Ultrasonography of Osteosarcoma in Dogs
Sonography Atlas of Dogs

Statistical Analysis

Among the 3 groups of dogs, differences in each individual bone resorption marker were analyzed using 1-way ANOVA and posthoc comparisons were made with a Tukey-Kramer multiple comparisons test. For each bone resorption marker, 95% confidence intervals were calculated for all 3 groups of dogs. All values were reported as mean ± standard deviation and range. Statistical analysis was performed by commercial computer software.^f Significance was defined as P < .05.

Basal Serum CTx Concentrations

In dogs with OSA, basal serum CTx concentrations were 454.7 ± 189.4 (range, 203.7–808.8) pg/mL. In dogs with orthopedic disorders and normal dogs, serum CTx concentrations were 359.6 ± 40.8 (range, 282.4–434.7) pg/mL and 324.7 ± 47.3 (range, 270.2–448.8) pg/mL, respectively. Basal serum CTx concentrations in dogs with OSA were statistically higher than those in dogs with orthopedic disorders and control dogs (Fig 1E, P < .01). No statistical difference in serum CTx concentrations was identified between dogs with orthopedic disorders and control dogs (P > .05). Although basal serum CTx concentrations were significantly increased in dogs with OSA, the lower and upper 95% confidence limits for OSA dogs (366.1 and 543.4 pg/mL, respectively) did overlap those of dogs with orthopedic disease (340.5 and 378.7 pg/mL, respectively).

Discussion

Although bone resorption markers universally quantify the breakdown products of type I collagen, our study findings suggest that their discriminatory capacity for supporting the diagnosis of focal malignant osteolysis in dogs with OSA is not equivalent. Of the 5 resorption markers studied, only urine NTx, serum NTx, and serum CTx were increased in dogs with OSA, but not in age- and weight-matched healthy control dogs or in dogs with confirmed orthopedic lameness. In addition, only urine NTx and serum NTx concentrations derived from dogs with OSA generated nonoverlapping 95% confidence intervals compared with dogs with orthopedic disorders and healthy dogs. Based on these findings, urine NTx and serum NTx appear to be the most discriminatory bone resorption markers for supporting the diagnosis of focal malignant osteolysis associated with appendicular OSA.

The documented increase in urine NTx, serum NTx, and serum CTx concentrations in dogs with OSA most likely reflects ongoing focal malignant osteolysis in these clinical patients, a pathologic process that is presumed to be absent in normal dogs and in dogs with osteoarthritic diseases. The findings from this study are in agreement with those of a previous investigation that demonstrated increases of a solitary bone resorption marker, urine NTx, in dogs with OSA in comparison with normal, healthy control dogs.¹² However, the current study provides additional information, because not only urine NTx but also serum NTx and serum CTx were determined to support the diagnosis of focal malignant osteolysis. Furthermore, basal concentrations of these 3 specific bone resorption markers were different between dogs with OSA and dogs with orthopedic lameness, a finding that has not been reported previously.

Interestingly, not every bone resorption marker evaluated in this study was capable of differentiating focal malignant osteolysis from either orthopedic disease or homeostatic bone turnover. No difference in basal urine CTx excretion could be identified among the 3 groups of dogs in this study, an unexpected finding given that CTx is a specific breakdown product of collagen type I, the major protein constituent of bone. Another unanticipated result was the similar urine DPD excretion identified in dogs with OSA and in those with orthopedic disorders. Given that chronic osteoarthritis results in articular cartilage breakdown, it would be predicted that orthopedically lame dogs may have increases in collagen type II metabolic products, but not DPD, which is a type I collagen major cross-link.

The identification of noninvasive, supportive diagnostic tests that aid in discriminating between the 2 most common causes of lameness (focal malignant osteolysis and osteoarthritis) in large breed, geriatric dogs may be of clinical importance to veterinarians and pet owners alike. Based upon calculated lower and upper 95% confidence limits, our findings suggest that quantifying basal concentrations of either urine NTx or serum NTx may help to differentiate malignant (OSA) from benign (osteoarthritis) causes of lameness in dogs. Already in humans, the practical assessment of urine NTx in patients being treated for osteoporosis has been facilitated through the development of disposable, handheld, devices^g for routine clinical use. With available technology, it may be possible to quickly and economically evaluate basal urine NTx concentrations in dogs with undetermined causes of lameness.

Although this study provides new information regarding the potential utility of 5 different bone resorption markers in dogs, several limitations should be highlighted. First, the sample sizes for the 3 groups were relatively small, but attempts were made to limit variations among groups, including age (skeletal maturity) and weight (skeletal mass). In addition, the orthopedic disorders that comprised one of our groups were restricted primarily to nonerosive arthropathies, usually cranial cruciate ligament rupture, which is not osteolytic in nature, not generally a primary differential diagnosis for focal malignant osteolytic processes, and highly variable in bone metabolism and clinical progression. As such, it is not surprising that for the most part, bone resorptive markers were not increased in this experimental group. Second, although some bone resorption markers were increased in dogs diagnosed with OSA, this patient population had advanced disease as demonstrated by radiographically evident osteolysis. It remains undetermined if bone resorption marker concentrations would provide similar differential information in dogs with smaller tumor burdens or lesser degrees of focal malignant osteolysis. Third, this investigation evaluated only osteolytic markers and did not assess counter-regulatory osteoblastic activities, both homeostatic and reparative, likely to be operative in the 3 groups of dogs studied. Although the prognostic value of bone alkaline phosphatase (bALP) activities in dogs with OSA has been described previously,^25,26 no study has characterized additional bone formation markers such as osteocalcin (OC), procollagen I N-peptide (PINP), and procollagen I C-peptide (PICP). Given that bALP, OC, PINP, and PICP are proteins produced by osteoblasts, their concentrations may be dysregulated in dogs with OSA, as they are in human cancer patients.^13,27–30 Fourth, it remains undetermined why discrepancies exist between serum and urine CTx concentrations in the 3 groups of dogs evaluated. Specifically, serum CTx was clearly increased in dogs with OSA, but its urine equivalent failed to differentiate dogs with OSA from normal controls and dogs with orthopedic disorders. Intuitively, bone resorption markers in the serum and urine should provide similar information regarding skeletal health, but this did not appear to be the case for all of the bone resorption markers evaluated in this study.

Because the clinical need to provide effective and durable pain relief for dogs suffering from malignant osteolysis and associated cancer pain increases, it will be necessary to validate and study surrogate biochemical markers of skeletal health. Findings from this study demonstrate that considerable variability exists among bone resorption markers for supporting the diagnosis of focal malignant osteolysis. Ultimately, information derived from similar studies may be useful not only for aiding the diagnosis of skeletal malignancy but also for assessing the effectiveness of novel palliative treatments.