appendicular tumors

BACKGROUND

Long bone tumors are relatively common in dogs (particularly middle to older large to giant breed dogs), but rare in cats. Most long bone tumors are malignant with the most common tumor being osteosarcoma, while others include chondrosarcoma, fibrosarcoma, and hemangiosarcoma. Osteosarcoma has a high metastatic potential in dogs, often to the lungs and other bone. Only 10%-15% of dogs with osteosarcoma have metastasis at diagnosis, but up to 90% will eventually develop metastasis.

DIAGNOSIS

Osteosarcoma is often suspected based on signalment (age and breed), clinical signs, tumor location, and radiographic features. There are no radiographic features which distinguish osteosarcoma from other types of primary bone tumors. However, osteosarcoma accounts for 85%-98% of all primary bone tumors. A fine-needle aspirate or core aspirate can be performed to diagnose a primary bone tumor and differentiate a primary bone tumor from other rare non-neoplastic diseases such as fungal osteomyelitis. ALP staining of fine-needle or core aspirates will frequently provide a definitive diagnosis of osteosarcoma. Alternatively, a core biopsy can be performed, but this is more invasive with a greater risk of complications (such as fracture) and between a 15% to 45% non-diagnostic rate. The need for a preoperative diagnosis is debatable as a knowledge of the tumor type does not change the initial treatment options; and biopsy, particularly a core biopsy, can have a negative impact on treatment of the primary bone tumor.

CLINICAL STAGING

The two most common sites of metastasis are the lungs and other bone, however metastasis to lymph nodes, skin, and multiple other sites have been reported. Thoracic radiographs or CT scans are recommended to assess the lungs for metastasis. A bone scan is the preferred method for evaluation of other bones for metastasis, but this imaging modality is not readily available. Whole-body radiographs or CT scans are alternatives, but detecting metastatic lesions with either of these imaging modalities is significantly less likely even when board-certified radiologists are reviewing these images. The regional lymph nodes should be palpated and aspirated.

TREATMENT

Treatment is divided into palliative and curative-intent. Palliative treatment options are aimed at controlling pain and include analgesia (pain killers), radiation therapy (with or without chemotherapy), and limb amputation. Curative-intent treatment options are recommended for dogs with no evidence of metastasis and include management of the local tumor (limb amputation, limb-sparing surgery, and stereotactic radiosurgery) and postoperative chemotherapy to decrease the risk of metastatic disease. Immunotherapy may also become available in the near future and a preliminary study in a small population of dogs has been very promising.

Limb Amputation

Limb amputation is the most commonly performed treatment for primary bone tumors in dogs. The decision to proceed with limb amputation is often difficult. However, cats have excellent function and the vast majority of dogs have good to excellent function following limb amputation. The only contraindication for limb amputation is pre-existing neurologic disease. Large breed dogs, obese dogs, and dogs with osteoarthritis are often good candidates for limb amputation. The majority of dogs can walk unassisted 1-2 days after surgery. The median time to maximal recovery is 4 weeks, but this time is significantly shorter for dogs with owners who are positive about the amputation procedure. There is no difference in recovery rates or mobility following amputation of either the fore or hind limb.  In a study published in 2015 in which 64 owners of dogs with amputated limbs were surveyed, 91% of owners perceived no change in their dog's attitude after limb amputation, 88% of owners reported complete or near complete return of their dog to pre-amputation quality of life, 78% indicated that their dog's recovery and adaption were better than they had expected; and 73% reported no change in their dog's recreational activities. 

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Limb-Sparing Surgery

Limb-sparing surgery is possible for dogs with tumors of the distal radius. Limb-sparing surgery involves removal of the bone tumor with appropriate margins and reconstruction using an endoprosthesis or a cortical allograft. Ablative limb-sparing surgery, where a segment of the bone is excised without subsequent reconstruction, is possible for tumors of the scapula, ulna, and, in some circumstances, pelvis (ischium). For distal radial limb-sparing surgery, limb function is good to excellent in 75%-80% of dogs. However, there is a high risk of complications with limb-sparing surgery such as infection (50%-70%), implant failure (30%-50%), and local tumor recurrence (15%-25%). A newer limb-sparing technique, called lateral manus translation, has a lower risk of both infection (15%) and implant failure (10%), but the overall risks of complications are still higher than for limb amputation.

Stereotactic Radiosurgery

Stereotactic radiosurgery (SRS) is a non-surgical limb-sparing option. This involves a form of radiation therapy which was first used for treatment of brain tumors in people. Radiation is typically delivered in two perpendicular planes, however the dose of radiation that can be delivered to the tumor is limited by radiation damage to adjacent normal tissue. SRS involves the delivery of radiation in multiple small increments circumferentially around the tumor resulting in high doses of radiation to the tumor, but very low doses to adjacent normal tissues. The advantages of this approach are that any limb can be preserved as bone tumors in any location can be treated with SRS, and the treatment time is relatively short with one to three treatments over a maximum of one week. The disadvantages include the risk of post-SRS bone fracture (which are more likely in lytic bone tumors, tumors that have had a preoperative core biopsy for diagnosis, and bone tumors with < 3mm cortical bone thickness) and limited availability (with North Carolina State University, Colorado State University, University of Florida, and University of California Davis being some of the universities with radiation facilities capable of delivering SRS).

PROGNOSIS

The prognosis for dogs with osteosarcoma is guarded. The median survival times for dogs treated with palliative modalities range from 3 to 5 months with most dogs dying as a result of uncontrollable pain. The combination of palliative radiation therapy and chemotherapy is associated with a median survival time of approximately 300 days in two studies involving a small number of dogs. The median survival time for dogs treated with limb amputation or limb-sparing surgery and chemotherapy is 10-14 months, with 50% of dogs alive at 1 year and 10%-30% dogs alive at 2 years. Poor prognostic factors include young dogs, location (proximal humerus), dogs with increased preoperative ALP levels, lymph node metastasis, and distant metastasis. For dogs with an infected limb-sparing surgery, the median survival time is significantly improved at almost 700 days. The mechanisms responsible for this improvement in survival time have inspired the investigation of immunotherapy as an adjunct to both surgery and chemotherapy for the management of dogs with osteosarcoma. A pilot study in a small number of dogs with appendicular osteosarcoma treated with limb amputation and postoperative chemotherapy followed by immunotherapy showed 1- and 2-year survival rates of 80% and 70%, respectively.

The prognosis for small breed dogs with osteosarcoma is also guarded, but better than large breed dogs. In a recent study of 51 small breed dogs (< 15 kg) with appendicular osteosarcoma, the median survival times were 112 days for untreated dogs, 257 days for dogs treated with limb amputation, and 415 days for dogs treated with limb amputation and chemotherapy. Poor prognostic factors included histologic grade and body weight (with heavier dogs having a worse prognosis).

The prognosis for dogs with chondrosarcoma is very good but dependent on histologic grade. The median survival times for dogs with grade I, II and III chondrosarcoma following amputation alone are 6 years, 2.7 years, and 0.9 years, respectively. The effect of chemotherapy for dogs with appendicular chondrosarcoma is unknown, but chondrosarcoma is considered chemoresistant in people. However, chemotherapy should be considered for dogs with grade III chondrosarcomas.

The prognosis for cats with malignant primary bone tumors is often good and limb amputation can be curative without chemotherapy.

Last updated on 6th March 2017