Rib Tumor Features and Outcome
Clinical features and outcome for dogs with rib tumors was consistent with other reports.1–6 The 2 most common rib tumors were primary OSA and CSA.3–6 Signalment was typically older, large breed dogs with no breed or sex predilection.2–6 Age is an inconsistent finding in dogs with rib tumors, with dogs reported to be either young (median, 2.0–4.5 years)2,4,5 or old (median, 7.0–9.0 years)3–6 on admission.
Clinical signs were similar to those in other reports, including visible or palpable chest wall mass, thoracic limb lameness, and altered breathing pattern.2–6 Thoracic limb lameness can be caused by mechanical interference or muscular or brachial plexus invasion from primary rib tumors arising from the 1st to 4th ribs.6 Causes for altered breathing include decreased lung volume and expansion because of the intrapleural extent of the chest wall tumor or pleural effusion.1,2,4,12
Radiographically, rib tumors were subjectively defined as primarily lytic, blastic, or mixed. There were no significant differences in radiographic patterns between different tumor types; hence, radiographic changes cannot be used to determine tumor type.2 Rib tumors were evenly distributed from ribs 2 to 8, and there was no predilection for the left or right chest wall. Others have reported that ribs 4–92,3,5,6 or 9–1313 and either the left or right chest wall were more commonly affected.2,3 Although we did not investigate the location of tumors along the rib, most rib tumors arise from or near to the costochondral junction.2,4,6 Advanced imaging was infrequently performed in our dogs; however, CT scans are recommended for both local and distant staging of chest wall tumors.14 Local staging assists in surgical planning by determining tumor size and location, extent of rib involvement (both number of ribs and dorsal and ventral extent), and whether there is adhesion or invasion into adjacent structures (lungs, pericardium, sternum, vertebra).14 Detection of metastatic pulmonary lesions is also significantly more sensitive using helical CT scans compared with survey radiographs, and this may be more pertinent in dogs with rib tumors because of superimposition of the lungs by the chest wall mass and pleural effusion.14,15 There was a relatively high incidence of bone metastasis in dogs with primary rib OSA (16%) and, similar to appendicular OSA in dogs,16 whole-body bone scans are recommended for detection of occult synchronous or metastatic disease and possibly determination of dorsal and ventral surgical margins for rib resection.
Surgical excision of rib tumors should include 1 rib cranial and caudal to the tumor, 3 cm of grossly normal bone dorsal and ventral to the tumor in the affected rib(s), and 3 cm lateral margins around all contiguous soft tissues, including biopsy tracts, pleura, muscle, and fascia.3–6 Noninvolved muscle should be preserved for autogenous reconstruction.14 In some human and veterinary reports, excision of the entire affected rib has been recommended for treatment of primary malignant sarcomas because of intramedullary spread of the tumor beyond the grossly palpable extent of the tumor.14,17 Intramedullary extension is also common in dogs with appendicular OSA, and advanced imaging modalities, such as CT, MRI, and whole-body bone scans, are recommended before limb-sparing surgery to determine surgical margins because of increased accuracy of these modalities compared with radiographs.18–20 Intramedullary extension of the tumor resulting in incomplete excision was not noted in these dogs, but excision of the entire affected rib(s) is a prudent recommendation nonetheless. If there is evidence of either adhesion or invasion of the rib tumor into adjacent structures, such as the lungs, pericardium, diaphragm, or vertebra,3 then these should be resected en bloc with the rib tumor. Adhesions should be excised en bloc rather than disrupted, because 57% of the tumor-associated adhesions have histologic evidence of invasion.21 In our dogs, partial lung lobectomy was performed in 25.6% of dogs and partial pericardectomy in 7.7% dogs. Partial lung lobectomy was performed more commonly in our dogs compared with other reports of chest wall resection for rib tumors in dogs.3–6,13 In humans, concurrent resection of any volume of lung with the rib tumor is associated with a significantly higher risk of respiratory complications and perioperative mortality22; however, respiratory complications are rare in dogs after chest wall resection,11,17,23,24 and en bloc partial lung lobectomy was not associated with an increased risk of postoperative complications in our dogs and in another report.5
Oncologic outcome in these dogs was similar to that reported in other studies. Local tumor recurrence was diagnosed in 7 dogs overall (15.9%), including 3 dogs with OSA, 2 dogs with CSA, and 1 dog with HSA. This is comparable to a local tumor recurrence rate of 10–25% reported in other studies of chest wall resection for treatment of rib tumors in dogs.4–6,25 Overall, local tumor recurrence was reported in 25% of dogs with incompletely excised and 13.3% of dogs with completely excised tumors. Four of 8 dogs with incompletely excised tumors survived <120 days after surgery and hence may not have had sufficient time to develop recurrence, as the median DFI for all tumor types in our study was 162 days. Decreased body weight significantly increased the risk of local tumor recurrence, but this was most likely associated with an increased difficulty in achieving appropriate surgical margins in smaller dogs.
Unlike Pirkey-Ehrhart et al,5 we did not identify an association between incomplete excision of the rib tumor and local tumor recurrence, but local tumor recurrence was 6.7 times more likely when there was histologic evidence of tumor cells at the surgical margins. A type II statistical error is the most likely explanation for this lack of statistical significance. Complete surgical margins is the most important prognostic factor for local tumor recurrence and overall survival after chest wall resection of rib tumors in both dogs and humans.5,6,14 As a result, aggressive chest wall resection is essential to achieve optimum postoperative survival for dogs with rib tumors. Furthermore, the intra- and postoperative morbidity associated with chest wall resection and reconstruction is relatively low.3,5,12
Primary rib OSA is the most common rib tumor in dogs. Estimated overall MST for dogs with primary rib OSA was 290 days. This included 19 dogs administered postoperative chemotherapy and 4 not administered chemotherapy. Postoperative survival time is short in dogs with rib OSA treated by chest wall resection alone with MSTs of 35–120 days and 6-month survival rates of ≤25%.2–6 Pirkey-Ehrhart et al5 showed that postoperative chemotherapy significantly improved median DFI and MST for dogs with primary rib OSAs from 60 to 225 days and 90 to 240 days, respectively. Despite the inclusion of a small number of dogs not administered chemotherapy, our estimated MST of 290 days supports the importance of postoperative chemotherapy in prolonging survival time in dogs with primary rib OSA.
Histologic subtype was not identified as a prognostic factor in our study despite 50% of the dogs with chondroblastic OSA being alive and disease-free 1335 and 2177 days postoperatively. However, the 2 remaining dogs with chondroblastic OSA died of lung metastasis at 200 and 256 days after surgery, and 1 dog alive and disease-free 2177 days postoperatively was diagnosed with lung metastasis 792 days after surgery, but these lesions resolved with an experimental gene therapy protocol. Histologic grade is prognostic for dogs with appendicular OSA,9,10 but was not prognostic for dogs with primary rib OSA.
Total serum ALP was identified as a prognostic factor in dogs with primary rib OSA. Estimated MST for dogs with total ALP within and greater than the reference interval was 675 and 210 days, respectively, with tumor-related deaths 7.9 times more likely in dogs with primary rib OSA and increased total ALP. Increased bone-specific and total serum ALP are also poor prognostic factors in dogs with appendicular OSA.7,8 The impact of increased ALP on survival time is probably specific to dogs with OSA, as ALP was commonly increased but not prognostic for dogs with rib CSA (50% of cases).
Estimated MST for dogs with primary rib CSA was not reached and was >3820 days (estimated mean, 1301 days), and survival was significantly better than that in all other primary rib tumors. Survival curves for dogs with primary rib CSA and OSA were not significantly different, but other studies have shown a significant difference in survival between these 2 types of primary rib tumors.3–5 Estimated MST for dogs with rib CSA was similar to other recent studies with reported MSTs of 1080 and 1750 days and was not reached (mean, 3097 days).5,6,11
Histologic grade has prognostic importance in dogs and people with appendicular CSA,11 but was not prognostic for primary rib CSA in our dogs or humans with rib CSA.26 In our study, 2 dogs with rib CSA developed lung metastasis (16.7%) 299 and 853 days postoperatively. Chemotherapy is unlikely to provide a survival benefit in dogs with rib CSA, because the metastatic rate is low to moderate (8–50%) and, when metastasis does occur, it usually occurs late in the course of disease.3,5,11 Furthermore, chemotherapy has no benefit in preventing metastasis or prolonging survival in people with CSA.27
In addition to tumor type and increased ALP, local tumor recurrence and distant metastasis were also identified as poor prognostic factors after chest wall resection for rib tumors in dogs. Estimated overall MSTs for dogs with no evidence of disease, local tumor recurrence, and distant metastasis were >3820, 162, and 290 days, respectively. Furthermore, tumor-related deaths were 28 times more likely with local tumor recurrence and 55.6 times more likely with metastasis. Pirkey-Ehrhart et al5 also found that local tumor recurrence and distant metastasis resulted in a significantly worse outcome for dogs with primary rib tumors. This highlights the necessity of appropriate preoperative staging and aggressive surgical resection to excise rib tumors with complete histologic margins, and the use of postoperative chemotherapy for rib tumors that are associated with a high risk of metastasis, such as OSA.