The latest from Dr. Sue Cancer Vet on feline injection-site sarcomas

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While teaching about almost every cancer in felines under the sun in a Fetch dvm360 conference session in Kansas City, Sue Ettinger, DVM, DACVIM (oncology), turned a spotlight on one condition that's been in the news for years now: soft-tissue sarcomas and injection-site sarcomas.

These are mesenchymal tumors-derived from nonepithelial, extraskeletal tissue and including tumors of muscle, fat and connective tissue-that are characterized by local aggressive behavior and high recurrence rates, according to Dr. Ettinger, who is a practicing veterinary cancer specialist, international speaker, author and media personality at Dr. Sue Cancer Vet.

What causes soft-tissue sarcomas?

While the incidence is only 1 in 10,000, the highest level of public awareness surrounds the association of vaccinations and soft-tissue sarcomas in cats. The most commonly implicated agents are rabies and feline leukemia vaccines, but other injections can also cause these tumors, including antibiotics and steroids.

Dr. Ettinger explains in her proceedings that she thinks of soft-tissue sarcomas like weeds with roots: “They have tentacle-like projections extending from the mass you can see and feel-and if you just remove the mass, the weed, and leave the roots, the tumor grows back like a weed.”

What do they look like? “No one, not even a cancer specialist like me, can look at or feel a mass and know tumor type or whether it's benign or malignant,” she writes. Sometimes the cats are younger (7 to 9 years of age) compared to cats with other kinds of tumors. Typical locations for injection-site sarcomas are interscapular, dorsal lumbar, flank or lateral thorax.

Doling out diagnostics

Your first step, according to Dr. Ettinger, is fine-needle aspiration for cytology, even though mesenchymal tumors often have low yield as they don't exfoliate well. Neoplastic fibroblasts can be difficult to differentiate from reactive ones, but cytology can rule out epithelial and discrete cell tumors. Reports cite 93% accuracy with high specificity for diagnosis of malignant mesenchymal tumor.

Why wait? Aspirate!

Dr. Ettinger talks worldwide to veterinarians and pet owners about the need for fine needle aspiration for tumors. Check out her pet-owner-facing PSA here.

“When a skin mass is the size of a pea-1 cm-and has been present for at least one month, do something,” she urges veterinarians. “Aspirate or biopsy, and treat appropriately!”

Staging diagnostics may include incisional biopsy for grade, complete blood count with serum chemistry profile, urinalysis, chest radiographs and abdominal ultrasound. Advanced imaging with computed tomography or magnetic resonance imaging can help determine local extent and aid in treatment planning.

“If the surgeon wants to do a CT scan for surgical planning, skip the chest X-rays,” she said to attendees, explaining that you'll have the chest in the scan.

Dr. Ettinger doesn't recommend simple excisional biopsy, as it's inadequate for local control.

Bearing down on the biopsy

Your biopsy report should be evaluated for grade, completeness of margins and mitotic index. Histologically, injection-site sarcomas are characterized by cystic areas, a transition zone with inflammation, aluminum-containing macrophages and myofibroblasts. Multinucleated cells may be present and correspond to high-grade tumors. For cases in which histologic margins are not complete, further local therapy is recommended-either a scar revision to get wide margins or adjuvant radiation.

Talking treatment

All cats with these sarcomas require aggressive local therapy (surgery with or without radiation). For high-grade soft-tissue and injection-site sarcomas, consider trimodal therapy for higher metastatic rates.

Surgery. Radical surgery is your treatment of choice when possible: “You want your first surgery to be your only surgery,” Dr. Ettinger says. Excision should involve gross tumor margins of at least 3 cm laterally and one fascial plane deep of normal tissue. Histologic margins of at least 5 mm and one fascial plane deep offer excellent local control. Tumor cells often invade beyond the pseudocapsule, which means that a sarcoma that shells out easily has most likely left microscopic disease behind and recurrence is likely.

Amputation may successfully control tumors on the distal limbs and tail. In some locations such as the distal limb, wide margins aren't possible or the tumor is too large to get wide and clean margins. Further therapy is likely needed after surgery to improve local control. Postoperative options include adjuvant external beam radiation, a second surgery for scar revision and chemotherapy. This emphasizes the need for early detection and identification of what the mass is prior to resection.  With adequate local control, median survival time exceeds three years, with some reports of up to five years.

Radiation. Monotherapy has varying control rates, and a better approach is often surgery with adjuvant radiation. Adjuvant radiation is recommended when wide surgical excision isn't feasible. Because complete excision can be a challenge in cats with injection-site sarcomas, surgery and radiation are often the standard of care for local control. In one report of cats with complete excision, 31% had recurrence. Media survival times often exceed two years for cats treated with combined surgery and radiation.

Chemotherapy. Consider patients with high-grade soft-tissue and injection-site sarcomas for chemotherapy, but remember that the benefit for these cancers is still not clear. Doxorubicin is considered the most effective chemotherapy to improve local control. 

Putting out a prognosis

Wide excision for naturally occurring sarcomas can offer a surgical cure, and the prognosis is good if complete excision can be achieved. This is most common for tumors on tails and distal limbs as well as small localized tumors. Another prognostic factor for injection-site sarcoma is whether the first excision is done by a referral surgeon, as these surgeries are likely more aggressive.

Unfortunately, complete excision can be challenging because of the locally aggressive nature and high recurrence rate with incomplete excision following surgery alone. For larger tumors treated with multimodal therapy (aggressive surgery, radiation and chemotherapy), cats can have an excellent long-term prognosis. However, a subset of cats develop metastasis, local recurrence, or both.

Median survival time with surgery followed by radiation is more than two years. The metastatic rate for cats with injection-site sarcoma is reported to vary from low to moderate. Although chemotherapy doesn't seem to improve outcomes following surgery and radiation, adjuvant doxorubicin may benefit cats for surgery alone. Cats with large nonresectable tumors or metastases at diagnosis receive poorer prognoses.

“Treatable but involved” is Dr. Ettinger's message to cat owners. “But I'm not trying to scare you away [from the cost of treatment].” Either way, “owners will need to monitor for recurrence,” she said in her session. “Monitor for lumps, and do something about it.”

For further information:

• Small Animal Clinical Oncology. 5th ed. St. Louis: Elsevier Saunders, 2013.

• Clinical Veterinary Advisor Dogs and Cats. 2nd ed. St. Louis: Elsevier Mosby, 2011.

• Cancer Management in Small Animal Practice. Saunders, 2010;275-282.

• Clinical Veterinary Advisor Dogs and Cats. 3rd ed. St. Louis: Elsevier Mosby, 2015;633-635.

• Novosad CA, et al. Retrospective evaluation of adjunctive doxorubicin for the treatment of feline mammary gland adenocarcinomas: 67 cases. J Am Hosp Assoc 2006 Mar/Apr;42(2):110-120.

• Hartmann K, et al. Feline injection-site sarcoma: ABCD guidelines on prevention and management. J Feline Med Surg 2015 Jul;17(7):606-613