrasonography (i.e., routine staging, abnormal biochemical profile values, palpable abdominal mass on physical examination, and other/miscellaneous); any abnormalities identified on the ultrasonogram; and diagnostic tool (cytology or histopathology) used to assess and diagnose the cause of abnormal findings. Additional data collected on each dog included whether treatment was performed following diagnosis; outcome (alive, dead, or lost to follow-up); and the date of death, euthanasia, or last follow-up (either last visit or last owner/veterinarian communication verifying the dog was alive).

Results

Eighty dogs with skeletal OSA were included in the study. Sixty-three dogs had a primary lesion identified in the appendicular skeleton, and the remaining 17 had lesions in the axial skeleton. Table 1 summarizes the included dogs’ data, subdivided by the reason for the abdominal ultrasonogram. The number of abnormal findings, the number of abnormal findings that were further investigated, and the number of dogs with neoplasia identified are displayed.

TABLE 1 Summary of the Reasons for Performing an Abdominal Ultrasonogram and Number of Abnormalities in 80 Dogs

*

One each of pheochromocytoma, lymphoma, hepatocellular carcinoma, and adenocarcinoma of the sublumbar lymph node

†

Abnormal serum biochemical profile values included seven cases with abnormal liver enzymes, two cases with abnormal renal values, one case with hypoglycemia, and one case with hypercalcemia.

§

Other reasons included previous history of other primary neoplasia (n = 5), illness (n = 5), and abdominal effusion (n = 1).

OSA, osteosarcoma.

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Thirty-six of the 80 ultrasonograms had abnormal findings, with 23 of the 36 abnormalities further investigated via either cytology or histopathology. The remaining 13 of the 36 ultrasonograms with abnormalities were not investigated due to either the owner’s wish not to further pursue diagnostics (5 of the 13) or due to low clinical suspicion of a significant abnormality (8 of the 13). Nineteen of the 23 ultrasonograms that were further investigated were deemed benign. None of the ultrasonograms revealed abdominal OSA metastases at initial diagnosis; however, 4 of the 80 ultrasonograms (5%) revealed another primary neoplasia.

Treatment was administered to 65 of the 80 dogs. Treatment modalities used were surgery, including either amputation or limb sparing procedures (n = 14), surgery with chemotherapy (n = 30), radiation therapy with chemotherapy (n = 12), surgery with chemotherapy and radiation therapy (n = 1), chemotherapy alone (n = 1), and palliative radiation (n = 12). Two of the 80 dogs (2.5%) were euthanized due to an abdominal abnormality (one dog with splenic hemangiosarcoma and one with undifferentiated sarcoma in the spleen). Due to abnormalities seen on ultrasonogram, both dogs had fine-needle aspirates of the spleen performed at the time of initial ultrasonography; however, the cytology was benign for both dogs. Of the 13 dogs that had ultrasonographic abnormalities without further investigation, 8 were treated for OSA. Follow-up data were available for six of those eight dogs. None were either euthanized or died as a result of an abdominal abnormality.

Discussion

Abdominal organ metastasis of skeletal OSA at diagnosis is rare; no cases were identified in this study. Although abdominal ultrasonography may be performed as part of the initial staging for OSA, it may be low yield in this setting. It is important to note that dogs with abnormal physical examinations may be at higher risk for having another primary neoplastic process than dogs with normal physical examinations; therefore, abdominal ultrasonography may be justified in those cases.

The absence of abdominal metastases of OSA identified at diagnosis in this study population is consistent with the lack of published literature regarding this particular form of metastasis. To the authors’ knowledge, no reports of dogs with abdominal organ metastases at diagnosis exist. An abstract evaluating the rate of abdominal metastases in a population of 123 dogs showed a similarly low rate of abdominal organ metastases in their population.¹²

In 5% of the dogs included in the current study, abdominal ultrasonography detected another malignancy. A study that evaluated a population of patients on initial presentation to an oncology department with various neoplasms found that it was uncommon (3%) to detect multiple distinct malignancies. In that study, OSA was not one of the most common primary tumors associated with concurrent malignancies, but 3.2% of the OSA population in the study had a concurrent malignancy identified, similar to the 5% found in the current study.¹³ Therefore, abdominal ultrasonography may aid in identification of other malignant processes that could be an important part of the decision-making process for the owner deciding between treatment options.

In the current study, an abdominal ultrasonogram was recommended either for routine staging or due to other abnormalities determined on physical examination and/or serum biochemical profile. In the setting of an identified abnormality, ultrasonography may be of a higher diagnostic yield. Two of the nine dogs (22%) that received an ultrasonogram due to a palpable abdominal mass on physical examination were diagnosed with another primary neoplasia. This is in direct contrast to the dogs that received an abdominal ultrasonogram as part of routine initial staging. Only one of those 49 dogs (2%) was diagnosed with another primary neoplasia. Based on the results of this study, patient selection for abdominal ultrasonography prior to OSA treatment is important.

It is also important to note the possible selection bias present surrounding the 13 dogs that did not have either cytologic or histopathologic confirmation of the abnormalities seen on abdominal ultrasonogram. Sixty-two percent of those dogs did not have either aspirates or biopsies performed due to the clinicians’ low suspicion of a significant abnormality. The fact that none of those dogs were euthanized due to abdominal abnormalities appears to indicate that the low index of clinical suspicion of abdominal malignancy was accurate.

One of the limitations of this study was the number of cases available for outcome assessment. Having a larger number of cases would further confirm the results of this study. Possible case selection bias may also be present as not every dog with OSA at the authors’ institution receives an ultrasonogram at the time of diagnosis. A prospective study where every dog diagnosed with OSA receives an abdominal ultrasonogram would eliminate this area of bias.

Conclusion

This study shows a low rate of abdominal metastases in a population of dogs with skeletal OSA. Further, abdominal ultrasonography as a part of initial staging is unlikely to reveal abdominal metastases from OSA and therefore may not be a useful staging tool for most dogs. However, in a certain population of dogs, such as those with palpable abdominal masses, abdominal ultrasonography can reveal additional disease processes that may affect the owner’s treatment decisions.