Introduction
Appendicular osteosarcoma is an aggressive malignancy with a guarded prognosis in dogs. Although the range of therapeutic options continues to expand, limb amputation or limb sparing followed by systemic chemotherapy remains the only potentially curative option for the disease, yet only 20% of dogs survive > 2 years after diagnosis.1 Whereas clinically evident distant metastasis is diagnosed at admission in only 10% of dogs with appendicular osteosarcoma, approximately 90% of dogs have occult micrometastases by the time osteosarcoma is diagnosed.2,3 Given the rapid growth behavior and metastatic potential of osteosarcoma, adjuvant platinum chemotherapy, with or without alternating doxorubicin, has become the standard of care for dogs undergoing limb amputation, yielding a survival benefit for many dogs.3
The concept that perioperative administration of chemotherapy may improve outcome is not new. A study4 in mice showed that the temporal relationship between tumor excision and chemotherapy is an important variable in determining outcome for osteosarcoma, with mice receiving chemotherapy at the time of surgery experiencing an enhancement of drug effect. This finding is most likely attributable to the recruitment of resting metastatic cells into the cell cycle once the primary tumor has been removed.5,6 In people with osteosarcoma, early initiation of chemotherapy after tumor excision is reported to provide a survival benefit, with data suggesting a worse outcome when chemotherapy is delayed.7,8
In dogs, no clear evidence exists that the TIamp-chemo is associated with better survival times. Adjuvant chemotherapy is commonly started 2 weeks after amputation to maximize patient recovery.1,9,10 A study11 in which survival times were compared for dogs beginning chemotherapy 2 and 10 days after amputation suggested no benefit to early initiation of chemotherapy. It is possible that these 2 time points did not represent the optimal interval for identification of survival benefits.
The purpose of the study reported here was to determine whether an optimal TIamp-chemo could be identified by retrospective analysis of a large cohort of dogs with appendicular osteosarcoma. We hypothesized that early delivery of adjuvant chemotherapy would be associated with a longer OST and TTP.
Materials and Methods
Animals
This multi-institution study was designed by the Italian Society of Veterinary Oncology. Dogs with appendicular osteosarcoma that underwent limb amputation followed by adjuvant chemotherapy between January 2012 and December 2019 at 9 European oncology centers were considered for inclusion in the study. Medical records and reported data were checked and reviewed at each participating oncology center before submission to the investigators for inclusion in the study.
Dogs were included if, in accordance with the current tumor node metastasis staging system,12 they had histologically confirmed appendicular osteosarcoma, no evidence of distant metastases prior to amputation, and a body weight > 15 kg; underwent tumor excision by amputation; received at least 2 cycles of adjuvant dose-intense chemotherapy; and had complete follow-up data. Dogs were excluded if the osteosarcoma involved the metacarpus, metatarsus, or phalanges because of the presumed better prognosis.13 Dogs with scapular osteosarcoma were also excluded because of the different surgical procedure performed (ie, partial or total scapulectomy vs limb amputation). All owners provided written informed consent to the use of their dogs’ medical records for the study.
Tumor staging and follow-up
To be included in the analysis, dogs were required to have undergone tumor staging by means of TBCT or 3-view thoracic radiography and abdominal ultrasonography, plus cytologic or histologic examination of the regional lymph node. When available, results of hematologic and serum biochemical analyses were also recorded. Routine monitoring for pulmonary metastasis by means of thoracic radiography occurred every 2 to 3 cycles of chemotherapy, unless clinical signs suggested the tumor had metastasized, in which case imaging was performed sooner. Follow-up abdominal ultrasonography occurred according to clinician and owner preference. Once chemotherapy was completed, dogs were followed up every 2 to 3 months by means of clinical examinations and thoracic radiography.
Data collection
For the analyses, data were collected from each dog’s medical record concerning variables hypothesized a priori to possibly influence TTP or OST, including age (categorized as ≤ 5 years vs > 5 years),14 sex, body weight, duration of clinical signs prior to diagnosis, site of osteosarcoma (proximal humerus vs other), serum ALP concentration (normal [unremarkable] vs high), monocyte count (normal vs high), lymphocyte count (normal vs high), type of imaging performed for staging (thoracic radiography vs TBCT), presence of lymph node metastasis (yes vs no), chemotherapy-related toxic effects per Veterinary Co-operative Oncology Group criteria15 (yes vs no; grades 3 and 4 vs others), surgical complications (none or minor vs major), TIamp-chemo, and times of noted tumor progression and death relative to osteosarcoma diagnosis, when applicable. Nine diagnostic laboratories provided services to the oncology centers participating in the study, resulting in different reference intervals for the same analytes; therefore, clinicopathologic data were classified as normal or high in accordance with applicable reference intervals. Minor surgical complications were defined as surgery-related events requiring only minorly invasive procedures (eg, management of the surgical wound, infection, or suture dehiscence). Major complications were defined as adverse events requiring a second surgery or resulting in failure of 1 or more organ systems.
Statistical analysis
Survival curves were generated with the Kaplan-Meier product limit method and compared between TIamp-chemo groups with the log-rank test. Survival estimates were computed as medians and 95% CIs. Cox proportional hazards regression was performed to evaluate the influence on TTP and OST of each of the predetermined potentially prognostic variables. For analysis, the continuous variables body weight and duration of clinical signs prior to diagnosis were reclassified as dichotomous variables with the median value used as the cut point. The TIamp-chemo was analyzed as a categorical variable (within 3, 5, 7, 10, 15, 20, 30, or > 30 days after amputation) and as a continuous variable. Variables with a P value < 0.10 in the initial analyses were further tested for independence of association with outcome in a multivariable Cox proportional hazards model.
The risks of tumor progression and death of the dogs within each interval category for chemotherapy initiation were then compared with those of the remaining dogs (eg, ≤ 3 days vs > 3 days and ≤ 5 days vs > 5 days) by means of Cox proportional hazards regression, and the interval with the highest, significant (P < 0.05) HR was selected as the optimal interval. For the continuous interval data, the Mann-Whitney U test was applied to assess interval differences between dogs that survived and did not survive to 1 year after amputation.
Distributions of the potential prognostic variables were compared between dogs that did or did not receive adjuvant chemotherapy within the selected optimal interval by means of the Fisher exact or χ2 test. Analyses were performed with the aid of a statistical software program.a Values of P < 0.05 were considered significant.
Results
Animals
A total of 168 dogs with appendicular osteosarcoma were included in the study. Overall, 86 (51.2%) dogs were male (36 castrated and 50 sexually intact) and 82 (48.8%) were female (52 spayed and 30 sexually intact). Median age was 8 years (range, 1 to 14 years); 34 (20.2%) dogs were < 5 years old, and 134 (79.8%) were > 10 years old. Median body weight was 32.2 kg (range, 16.2 to 75 kg). Breed was classified as purebred (n = 134 [79.8%]) or mixed breed (34 [20.2%]). The most common breeds were Rottweiler (n = 27 [16.1%]), German Shepherd Dog (13 [7.7%]), Boxer (11 [6.5%]), Greyhound (7 [4.2%]), Great Dane (5 [3.0%]), and Schnauzer (5 [3%]). All other breeds were each represented by < 3% of dogs.
Clinical findings
All dogs were lame at the time of presentation; median duration of clinical signs was 28 days (range, 2 to 120 days). Sites of osteosarcoma included the distal aspect of the radius (n = 44 [26.2%]), proximal aspect of the humerus (38 [22.6%]), distal aspect of the femur (33 [19.6%]), distal aspect of the tibia (22 [13.1%]), proximal aspect of the femur (13 [7.7%]), proximal aspect of the tibia (11 [6.5%]), and distal aspect of the ulna (7 [4.2%]).
Serum ALP activity was measured in 99 (58.9%) dogs before amputation and exceeded the upper reference limit in 22 (22.2%) of those dogs. Monocyte and lymphocyte counts were recorded for 98 (58.3%) dogs; 11 (11.2%) of those dogs had monocytosis, and 1 (1.0%) had lymphocytosis.
No dog had evidence of intrathoracic or abdominal metastasis before beginning chemotherapy. Tumors were staged by means of TBCT in 89 (53.0%) dogs, whereas 79 (47.0%) dogs underwent staging by 3-view thoracic radiography and abdominal ultrasonography. Histologic examination of the excised lymph node was performed in 98 (58.3%) dogs and cytologic examination of the regional lymph node in 70 (41.7%) dogs. Overall, 7 (4.2%) dogs had histologically confirmed regional lymph node metastasis.
Amputation, adjuvant chemotherapy, and adverse events
The osteosarcoma was removed surgically via coxofemoral disarticulation (n = 66 [39.3%]), forequarter amputation (57 [33.9%]), scapulohumeral joint disarticulation (32 [19.0%]), and hemipelvectomy in conjunction with pelvic limb amputation (13 [7.7%]). Overall, 45 (26.8%) dogs experienced surgical complications, 38 of which were classified as minor and 7 as major. Minor complications included seroma (n = 24), hematoma (4), mild wound infection (4), edema (2), partial suture dehiscence (1), tracheitis (1), otitis externa attributed to wearing of an Elizabethan collar (1), and contralateral muscle injury (1). Major complications included wound dehiscence requiring a second surgery (n = 2), phantom limb syndrome (1), disseminated intravascular coagulopathy (1), soft tissue necrosis requiring a second surgery (1), Escherichia coli infection requiring a second surgery (1), and life-threatening bleeding from the amputation site (1). All dogs with surgical complications recovered fully.
All dogs received adjuvant chemotherapy IV; 107 (63.7%) received carboplatin alone (median, 4 doses; range, 2 to 6 doses), 12 (7.1%) received doxorubicin alone (median, 4 doses; range, 3 to 5 doses), 11 (6.5%) received cisplatin alone (median, 3 doses; range, 3 to 4 doses), 33 (19.6%) received alternating doses of a platinum compound (cisplatin [n = 25] or carboplatin [8]) and doxorubicin (median, 4 doses; range, 2 to 8 doses), 4 (2.4%) received both cisplatin and dacarbazine (median, 4 doses; range, 4 to 6 doses), and 1 (0.6%) received alternating doses of carboplatin and epirubicin (3 doses each). Carboplatin was administered at a median dose of 300 mg/m2 (range, 210 to 300 mg/m2), doxorubicin at 30 mg/m2 (range, 15 to 30 mg/m2), cisplatin at 70 mg/m2 (range, 60 to 70 mg/m2), dacarbazine at 200 mg/m2/d for 5 consecutive days, and epirubicin at 30 mg/m2.
Data on toxic effects of chemotherapy were available for 156 (92.9%) dogs, of which 56 (35.9%) experienced adverse events. Overall, there were 40 episodes of toxic effects involving the bone marrow (27 grade 1, 6 grade 2, 5 grade 3, and 2 grade 4 toxic effects), 17 episodes of toxic effects involving the gastrointestinal tract (7 grade 1, 9 grade 2, and 1 grade 3), 3 episodes of grade 1 lethargy, and 1 episode of grade 2 toxic effects involving the kidneys. Overall, 8 dogs (14.3% of the 56 dogs experiencing toxic effects and 5.1% of all 156 dogs with available data) had grade 3 or 4 toxic effects. Two dogs with grade 4 toxic effects involving the bone marrow required a dose decrease and hospitalization. No dogs required postponement of dose administration.
Outcomes
Of the 168 dogs, 148 (88.1%) died and 20 (11.9%) remained alive at the point of data analysis (median follow-up time, 323 days; range, 115 to 1,196 days). Among the dogs that died, 135 (91.2%) died of tumor-related causes. The 1-, 2-, and 3-year survival rates were 38.0% (60/158), 11.2% (17/151), and 6.0% (9/149), respectively. Median TTP was 230 days (95% CI, 178 to 282 days), and median OST was 278 days (95% CI, 226 to 330 days).
Time interval between amputation and adjuvant chemotherapy
Median TIamp-chemo was 14 days (range, 1 to 210 days). Specifically, 40 dogs received adjuvant chemotherapy within the first 3 days after amputation, 52 within 5 days, 61 within 7 days, 69 within 10 days, 99 within 15 days, 127 within 20 days, 152 within 30 days, and the remaining 16 after 30 days.
Dogs experiencing no surgical complications (n = 123) began receiving chemotherapy a median of 10 days (range, 1 to 210 days) after amputation. Chemotherapy was initiated in dogs experiencing minor surgical complications a median of 16 days (range, 2 to 49 days) after amputation, whereas a significantly (P < 0.001) longer TIamp-chemo was observed for dogs experiencing major complications (median, 28 days; range, 24 to 41 days). Among the 123 dogs with no surgical complications, 10 (8.1%) began receiving chemotherapy > 30 days after amputation.
Dogs that survived for at least 1 year after amputation began receiving adjuvant chemotherapy significantly (P = 0.004) earlier (median, 6 days) than did nonsurvivors (median, 14 days). The TIamp-chemo associated with the best survival benefit was 5 days: median TTP for dogs with a TIamp-chemo ≤ 5 days was significantly (P = 0.005) longer than that for dogs with a TIamp-chemo > 5 days (Figure 1; Table 1).
Kaplan-Meier curves of TTP for 168 dogs with appendicular osteosarcoma that began receiving adjuvant chemotherapy ≤ 5 days (dotted line; n = 52; median, 375 days) or > 5 days (solid line; 116; 202 days) after amputation of the affected limb (P = 0.005). Tick marks represent censored dogs.
Citation: Journal of the American Veterinary Medical Association 259, 7; 10.2460/javma.259.7.749
Kaplan-Meier curves of TTP for 168 dogs with appendicular osteosarcoma that began receiving adjuvant chemotherapy ≤ 5 days (dotted line; n = 52; median, 375 days) or > 5 days (solid line; 116; 202 days) after amputation of the affected limb (P = 0.005). Tick marks represent censored dogs.
Citation: Journal of the American Veterinary Medical Association 259, 7; 10.2460/javma.259.7.749
Kaplan-Meier curves of TTP for 168 dogs with appendicular osteosarcoma that began receiving adjuvant chemotherapy ≤ 5 days (dotted line; n = 52; median, 375 days) or > 5 days (solid line; 116; 202 days) after amputation of the affected limb (P = 0.005). Tick marks represent censored dogs.
Citation: Journal of the American Veterinary Medical Association 259, 7; 10.2460/javma.259.7.749
Comparison of various characteristics for 168 dogs with appendicular osteosarcoma that began receiving adjuvant chemotherapy ≤ 5 days or > 5 days after amputation of the affected limb.
| Characteristic | ≤ 5 days (n = 52) | > 5 days (n = 116) | P value |
|---|---|---|---|
| Sex (No. [%]) | 0.65 | ||
| Male | 28 (54) | 58 (50) | |
| Female | 24 (46) | 58 (50) | |
| Age (No. [%]) | 0.15 | ||
| ≤ 5 y | 14 (27) | 20 (17) | |
| > 5 y | 38 (73) | 96 (83) | |
| Body weight* (No. [%]) | 0.02 | ||
| ≤ 32 kg | 33 (63) | 50 (43) | |
| > 32 kg | 19 (37) | 66 (57) | |
| Duration of clinical signs*† (No. [%]) | 0.92 | ||
| ≤ 28 d | 9 (50) | 52 (51) | |
| > 28 d | 9 (50) | 49 (49) | |
| Tumor site (No. [%]) | 0.20 | ||
| Proximal aspect of the humerus | 15 (29) | 23 (20) | |
| Other | 37 (71) | 93 (80) | |
| Serum ALP activity† (No. [%]) | 0.18 | ||
| Normal | 14 (93) | 63 (75) | |
| High | 1 (7) | 21 (25) | |
| Monocyte count† (No. [%]) | 0.68 | ||
| Normal | 16 (94) | 71 (88) | |
| High | 1 (6) | 10 (12) | |
| Lymphocyte count† (No. [%]) | 0.17 | ||
| Normal | 16 (94) | 81 (100) | |
| High | 1 (6) | 0 (0) | |
| Modality used for tumor staging (No. [%]) | < 0.001 | ||
| TBCT | 15 (29) | 74 (64) | |
| Rad + US | 37 (71) | 42 (36) | |
| Regional lymph node metastasis (No. [%]) | 0.44 | ||
| No | 51 (98) | 110 (95) | |
| Yes | 1 (2) | 6 (5) | |
| Surgical complications (No. [%]) | 0.10 | ||
| None or minor | 52 (100) | 109 (94) | |
| Major | 0 (0) | 7 (6) | |
| Chemotherapy-related toxic effects† (No. [%]) | 0.002 | ||
| No | 38 (83) | 62 (56) | |
| Yes | 8 (17) | 48 (44) | |
| Grade 3 or 4 toxic effects† (No. [%]) | 0.44 | ||
| No | 45 (98) | 103 (94) | |
| Yes | 1 (2) | 7 (6) | |
| Median (95% CI) TTP (d) | 375 (162–588) | 202 (146–257) | 0.005 |
| Median (95% CI) OST (d) | 445 (345–545) | 239 (187–291) | 0.003 |
| Survival rate (proportion [%]) | |||
| 1 y | 29/50 (58) | 31/108 (29) | < 0.001 |
| 2 y | 9/44 (20) | 8/107 (7) | 0.03 |
| 3 y | 5/43 (12) | 4/106 (4) | 0.12 |
The cut point used to dichotomize this continuous variable represents the median value for all dogs.
Data were unavailable for some dogs.
Rad + US = 3-view thoracic radiography and abdominal ultrasonography.
Median OST for dogs with a TIamp-chemo ≤ 5 days was significantly (P = 0.003) longer than that for dogs with a TIamp-chemo > 5 days (Figure 2). Dogs with a TIamp-chemo > 5 days (vs ≤ 5 days) had a 1.7-fold increase in the hazard (interpreted as risk) of both tumor progression and death (Table 2), without controlling for other factors. On the other hand, dogs with a TIamp-chemo ≤ 5 days had significantly higher 1- and 2-year survival rates (Table 1). Median TTP and OST for the 16 dogs with a TIamp-chemo > 30 days were 136 days (95% CI, 122 to 150 days) and 169 days (95% CI, 102 to 236 days), respectively. A longer duration of clinical signs was the only other variable significantly associated with an increased risk of tumor progression, without controlling for other factors, whereas age > 5 years was associated with an increased risk of nonsurvival (Table 2).
Kaplan-Meier curves of OST for the dogs of Figure 1. Median OST was significantly (P = 0.003) longer for dogs with a TIamp-chemo ≤ 5 days (445 days) versus dogs with a TIamp-chemo > 5 days (239 days). See Figure 1 for remainder of key.
Citation: Journal of the American Veterinary Medical Association 259, 7; 10.2460/javma.259.7.749
Kaplan-Meier curves of OST for the dogs of Figure 1. Median OST was significantly (P = 0.003) longer for dogs with a TIamp-chemo ≤ 5 days (445 days) versus dogs with a TIamp-chemo > 5 days (239 days). See Figure 1 for remainder of key.
Citation: Journal of the American Veterinary Medical Association 259, 7; 10.2460/javma.259.7.749
Kaplan-Meier curves of OST for the dogs of Figure 1. Median OST was significantly (P = 0.003) longer for dogs with a TIamp-chemo ≤ 5 days (445 days) versus dogs with a TIamp-chemo > 5 days (239 days). See Figure 1 for remainder of key.
Citation: Journal of the American Veterinary Medical Association 259, 7; 10.2460/javma.259.7.749
Results of Cox proportional hazards regression showing the risk of tumor progression and nonsurvival over the study period for the 168 dogs of Table 1 with various characteristics, without controlling for other factors.
| Characteristic | Tumor progression HR (95% CI) | Nonsurvival P value | Nonsurvival | P value |
|---|---|---|---|---|
| Female (vs male) | 0.9 (0.6–1.2) | 0.51 | 1.0 (0.7–1.4) | 0.92 |
| Age > 5 y (vs ≤ 5 y) | 1.4 (0.9–2.1) | 0.14 | 1.5 (1.0–2.3) | 0.050 |
| Body weight* > 32 kg (vs ≤ 32 kg) | 1.0 (0.7–1.4) | 0.98 | 1.1 (0.8–1.5) | 0.50 |
| Duration of clinical signs*† > 28 d (vs ≤ 28 d) | 1.5 (1.0–2.3) | 0.03 | 1.5 (0.9–2.2) | 0.06 |
| Tumor located in proximal aspect of the humerus (vs other site) | 1.3 (0.9–1.9) | 0.19 | 1.3 (0.9–1.9) | 0.14 |
| High (vs normal) serum ALP activity† | 1.1 (0.7–1.8) | 0.66 | 1.1 (0.7–1.8) | 0.66 |
| High (vs normal) monocyte count† | 1.7 (0.8–3.2) | 0.12 | 1.6 (0.8–3.2) | 0.15 |
| Rad + US (vs TBCT) used for staging | 1.1 (0.8–1.5) | 0.60 | 1.1 (0.8–1.6) | 0.41 |
| Regional lymph node metastasis (yes vs no) | 0.9 (0.4–2.2) | 0.87 | 1.2 (0.5–2.6) | 0.68 |
| Major (vs no or only minor) surgical complications† | 1.2 (0.9–1.8) | 0.24 | 1.2 (0.9–1.8) | 0.23 |
| Chemotherapy-related toxic effects† (yes vs no) | 0.8 (0.6–1.2) | 0.39 | 0.9 (0.6–1.2) | 0.41 |
| Grade 3 or 4 toxic effects† (yes vs no) | 0.6 (0.3–1.2) | 0.13 | 0.5 (0.2–1.1) | 0.09 |
| TIamp-chemo > 5 d (vs ≤ 5 d) | 1.7 (1.2–2.4) | 0.005 | 1.7 (1.2–2.5) | 0.003 |
See Table 1 for remainder of key.
On multivariable analysis, a TIamp-chemo > 5 days (HR, 1.6; 95% CI, 1.0 to 2.3; P = 0.03) and duration of clinical signs > 28 days (HR, 2.8; 95% CI, 1.4 to 5.4; P = 0.002) were significantly associated with an increased risk of tumor progression. The TIamp-chemo was the only variable that retained a significant association with OST on multivariable analysis (Table 3).
Results of multivariable Cox proportional hazards regression showing the risk of nonsurvival over the study period for the 168 dogs of Table 1 with various characteristics, controlling for other factors.
| Characteristic | HR (95% CI) | P value |
|---|---|---|
| Age > 5 y (vs < 5 y) | 1.1 (0.5–2.6) | 0.72 |
| Duration of clinical signs | 1.5 (0.9–2.2) | 0.07 |
| > 28 d (vs ≤ 28 d) | ||
| TIamp-chemo > 5 d (vs ≤ 5 d) | 2.3 (1.2–4.5) | 0.01 |
| Grade 3 or 4 toxic effects (yes vs no) | 1.1 (0.5–2.6) | 0.72 |
Examination of each potential prognostic variable separately between dogs with a TIamp-chemo ≤ 5 days versus > 5 days revealed that the 2 groups were well balanced except for body weight (lower proportion of dogs > 32 kg in the ≤ 5 days group), imaging modality use for tumor staging (lower proportion of dogs undergoing TBCT in the ≤ 5 days group), and chemotherapy-related toxic effects (lower proportion of dogs in the ≤ 5 days group; Table 1).
Discussion
Although the benefit of adjuvant chemotherapy for dogs with resected osteosarcoma has been documented, the optimal timing after limb amputation has not been previously defined. In the present retrospective study, dogs with appendicular osteosarcoma without distant metastases that began receiving adjuvant chemotherapy earlier after amputation than other dogs lived significantly longer than those other dogs. The clearest difference was observed when comparing dogs with a TIamp-chemo ≤ 5 days (median OST, 445 days) with those with a TIamp-chemo > 5 days (median OST, 239 days).
These findings have several possible biological explanations. The ultimate goal of adjuvant chemotherapy is to decrease the chance of metastasis development by eradicating neoplastic cells after tumor excision. According to the mathematic model by Goldie and Coldman,16 drug sensitivity of neoplastic cells is related to their rate of spontaneous mutation toward a drug-resistant phenotype. Because the tumor mutation rate increases over time, a longer interval between tumor excision and chemotherapy may increase the occurrence of a chemoresistant cell phenotype. Also, a long interval between tumor excision and chemotherapy may facilitate the proliferation of residual malignant cells. This possibility has been documented in mice, wherein excision of the primary tumor leads to an increase in the number of circulating tumor cells, thereby accelerating the progression of the micrometastatic burden.6 Other reported findings suggest that primary tumor removal accelerates angiogenesis by release of oncogenic growth factors and permits tumor growth through induction of immunosuppression.17–21 For these reasons, chemotherapy is believed to be more effective if initiated when the tumor burden is low and when cancer cells are multiplying most rapidly. The reason why chemotherapy was delayed in some dogs of the present study remains unclear but may have included a delay in referral, surgical complications, logistical issues, or oncologist or owner choice.
Interestingly, although the 1- and 2-year survival rates in the present study were significantly higher for dogs receiving early versus delayed adjuvant chemotherapy, this advantage was no longer evident at 3 years. Surviving dogs were too few for meaningful comparisons at 3 years, so the results for this time point should be regarded with caution. However, if this loss of a differential effect of the interval between amputation and adjuvant chemotherapy is real, it would highlight that osteosarcoma remains a largely incurable disease for which new treatment strategies are greatly needed.
Overall, 45 (26.8%) dogs had some form of surgical complication in the present study. The observation that 8 of these dogs had a TIamp-chemo > 30 days was not unexpected because animals with postoperative complications would likely require more time for recovery, but what was most surprising was that 8% of the dogs with no reported complications had a TIamp-chemo > 30 days. Although the reasons behind this were not investigated, possibilities include a delay in obtaining the histopathological diagnosis owing to sample decalcification or a delay in making a referral to an oncologist after diagnosis.
Some authors have argued that the risk of perioperative chemotherapy-induced toxic effects may be maximized due to immunodepression after surgery; therefore, a brief interval between tumor excision and chemotherapy may cause severe adverse events.22 In the present study, initiation of adjuvant chemotherapy within 5 days after amputation was feasible, with no increase in incidence of grade 3 or 4 toxic effects. We also found that the risk of chemotherapy-related toxic effects was not increased to the same extent as the risk of nonsurvival in the group with delayed initiation of chemotherapy. Multidisciplinary treatment strategies are needed to reduce the incidence of postoperative complications and promote timely adjuvant chemotherapy.23
Various chemotherapy protocols were used in the present study; however, previous reports3,10 have indicated similar outcomes regardless of protocols, number of doses, and intertreatment interval.
A further unanswered question is whether an ideal timing exists for initiation of adjuvant chemotherapy, after which the benefit of such treatment decreases. In the present study, dogs receiving chemotherapy > 30 days after amputation had a median TTP and OST of 136 and 169 days, respectively. For amputation alone, a median survival time of 119 to 175 days has been reported in dogs with osteosarcoma,14,24,25 with most animals euthanized because of metastatic disease. A crude comparison with those data indicates that the survival benefit of adjuvant chemotherapy is almost lost with a delay of approximately 4 weeks from amputation, and it is questionable whether systemic treatment is appropriate after this time.
The suggestion that the survival benefit of adjuvant chemotherapy is time-dependent is not new. In addition to the aforementioned explanation introduced by Goldie and Coldman,16 the mathematical model by Harless and Qiu26 indicates the effectiveness of chemotherapy is inversely proportional to the tumor burden requiring eradication, which, in turn, is a function of the interval between tumor excision and initiation of chemotherapy.
In people with high-grade appendicular osteosarcoma, neoadjuvant chemotherapy followed by tumor excision and adjuvant chemotherapy is the most frequently used strategy. Neoadjuvant chemotherapy is aimed at treating micrometastatic disease early, thereby reducing the risk of distant metastasis, and provides an indication of tumor responsiveness to chemotherapy.27 In dogs with appendicular osteosarcoma, chemotherapy reportedly does not significantly increase survival time when initiated preoperatively or intraoperatively14,24,25; nevertheless, this finding has not been confirmed through prospective studies and merits further investigation.
Multivariable analyses in the study reported here revealed that a long duration of clinical signs was significantly associated with an increased risk of tumor progression. This was not unexpected given that dogs that had experienced clinical signs for a longer period were more likely to have had time for micrometastases to become established at distant sites and, therefore, their disease would progress sooner.
Findings of the present study need to be interpreted in the context of the strengths and limitations of the study design. Although every effort was made to reduce any selection bias, the retrospective design resulted in the absence of some information, such as serum ALP activity and lymphocyte and monocyte counts, from the medical records. Dogs were included only if they received at least 2 doses of chemotherapy because the response to only 1 dose would have been difficult to evaluate, given that the first restaging by means of imaging was suggested after 2 to 3 doses of chemotherapy.
In the present study, approximately 50% of dogs underwent staging by thoracic radiography and abdominal ultrasonography, which may have led to an underestimation of tumor stage. However, the choice of imaging technique did not appear to affect tumor progression or nonsurvival. Furthermore, only 28% of dogs that received chemotherapy ≤ 5 days after amputation underwent TBCT, compared with 64% of dogs that received chemotherapy at a later time point, and this difference was significant. Because CT has superior resolution for the detection of metastasis, this observation reinforced the finding of improved survival times for dogs that began receiving adjuvant chemotherapy earlier versus later after amputation. Although restaging with thoracic radiography was recommended every 2 to 3 doses of chemotherapy for all dogs, the actual frequency of restaging varied, and this lack of standardization may have led to an overestimation of TTP.
Another study limitation was that only 60% of dogs underwent histologic examination of the regional lymph node, even though all dogs underwent cytologic examination. Therefore, the true prevalence of lymph node metastasis may have been underestimated. Similarly, slides were not reviewed, and histologic grade and osteosarcoma subtypes were not recorded for many dogs, which also could have biased outcome measurements. Furthermore, the lack of necropsy data and standardized diagnostic testing to determine cause of death may have biased outcome estimates. Despite the potential flaws in the accuracy of the information collected, we do not believe these shortcomings invalidated the study conclusions.
In conclusion, findings of the present study supported our hypothesis that initiation of adjuvant chemotherapy within 5 days after amputation of the affected limb is associated with a significant survival benefit in dogs with nonmetastatic appendicular osteosarcoma. These results suggested that the timing of chemotherapy initiation is an important variable and that efforts should be made to minimize postsurgical recovery time. Given the low prevalence of chemotherapy-related toxic effects among dogs receiving chemotherapy ≤ 5 days after amputation, we believe adjuvant chemotherapy should be considered, in accordance with a patient’s condition, at the earliest possible time after tumor excision, as in people.7,8 Additional research is warranted to verify these results.
Acknowledgments
No third-party funding or support was received in connection with this study or the writing or publication of the manuscript. The authors declare that there were no conflicts of interest.
Findings of this study were presented in part at the 30th ECVIM-CA Annual Congress.
The authors thank Adele Barbanera, Paolo Guazzi, and Maurizio Annoni for contributing cases to the study.
Footnotes
SPSS Statistics, version 19, IBM Corp, Armonk, NY.
Abbreviations
| ALP | Alkaline phosphatase |
| HR | Hazard ratio |
| OST | Overall survival time |
| TBCT | Total body computed tomography |
| TIamp-chemo | Time interval between amputation and adjuvant chemotherapy |
| TTP | Time to tumor progression |
References
- 1. ↑
Culp WT, Olea-Popelka F, Sefton J, et al. Evaluation of outcome and prognostic factors for dogs living greater than one year after diagnosis of osteosarcoma: 90 cases (1997–2008). J Am Vet Med Assoc 2014;245:1141–1146.
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Withrow SJ, Powers BE, Straw RC, et al. Comparative aspects of osteosarcoma. Dog versus man. Clin Orthop Relat Res 1991;270:159–168.
- 3. ↑
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