Treating and Managing Vaccine-Associated Sarcomas

Dennis W. Macy DVM,MS

Colorado State University, Fort Collins, Colorado 80523

Recommendations for preventing or reducing the incidence of vaccine-associated tumors in cats are controversial. Recommendations include a change in vaccination site location, decreased use of polyvalent vaccines, use of nonadjuvanted vaccines, avoiding the use of aluminum based adjuvants, and not overvaccinating, among others.^8,33,34

Vaccine site location recommendations have also recently been changed. The National Feline Vaccine-Associated Sarcoma Task Force recommends that no vaccine be given in the interscapular space, that rabies vaccine be administered subcutaneously in the distal right rear leg, that FeLV vaccine be administered subcutaneously in the distal left rear leg, and that all other vaccines be administered subcutaneously in the right shoulder region.^8,33,34 It appears from our research and reports by others that intramuscular and subcutaneous administration both result in local inflammation and tumor production. Subcutaneous sites are recommended for all vaccines since they will result in earlier detection of these growths if they occur. The reasoning behind these vaccine site administration recommendations is not based so much on prevention but rather on earlier diagnosis and potentially a higher cure rate when treated surgically.

It would appear that the problem of vaccine-site associated sarcomas will be with us for some time, and the question of what to do with postvaccination lumps is a practical one. Some rabies and FeLV vaccines produce postvaccinal lumps in 100% of the vaccinates but, fortunately, it has been observed that most resolve in two to three months. Very few vaccine-associated tumors occur sooner than three months following vaccination. Given these observations, the Vaccine-Associated Feline Sarcoma Task Force currently recommends that any vaccine site lumps > 2 cm in diameter or those that are present after three months from the time of vaccination or be biopsied. A biopsy will determine the magnitude of the surgery, i.e., lumpectomy versus radical surgery. We do not recommend excising the mass prior to biopsy. Attempts at simple excision of these tumors is seldom curative and ultimately leads to local recurrence with a more difficult second surgical attempt. Even attempts at aggressive wide surgical excision are often incomplete and result in a 30%-70% failure rate.^35,36 Treatment failure is most likely due to underestimating the extent of the primary disease. MRI or contrasting CT examination often show the magnitude of the surgery that may be needed in order to achieve complete removal and are recommended when available (Figure 3). The interscapular location and infiltrative nature of these tumors makes excision difficult even with partial scapulectomy, excision of epaxial muscles, and removal of dorsal cervical vertebral processes. There appears to be an advantage to refer these cases to a qualified surgeon before the second surgery is contemplated. After a second surgical intervention has been attempted, there appears to be no improvement in survival with referral to specialist surgeons. Rear leg amputation has a higher rate of cure than surgery in the interscapular space for vaccine-site associated sarcomas. Given the incomplete removal of these tumors even with aggressive surgery, radiation is often used before or after surgery. The interim survival analysis in one study indicates a doubling of survival when radiation is combined with surgery.^32,35 In another study in which radiation therapy was given prior to surgery and complete margins were obtained, the median disease free interval was 700 days.³⁷ Although the combination of surgery and radiation has increased tumor control rates in some studies, a significant number of cats still fail to respond to this combination. Several chemotherapy agents, including carboplatin, doxorubicin, cyclophosphamide, and the combination of cyclophosphamide and vincristine have been used in cats with vaccine-associated tumors.³⁸ Most chemotherapeutic attempts result in partial responses, but some complete responses have been observed with these drugs. Immunomodulators have been used systemically or intralesionally, but response has been limited.³⁹ Although the vast majority of vaccine-site associated sarcomas are only locally invasive, approximately 10%-20% will metastasize to the lungs or other sites.^40-42