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An approximately 9-year-old FS Beagle presented to her primary veterinarian for evaluation of a mass on her thoracic wall. The owner first noticed it a few days prior to the appointment. On physical examination, the patient was noted to have a hard lump on the right lateral ventral thorax near the axilla that measured approximately 1.5cms in diameter. Orthogonal view thoracic radiographs were performed and per the medical records the mass was soft tissue opacity and could only be identified on the ventrodorsal view. A fine needle aspirate of the mass was performed and cytology revealed a mesenchymal neoplasm with matrix and blood with the comment that findings suggest a matrix producing mesenchymal neoplasm, including chondrosarcoma or an extraskeletal osteosarcoma.
The patient was scheduled for biopsy and possible mass removal 6 days later. Preoperative chemistry and PCV were unremarkable. Surgical exploratory of the region was performed and it was noted that the mass was underneath the thoracic musculature and attached to the external aspect of the rib cage. Also, the intercostal muscles were visible adjacent to the mass. Due to concern for inadvertent entry into the thoracic cavity if removed, an incisional biopsy was performed.
Histopathology from the mass revealed a well-differentiated chondrosarcoma with a few scattered mitotic figures (1 per 10 high powered fields). Referral to an oncologist was recommended.
The owner had a phone consultation with an oncologist 8 days post biopsy and
a CT scan was recommended to help determine resectabilty and assist in surgical planning. The owner opted to pursue the CT scan, which revealed an aggressive monostotic osteolytic lesion arising from the distal aspect of the right 5th rib that was characterized by expansile lysis of the cortical, and medullary bone surrounded by a soft tissue attenuating mass measuring 1.7 x 2.8 x 3.2 cm. The mass did not come in contact with the 4th or 6th ribs and neither exhibited any periosteal reaction. The tracheobronchial and cranial mediastinal lymph nodes were normal and there was no evidence of pulmonary metastatic disease.
Based on the CT findings, mass resection with partial removal of the right 4th, 5th and 6th ribs was recommended. Approximately 3 weeks later, the owners had a surgical consultation and the recommended procedure, possible complications and postoperative care were discussed. The owner opted to admit the dog for surgery the same day. Preop bloodwork was performed which was unremarkable and the mass and previous surgery site including scar and biopsy site were resected with margins. A thoracostomy tube was placed and a latissiuus dorsal flap was used to assist in closing the thoracic wall defect. The patient recovered smoothly from anesthesia and did well postop. She was discharged from the hospital at 2 days postop and histopathology showed chondrosarcoma grade 1 with margins free of tumor (measured 8mm-1.5 cm). No further treatment was recommended and at 1 month postop the patient was doing well with no evidence of recurrence.
Primary rib tumors are uncommon but when they occur they are most often malignant sarcomas with osteosarcoma and chondrosarcoma being the most common. Other reported rib tumors include hemangiosarcoma, fibrosarcoma, soft tissue sarcoma and leiomyosarcoma.
On physical examination, rib tumors are often firm and immobile on palpation.
If concerned about a rib tumor, radiographs should be performed to not only evaluate the rib mass but also to look for possible metastatic spread to the lungs. Important to note is that the amount of bone lysis or production on radiographs has not been shown to help in distinguishing between the two most common rib tumors, osteosarcoma and chondrosarcoma.
In addition to thoracic radiographs, full blood work is recommended to evaluate the overall health of the patient. In one study ALP was not indicative of tumor type but if elevated was significantly associated with a decreased survival in dogs with rib osteosarcoma. Abdominal ultrasound can also be considered as a staging diagnostic and is more important with certain types of rib neoplasia such as hemangiosarcoma.
The next diagnostic step is ideally a CT of the thorax to help evaluation the full extent of the mass and further investigate the lungs for any evidence of metastatic disease. Thoracic lymph nodes can also be visualized, although this would not typically be the first place for spread. Surgical planning can then be based off of the CT scan.
Figure 1: CT image of aggressive monostotic osteolytic lesion arising from he distal aspect of the right 5th rib
Nuclear scintigraphy has also been reported to be used in patients with rib osteosarcoma to help determine if the rib mass is a metastatic lesion or look for metastasis of the mass. In one study of rib osteosarcoma, a 16% bone metastatic rate was noted from a primary rib osteosarcoma.
Incisional biopsy can be performed either via Tru-cut biopsy or surgical wedge biopsy prior to surgical removal. The advantage of the surgical biopsy over the incisional biopsy is that a larger sample can be obtained and it has been shown that the larger the biopsy sample the increased chance of a correct diagnosis. A biopsy is beneficial in helping determine prognosis with different treatments not the best treatment option, because in general, surgery is the initial recommended treatment regardless of diagnosis if no metastatic lesions are noted.
With surgical resection of rib tumors, the tumor is removed en bloc with margins that include one rib cranial and one rib caudal to the lesion as well as 3 cms dorsal and ventral to the mass. Advanced imaging results help determine if the overlying skin and which overlying muscle(s) needs resected. If the mass was biopsied prior to surgery, it is important to make sure the entire biopsy tract is included in removal. The reported maximum number of ribs that can be removed is 6 but up to 7 has been reported to be performed successfully. Removing more than 6 though increases the risk of severe respiratory compromise and dysfunction postop.
For closure, the use of an autogenous (such as a latissimus dorsi flap) or prosthetic technique (mesh) can be used. The latissimus dorsi flap is the most commonly used local tissue flap and it can be used as a muscle flap or myocutaneous flap. In one study, use of mesh alone was associated with an increased risk of postop complications compared to local tissue flap closure and therefore closure with a local tissue flap or a composite closure using a combination of a autogenous and prosthetic technique is preferred. Diaphragmatic advancement can be used with caudal thoracic wall defects and the use of omentum tunneled into the defect from the subcutaneous tissue or through the diaphragm has been reported.
Use of spinal plates to help with chest wall stability has been reported and it is recommended to suture the muscle flap or mesh taut to help maintain rigidity however absolute rigid reconstuction of the thoracic wall has not been shown to be necessary.
Tumor type is important in determining prognosis. Rib osteosarcoma has similar biological behavior of appendicular osteosarcoma. In one study, rib osteosarcoma treated with surgery alone had a mean survival time of 90 days. Postop chemotherapy can prolong survival and treatment with surgery and postoperative chemotherapy had a mean survival time of 240 days.
Chondrosarcoma, the second most common primary rib tumor has a better prognosis than osteosarcoma with reported median survival times of even greater than 3820 days in one study. If clean margins without metastatic disease are found, adjuvant chemotherapy is not commonly recommended for chondrosarcoma.
Submitted by: Lauren May, DVM
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