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Treatment of feline injection-site sarcoma with surgery and iridium-192 brachytherapy: retrospective evaluation of 22 cats

Bloch J, Rogers K, Walker M, Dawson J, Wilson-Robles H. Treatment of feline injection-site sarcoma with surgery and iridium-192 brachytherapy: retrospective evaluation of 22 cats. J Feline Med Surg. 2019 Apr 15;:1098612X19844345.

Injection-site Sarcoma (ISS) is a rare but well documented disease of cats that consists of a highly aggressive, invasive and metastatic cancer that occurs at a site of previous injections. Classically this disease has been associated with vaccination, but ISS may occur at any site of previous inflammation. Because of the highly invasive nature of the this tumor, surgically resection is notoriously difficult, with current recommendations 5cm margins and 2 fascial planes for resection. This may be difficult to attain, and even when approached appropriately there is often residual disease. Post-operative radiation therapy has been shown to improve time to disease recurrence and overall survival, however results have still been less than desirable, and this requires many hospital visits and repeated anesthesia.

Implantation of radioactive iridium at the surgical site provides an opportunity for a high radiation dose to be delivered to any residual tumor with a single anesthesia and  minimal exposure of surrounding tissue to radiation.

The purpose of this study was to determine if brachytherapy with implanted iridium-192 had comparable outcomes to those reported for other adjuvants to surgery in the management of ISS. The study was designed as a retrospective observational study.

Records of 22 cats diagnosed with ISS and treated with surgery and brachytherapy were reviewed. Cats were treated in the 1995-2008 time span. Each cat had surgery followed by implantation of iridium-192. Cats received adjuvant chemo at the discretion of the attending oncologist and owner. Time to progression, disease-specific survival, complications, risks factors, and comorbidities were recorded.

11 cats were neutered males and 11 spayed females, with a mean age of 9.8 years. Tumors represented a variety of sarcoma types all suspected to be injection induced. Implantation of iridum occurred a median of 4.5 days after surgery, with doses and techniques determined by radiation oncologists. Median dosage delivered was 50Gy over a median of 7 days.

Complications associated with radiation therapy occurred in 90% of treated cats. The most common (77%) was alopecia and leukotricia. Other complications included serous drainage, seroma formation, and skin infections (22%, 18%, and 9% respectively). The most serious complication was wound dehiscence in 77% of cats. All adverse effects other than leukotricia responded to therapy.

4 cats received doxorubicin chemotherapy as well as radiation and surgery.

Median time to progression for all cats was 619 days, with 54% having local recurrence and 13.67% having distant recurrence. Local recurrence was within the radiation field in 50% and at the margin in 41%.

Median disease specific survival was 1242 days. There was no correlation with survival and recurrence, clean margins, location of recurrence, number of surgeries, or administration of chemotherapy.

These numbers are very comparable to previously reported data for cats managed with traditional radiation therapy.

The small number of cats enrolled in the study, as well as the variability in timing of implantation, additional concurrent therapies (ie doxorubricin) and retrospective nature of the analysis are some limitations present in this data. Further work on larger numbers of cats in a prospective setting would be recommended.

The authors of this study conclude that the use of iridium 192 brachytherapy may be a successful tool in the management of ISS, and that results appear to be comparable to traditional external beam radiation. Further work should be done on whether the adverse effects and cost of this therapy are lower than traditional radiation, and to determine if combination with other modalities (ie chemotherapy) results in extended lifespans. Iridium brachytherapy represents another tool that may be used in the management of ISS in cats.

See Also

Cronin K, Page RL, Spodnick G, et al. Radiation therapy and surgery for fibrosarcoma in 33 cats. Vet Radiol Ultrasound 1998; 39: 51–56.

Cohen M, Wright JC, Brawner WR, et al. Use of surgery and electron beam irradiation, with or without chemotherapy, for treatment of injection-site sarcomas in cats: 78 cases (1996–2000). J Am Vet Med Assoc 2001; 219: 1582–1589.

Phelps HA, Kuntz CA, Milner RJ, et al. Radical excision with five-centimeter margins for the treatment of feline injection-site sarcomas: 91 cases (1998–2002). J Am Vet Med Assoc 2011; 239: 97–106.