INCIDENCE, SIGNALMENT AND ETIOLOGY

Soft tissue sarcomas that occur in the subcutis of the dorsal neck/interscapular area, flank/paralumbar area, dorsolateral thorax, and femoral musculature have been increasing in frequency.¹ These sarcomas have been termed injection site sarcomas or vaccine associated sarcomas due to their anatomic location at common sites of SC injection. One study showed an eightfold increase in the number of sarcomas diagnosed from 1988 to 1994 and a similar increase in the ratio of injection site to noninjection site sarcomas.² In the 5 years from 1984 to 1988, injection site sarcomas outnumbered sarcomas at other sites only once, while between 1989 and 1992 they were the most common type.² This coincides with widespread rabies vaccination and availability of FeLV vaccines. In three large studies encompassing 773 cats with sarcomas, 489 tumors occurred at injection sites.^1-3

Injections site sarcomas occur in younger cats than do sarcomas at noninjection sites, with a peak at 6 to 7 years of age.^1,3,5 The signalment of affected cats is otherwise similar whether the sarcoma is injection site related or not.¹

These tumors are most commonly fibrosarcomas but may also be described as osteosarcoma, malignant fibrous histiocytoma (histiocytic sarcoma), giant cell tumor, myofibroblastic sarcoma, rhabdomyosarcoma, leiomyosarcoma, chondrosarcoma, undifferentiated sarcoma, neurofibrosarcoma/nerve sheath tumor, and liposarcoma. The majority of injection site sarcomas have been associated with vaccination, particularly rabies and FeLV vaccines.^2,3,5 Sarcomas have also been associated with feline panleukopenia, herpesvirus, and calicivirus vaccination^48,49 in countries where vaccination for rabies and FeLV are not common. Sarcomas appear to arise months to years after vaccination.^{2, 5} Injection site sarcomas have also arisen at the site of antibiotic administration, SC fluid administration,^2,7-9 long-acting corticosteroid injection, 6 or lufenuron injection (Program® 6 month injectable).¹⁰

There have been many estimates of the incidence of sarcomas that occur following vaccination. Most have been based on estimates of the number of cats truly vaccinated, and all rely on what could be incomplete or biased reporting of tumors.

In California, one study estimated that between 10 and 12 cats/100,000 would develop a sarcoma at a vaccination site.² A nationwide study involving 235 practitioners affiliated with the American Association of Feline Practitioners looked at the 1992 incidence of sarcomas in vaccinated cats.¹¹ This study found that 48% (158 of 329) were at vaccination sites and the incidence was 36 cats/100,000. Other estimates have ranged from 10/100,000 to 100/100,000.¹² One small study showed the risk of sarcoma formation to be 130/100,000 vaccinations.⁶

Injection sites sarcomas are often associated with an inflammatory infiltrate, primarily macrophages, that are frequently reported to contain bluish "foreign material^4,5 and may include giant cells.⁴ In one study, aluminum was associated with this inflammation, leading to speculation that aluminum containing adjuvants in vaccines may cause inflammatory changes that lead to carcinogenesis.¹³ Trials with adjuvanted killed vaccines in 36 cats showed that 80% to 100% of cats had a local inflammatory reaction following vaccination and that rabies vaccination created a larger inflammatory mass than did vaccination for FeLV.^50,54 Although reactions were most common with aluminum adjuvant vaccines, less common with non-aluminum adjuvant vaccines, and not seen with nonadjuvant vaccines, the size of the reaction was not related to the presence of aluminum. IM injection has been shown to cause the same reaction as SC injection,⁸ and sarcomas have occurred at IM injection sites,^6,14 although they appear to be less common at this location.²

One study showed an increasing risk of sarcoma formation with the use of killed vaccines.⁶ The risk of sarcoma formation appears to be greatest following FeLV vaccination, with vaccinated cats being nearly three times more likely to develop a tumor than cats not receiving FeLV vaccine.² Rabies vaccination is less strongly associated with sarcoma formation, with vaccinated cats about twice as likely to develop a tumor.² Since adjuvants are used with killed vaccines and appear responsible for much of the inflammation, some investigators have suggested the preferential use of modified-live vaccination. However, some sarcomas have been associated with modified-live virus vaccination.⁶

The risk of sarcoma development increases with the number of vaccines given at a site. In one study, cats receiving three to four vaccinations in the interscapular region were nearly twice as likely to develop a sarcoma than if they received one vaccine at that site.²

In one study of cats with sarcomas, vaccines received were cataloged according to their manufacturer. There were seven manufacturers of FVRCP vaccines, five manufacturers of FeLV vaccines, and eight manufacturers of rabies vaccines.² There was no association between manufacturers and vaccine formation in this or other studies.³

It has been known that inflammation may cause sarcoma development in cats because ocular sarcomas at the site of ocular trauma have been described^15,16. Fibrosarcomas have also been induced by trauma in cats infected with FeSV.¹⁷ FeSV requires FeLV as a "helper" virus in order to proliferate, so if FeSV were to be implicated in injection sarcoma carcinogenesis, affected cats would be FeLV positive. Of 169 cats with sarcomas in one study, only 8 were FeLV positive and only 4 of these had injection site sarcomas.² In addition, of 36 cats tested for FIV, only 3 were antibody positive and only 1 of these had an injection site sarcoma. In a second study, all tested cats were FeLV and FIV negative.⁴⁵ Immunohistochemical staining of 130 injection site sarcomas showed no evidence of FeLV gp70, and polymerase chain reaction showed no evidence of FeLV in 100 injection site sarcomas.¹⁸ It seems unlikely that FeSV is involved in injection sarcoma development.

The development of sarcomas does seem to be related to the presence of oncogenes and mutations in tumor suppressor genes. Approximately 75% of injection sarcomas were found to contain p53 and c-kit oncogene.¹⁹ Approximately one third of tumors contained both p53 and mdm-2, and these tumors were histologically more anaplastic, perhaps explaining the aggressive biologic behavior of some injection site sarcomas.

The current thinking is that individual cats may respond to inflammatory changes in a manner that predisposes to sarcoma formation. This is supported by the discovery of cats that have developed sarcomas at more than one injection site and the finding of injection site sarcomas in related cats .⁸ Current studies to investigate the cytogenetics of injection site sarcomas may provide more information. Up-to-date information regarding research can be obtained from the Vaccine Associated Fibrosarcoma Task Force (http://www.avma.org/vafstf/default.asp).

TREATMENT

Surgery

Surgical excision has been the principal treatment modality for soft tissue sarcomas. Due to the extensive infiltration and invasion of surrounding normal tissue, it is necessary to resect a wide and deep (more than 2 cm) margin of normal tissue in all surgical planes around the palpable tumor. While this is often possible in larger species such as dogs and humans, it is rarely possible in cats, particularly following an initial unsuccessful attempt. Exceptions to this would be tumors on a distal limb or other extremity that can be amputated. For this reason the first attempt at surgical removal should be the definitive one, and wide surgical margins that include bone, muscle, and other structures should be obtained. For example, a cat with soft tissue sarcoma in the interscapular space would require resection that encompasses 2 cm or greater circumferential margins of normal tissue, including the dorsal process of the scapulae and dorsal spinous processes. The aim should be to remove the tissue "en bloc" without incising tumor tissue itself.

The surgeon should be aware that as soft tissue sarcomas grow, they compress a cuff of tumor cells to form a pseudocapsule, thereby giving the false impression that the sarcoma is encapsulated.

Because aggressive surgeries require specific surgical skills and adequate planning, the preferred approach is an incisional or Tru-cut needle biopsy prior to consulting with an experienced surgeon and/or radiation oncologist. As stated earlier, pretreatment staging should include a CT scan to more clearly define the tumor margins and areas of infiltration.

In a study of 84 cats surgically treated for soft tissue sarcoma, 60 cats (70%) had tumor recurrence an average of 3.5 months later.³⁰ Tumors recurred as soon as 2.5 weeks and as long as 1.5 years after surgery. Only 34 of the 60 cats had a second surgery; the other 26 were euthanized. Of these 34 cats, 27 (80%) had a second recurrence and 12 of these were euthanized. Of the 84 cats originally treated, only 9 cats became disease free and only 4 of those were disease free longer than 18 months.³⁰ A similar recurrence rate of over 80% was seen in another study in which the median tumor free period following surgical excision was 4 months.²¹ Only 10% of 61 cats were tumor free a year after surgery in another study²⁸; these cats had all been treated by wide excision or amputation.

Tumors that involve the limb often recur after an attempted local excision, but the likelihood of long-term control following amputation is high. Similarly, tumors of the pinnae or even nictitans may be removed with complete surgical margins. For more extensive limb tumors that approach the pelvis, a hemipelvectomy may be required; if complete margins cannot be obtained, a rapid recurrence should be expected. Procedures that are less aggressive than amputation, such as scapulectomy, may leave tumor cells and lead to recurrence.

Aggressive surgical excisions in other areas may lead to long-term tumor control even after other methods have failed. However, the first surgery should be considered definitive rather than relying on a second or third surgery for salvage. Wide resection on the chest wall or flank may require rib or body wall removal and the use of propylene mesh. Even with extensive surgery and reconstructive attempts, recurrences may still occur.

Radiation Therapy

Most early studies report very little efficacy for radiation therapy in reducing recurrence rates for soft tissue sarcoma. A combination of minimal surgical excision and low doses of radiation therapy probably contributed to the apparent ineffectiveness.^24-26 More recent studies suggest that the treatment of choice for this tumor type is aggressive surgery in combination with high doses of pre-or postoperative radiation therapy.

In two studies, cats were treated with brachytherapy using iridium-192 (¹⁹²Ir) implants after surgery. The dose in one study was 60 Gy²⁴ but was not provided in the other.²¹ The recurrence rate in one study was 70% (11 of 16 cats), with a median survival of 8 months²⁴; in the other group of cats, 50% of tumors recurred and the median disease free interval was 12.5 months,²¹ which was better than for cats in the same study treated with surgery alone (discussed previously).

Selection of cats with poor prognostic factors may have influenced outcome in another study in which brachytherapy or cobalt-60 teletherapy resulted in a median disease free interval of only 4.5 months.²⁵ Information about radiation doses was not provided.

In a study of 31 cats treated with orthovoltage radiation to a dose of 51 to 60 Gy following incomplete surgical excision, median tumor free interval was 18 months and median survival was 22 months.²⁸ Acute toxicities were mild, but systemic toxicities led to the euthanasia of 6 cats, 2 due to pneumonitis and 4 due to renal failure. These toxicities occurred because of radiation of underlying structures when injection site sarcomas of the interscapular area or the flank were treated.

High-dose radiation therapy (57 Gy) was used to treat 25 cats with soft tissue sarcomas. An electron beam was used to deliver the radiation to most cats; this method delivers radiation to superficial tissues while sparing the underlying normal tissue, thereby avoiding toxicity. Median survival for all cats was 700 days.²⁹ In this small group of cats, the administration of doxorubicin chemotherapy was not associated with longer survival, but further studies are warranted as other anecdotal experience indicates that chemotherapy may improve survival. Increasing numbers of surgeries prior to radiation was not associated with decreased survival.

Sarcoma recurrence after radiation may be due to improved tumor cell survival along the relatively hypoxic surgical scar. Hypoxia reduces the effectiveness of radiation therapy. One study investigated the efficacy of presurgical radiation therapy to an area surrounding the tumor followed by wide surgical excision.²⁵ The tumor recurred in 11 of 33 cats, and 8 of 33 developed metastases (4 also experienced tumor recurrence). The tumor was more likely to recur quickly in cats in which surgical excision was incomplete after radiation therapy. Tumors recurred in the 5 cats with incomplete resection a median of 3.5 months after surgery, while those with complete resection were tumor free a median of 23 months after surgery.²⁵ Tumor volume at the time of radiation did not influence recurrence or survival, implying that even large tumors could be treated in this manner. Some cats developed transient pneumonitis, and wound dehiscence occurred and was repaired in 4 of 33 cats. It appears that radiation may act to "sterilize" the margins of a tumor, enabling a more effective surgical excision. Radiation therapy followed by aggressive surgery may be the treatment of choice for feline soft tissue sarcomas.

Chemotherapy

There is very little information regarding chemotherapy in the treatment of soft tissue sarcomas in cats. The reportedly low metastatic rate has meant that chemotherapy is rarely used in an adjuvant setting. The higher metastatic rates now reported and the reduction in the rate of local recurrences following the use of surgery and radiation therapy indicate that chemotherapy may have an increasing role in the management of soft tissue sarcomas in cats.

Drugs that anecdotally appear to have no efficacy are vincristine, methotrexate, and cyclophosphamide. Doxorubicin has been used with apparent success to treat cats with local recurrence after surgery.³¹ Mitoxantrone did not influence tumor recurrence in a cat with soft tissue sarcoma.²² The use of carboplatin chemotherapy did not seem to improve survival rates in another study,²⁴ although some oncologists believe that this drug is helpful. Similarly, studies are currently in progress to investigate the efficacy of ifosfamide, a drug that is very effective against soft-tissue sarcomas in humans.

In an investigation of a novel approach to treat recurrent sarcomas, IV bleomycin (0.5 mg/kg) was combined with electric stimulation of sarcoma and immunotherapy.²⁴ Tumor regression was seen in only one cat, but survival appeared to be prolonged (5 months) compared with untreated cats (0.7 months).

In another study, either doxorubicin, mitoxantrone, or carboplatin was administered to seven cats in which sarcoma recurred after surgery and radiation.²⁵ The median survival for these cats was 3.5 months, and two cats were alive 10 to 22 months after treatment. Doxorubicin and carboplatin need to be further evaluated in the treatment of soft tissue sarcomas in cats, particularly as an adjuvant to surgery and radiation therapy.

Immunotherapy

Nonspecific immunomodulation using a mixed bacterial vaccine or levamisole had no obvious effect on recurrence rates or survival following surgical excision of soft tissue sarcomas.

Acemannan is another nonspecific immunomodulator that has been evaluated in a small number of cats with fibrosarcoma. Cats were injected with 2 mg/kg intralesionally weekly for 6 weeks prior to surgery and megavoltage radiation therapy (60 Gy). The cats then received 1 mg/kg intraperitoneally weekly for 6 weeks and then monthly for 1 year. Of four cats so treated, one had tumor recurrence 8 months after surgery but the other three had no recurrence for 14 to 19 months after surgery.³³ The true contribution of acemannan to survival in these cats is difficult to evaluate.

Tenogeneic cells (Vero hIL-2) that secrete human recombinant interleukin-2 (hrlL-2) were infiltrated around the tumor at the time of surgical resection and implantation of ¹⁹²Ir seeds for brachyradiotherapy.²⁴ This infiltration was repeated 5 days later and another five times over the next 2 months. Of 16 cats treated by this protocol, two had local recurrence and three had metastases, with an overall median survival of 16 months. In comparison, 11 of 16 cats that did not receive Vero hlL-2 cells had tumor recurrence and a median survival of 8 months. Antibodies to the cells were detected after 5 days of treatment, and most cats had a local inflammatory reaction to injection. One cat developed anaphylaxis.³³

Immunotherapy may contribute to longer survival in cats treated with local therapies for fibrosarcoma. Further studies involving the use of specific immunomodulators are needed to define the role of immunotherapy.

References

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