Current treatment options for dogs with appendicular osteosarcoma - Veterinary Medicine
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Current treatment options for dogs with appendicular osteosarcoma
Your therapeutic choices for dogs with osteosarcoma in their limbs continue to expand. Deciding which option is best comes down to the site and extent of the tumor and the owner's preference.


VETERINARY MEDICINE


Endoprosthesis


FIGURE 3. A photograph of the cranial aspect of a dog forelimb during a radial endoprosthesis limb-sparing procedure (left) and a postoperative lateral radiograph of the forelimb (right).
A radial endoprosthesis (Veterinary Orthopedic Implants, South Burlington, Vt.) has been developed as an alternative to cortical bone allografting. This prosthesis couples a specially designed limb-sparing bone plate with a metal spacer that spans the defect between the radial osteotomy site and the radiocarpal bone (Figure 3). The articular surface of the radial carpal bone is also removed, and carpal arthrodesis is performed. Advantages of this technique include the lack of the need for a bone bank, less technical difficulty, and a shorter operative time.22 Observations from clinical experience indicate that endoprostheses may be associated with a lower infection rate and that the infections may be more superficial and easier to resolve.22 A theoretical disadvantage is that the host bone is not incorporated into the endoprosthesis. This disadvantage maybe negated, however, by the relatively short survival times typical of most dogs affected with osteosarcoma. In addition, such concerns are not unique to endoprostheses because there is also some question as to whether cortical allografts are effectively incorporated and replaced by recipient bone as well.

RADIATION THERAPY


FIGURE 4. A computed tomography image in the dorsal plane of an osteosarcoma in a dog distal tibia. The stereotactic radiosurgery treatment plan is superimposed, and the dose distribution is represented by isodose lines. The image represents only a single slice of the treatment plan. The 75% isodose line (30 Gy) is shown in blue, the 50% isodose line (20 Gy) in red, and the 25% isodose line (10 Gy) in green. Although they appear as lines on the two-dimensional display, isodose lines represent three-dimensional dose shells surrounding the target volume. A transosseous Kirschner wire and fiberglass cast material (used temporarily to anchor the radiation targeting device to the bone) are visible in the top portion of the image.
Radiation therapy has been used extensively to treat appendicular osteosarcoma.31,42 Radiation therapy administered before surgery, when given in moderate doses (32 Gy), has been shown to significantly reduce the local tumor recurrence rate after cortical allograft limb-sparing surgery.42 Previous studies, however, have shown that preoperative radiotherapy at higher doses (36 to 52 Gy) causes unacceptable rates of fixation device failure and graft complications.31 As survival is often limited by distant metastasis rather than local recurrence, preoperative radiation has not been shown to increase long-term survival. 42

Palliative radiation therapy is another alternative to amputation and limb-salvage procedures. This treatment can effectively palliate the pain associated with appendicular osteosarcoma.43-45 Previously described palliative protocols involve three fractions of 10 Gy given at Days 0, 7, and 21.43,44 A more effective protocol has recently been described that uses four 8-Gy fractions with one fraction given every seventh day.45 This new protocol eliminates the two-week gap between the seventh and 21st day and has been shown to provide a higher response rate and longer survival times.45 Using the four-fraction protocol resulted in a 92%45 response rate vs. 74%44 or 80%43 reported using the three-fraction protocol. The median duration of response using the four-fraction protocol is 95 days, with a median survival time of 313 days45—much higher than that observed using the previously described protocols.43,44

Recently, stereotactic radiosurgery has been used to treat dogs with appendicular osteosarcoma.46 Unlike fractional therapy, stereotactic radiosurgery delivers the entire radiation treatment (about 30 Gy) through a single, large dose in a highly targeted manner. The precise nature of stereotactic radiosurgery allows the delivery of a radiation dose that is conformed to the shape of the tumor target, and damage to the normal surrounding tissues is minimized by a steep dose gradient (Figure 4).47 Advantages of administering a single, large fraction include fewer anesthetic episodes and possibly a greater biologic effect on tumor tissue when compared with an equivalent total dose delivered in multiple (e.g. three 10 Gy) fractions.48

Preliminary results with stereotactic radiosurgery in lower extremity (radius and tibia) tumors have shown the ability to provide long-term local control (> 2 years), especially when it is possible to surround the entire tumor with the 30-Gy isodose line during treatment planning and when combined with chemotherapy.46 Currently, carboplatin (300 mg/m2) is given intravenously just before stereotactic radiosurgery as a radiation sensitizer, and carboplatin alone or in combination with doxorubicin is given adjunctively during the convalescent period. Upper extremity (humerus and femur) tumors have also been treated by one of the authors (Farese JP: Unpublished data; currently being evaluated for response) and may become the ideal application of stereotactic radiosurgery since limb-sparing surgery is not performed routinely in these tumor locations. The prominent muscle mass between the tumor and the skin in upper extremity locations enables even larger radiation doses to be used without causing marked radiation skin injury.44


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Source: VETERINARY MEDICINE,
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