Treatment

Post-surgical treatment for FISS is perhaps the most harrowing and confusing part of the whole process. The treatment you choose for your cat will have a tremendous impact on their prognosis. As a pet owner, knowing this is incredibly difficult because there are so many barriers to cancer treatment like cost, geographic availability, and your cat’s general health. The decision to pursue post-surgical treatment cannot be made lightly. The best thing you can do is meet with your vet, become familiar with your options and their potential outcomes, and consider your cat’s best interest.

An important step in choosing the best treatment option is determining whether the cancer has metastasized. If the cancer has spread to your cat’s lungs or other areas of the body, at-home palliative care may be your best option. FISS requires aggressive, localized treatment. Ultimately, once your cat has reached this progression of FISS curative treatment outcomes are unlikely and the kindest thing to do is let them live the rest of their peacefully without incessant vet visits.

Even with the combination of aggressive surgery and radiation therapy, there is still a 20 to 40% chance the tumor will grow back and a 25% chance it will metastasize. FISS is highly unpredictable and aggressive. Without treatment, some cats may never have tumor reoccurrence. Some tumors could regrow within months after treatment. There is still so little known about this disease. You have to trust your gut and your heart and know you will make the best decision for your cat.

Multimodal vs. Monomodal Treatment Options

Monomodal refers to using only one treatment option, while multimodal refers to using two or more treatment options together. Surgery is typically considered the first ‘course of treatment’ for FISS. Due to financial or other barriers, it could be the only treatment for many cats diagnosed with FISS. However, surgery alone is rarely curative and without additional treatment tumor reoccurrence within six months to a year is likely. Research has shown that surgery in combination with another treatment — typically radiotherapy — is significantly more effective at prolonging the disease-free interval time and survival time in cats with FISS. After reading most of the literature available on FISS treatment options, this is generally what the median disease-free intervals and survival times look like across a few treatment plans:

SURGERY ALONE:

  • Disease free for less than 1 year following surgery

  • Survival time 1 to 2 years following surgery

SURGERY AND COARSE-FRACTIONED (PALLIATIVE) RADIOTHERAPY with clean surgical margins

  • Disease free for 1 to 2 years after completing radiotherapy

  • Survival time 3 to 5 years after completing radiotherapy

SURGERY AND DEFINITIVE RADIOTHERAPY with clean surgical margins

  • Disease free for 3 to 5 years after completing radiotherapy

  • Survival time 5+ years after completing radiotherapy

The way I think of it, your disease-free interval and survival time doubles when you add radiotherapy in addition to surgery, and doubles again if you choose definitive radiotherapy over the palliative option. It’s important to note that these are based on cats with clean surgical margins. These numbers decrease significantly if the tumor was not completely excised or if the surgical margins were dirty.

Even after radiotherapy in combination with surgery, the 1-year tumor reoccurrence rate is still between 30 and 40% and increases as the time since radiation therapy increases. This disease is incredibly aggressive and unpredictable. There is no treatment plan that guarantees a cancer-free life for your cat. However, there is evidence that shows surgery and radiation treatments together can significantly increase the time until tumor reoccurrence.

Radiation

According to Momo’s radiation oncologist, approximately 60 to 80% of cats will have tumors that never grow back after the combination of surgery and radiation therapy.

WHAT IS IT?

Radiation therapy is a type of cancer treatment which aims a beam of radiation, usually x-rays, directly at the cancer while protecting healthy tissues from high doses of radiation. The dose of radiation typically administered per treatment is about three times greater than that of a normal radiograph. Radiation therapy damages cancer cells (and other cells) by destroying their genetic material, which controls how cells grow and divide. Healthy cells may be damaged along with cancer cells during radiation therapy, but healthy cells can repair themselves more easily than cancer cells. The goal of radiation therapy is to treat the cancer while harming as few healthy cells as possible.

Radiation therapy in combination with surgery is typically the go-to treatment for FISS. Radiation is a highly-localized treatment which targets a specific area with millimeter precision, which works well for FISS since it is a very localized disease. External beam radiation is most commonly administered to cats with FISS. This treatment uses a large machine called a linear accelerator which aims high-energy beams at a precise point on the body.

Prior to beginning radiation therapy,  your veterinarian will require your cat undergo a CT scan to map out the radiation field and target areas. This helps to plan the radiation therapy with the precision necessary. Each treatment typically takes about 20 minutes. Your cat will be sedated since the radiation has to be administered within millimeters, meaning your cat has to be completely still. Once treatment is over your cat will be ready to go home.

PRE-OPERATIVE RADIATION THERAPY

Radiation therapy can be performed prior to surgical excision of the tumor. The advantage of doing so is shrinking the tumor, as well as it’s microscopic tentacles which invade the surrounding tissue. This can increase the change of completely excising the tumor, especially in areas where complete excision is more difficult.

POST-OPERATIVE RADIATION THERAPY

Radiation therapy after surgery is highly recommended, even if clean margins are achieved. This is due to the undetectable microscopic disease that may be left behind in the cat. Radiation therapy can help kill any cancerous cells that remain in the tissues surrounding the tumor and significantly increases the disease-free interval and survival time as compared to surgery alone. As the time between surgery and the start of radiotherapy increases, the disease-free interval and survival time decreases, so beginning treatment soon after surgery is important.

RADIATION AS A SOLE TREATMENT OPTION

Based on what I’ve read, this is not advised. Radiation can help shrink the tumor, or possibly eliminate it if it is small enough, but the reoccurrence rate following a radiation-only approach is generally high. This could be a good option for a cat with an inoperable FISS tumor, or who otherwise is not fit for surgery. But it is considered to be a palliative option rather than a curative one.

Types of Radiation Therapy

Coarse-Fractioned Radiotherapy

This option is sometimes referred to as “palliative radiation therapy” with the goal of delaying/stunting the growth of the tumor, but depending on many factors including the surgical margins achieved, this treatment could result in a curative outcome. Coarse-fractioned radiation therapy uses larger doses of radiation per fraction (treatment session). This means higher doses are given over fewer treatment sessions. For FISS, coarse-fractioned radiation treatments usually occur once a week for four to six weeks.

Because an overall lower dose of radiation is administered as compared to definitive radiotherapy, there are less side effects associated with this treatment option. Short term side effects include small amounts of redness or skin flakiness associated with the surgical incision. Long term side effects could include hair loss or whitening; scarring of the skin, lungs, or muscle; bone brittleness; and radiation induced cancers. However, fewer treatment sessions decreases the risk of long term side effects tremendously. Most cats tolerate this treatment very well and may not show any side effects.

Definitive Radiotherapy

Definitive radiotherapy, also known as full-course radiotherapy, is considered to be a curative option for FISS patients. The goal of this treatment option is to sterilize any cancer cells that remain after surgery. Definitive radiation usually consists of 18 treatment sessions over 3 to 4 weeks. This means your cat will be at the veterinary hospital everyday Monday through Friday to receive radiation treatments. The doses are smaller but more frequent, exposing them to an overall higher amount of radiation. Because of this, doses must be fractionated because normal tissues tolerate smaller doses per fraction better.

Side effects are generally greater when undergoing definitive radiation therapy due to the higher overall radiation amount. Acute side effects may include skin irritation (similar to that of a sunburn), hair thinning or loss, and a temporary soft cough if the lungs are in the treatment field. In the three to four months following treatment, fur that grows back will most likely by white or gray. Long term side effects that may occur six months to a year after treatment include hair loss or thinning; scarring of the skin, muscles, or lungs; brittle bone or fracture within the treatment field; spinal cord degeneration if in the treatment field; and radiation-induced cancers. Essentially, side effects are more likely to be seen in areas within the treatment field. If your cat’s tumor was removed from it’s hips, it could experience bone brittleness in its hip bones. If it’s tumor was removed from its shoulders/chest, the lungs may become scarred.

Stereotactic Radiotherapy

Stereotactic Radiotherapy (SRS/SRT) is a new, advanced treatment option available to FISS patients. This method achieves even greater precision than conventional radiation treatments. The goal during SRT is to only target cancerous cells and avoid normal tissue to the maximum extent possible. SRT is ablative, meaning it will kill any living tissue where the beam is focused. Because of the incredibly precise, high dose of radiation that can be administered with little impact to normal tissues, SRT can result in a curative outcome with only one to five radiation treatments.

This is an incredible advancement in pet cancer treatment, but it seems like there is still a lot to learn about it. When visiting Michigan State University’s veterinary radiation oncology clinic, I was informed that SRT radiation was only an option for bulky disease (a tumor must be present). However, I have also heard from others, especially those that have visited PetCure Oncology clinics, that SRT was given post-surgically with curative intent to their cats. I haven’t been able to find more information about this matter and whether SRT is an appropriate post-surgical treatment option for FISS patients.

As you can imagine, visiting the veterinary hospital everyday for multiple weeks can take a tremendous toll on both pet and owner. The ability to achieve curative results with very few fractions could change how pet cancer treatment is approached. At this time, the number of veterinary clinics that offer SRT is limited. PetCure Oncology is a nationwide vet clinic that specializes in this treatment. In Michigan, where I’m located, there are only two clinics in the state that offer SRT.

Chemotherapy

Chemotherapy is a drug treatment that uses powerful chemicals to kill fast-growing cells in the body. There is little research available that suggests chemotherapy alone is an effective treatment for FISS. In combination with other treatments it can be used as a palliative treatment. Chemotherapy drugs that have been used in FISS multimodal treatment approaches included doxorubicin, ifosfamide, carboplatin, cyclophosphamide, mitoxantrone, and vincristine; however, doxorubicin is typically the drug of choice for treating FISS. As part of a multimodal treatment plan including surgical excision and radiotherapy, limited research shows that chemotherapy may increase disease-free interval times. An important consideration is the side effects that accompany chemotherapy drugs. These drugs are highly toxic and unlikely radiotherapy, which is precisely aimed at the area of concern, chemotherapy is administered to the whole body. The use of doxorubicin in cats can cause anemia, myelosuppression, and nephrotoxicity. This makes it an inappropriate treatment option for cats with other chronic conditions such as chronic kidney disease or autoimmune disorders.

Chemotherapy may be a useful tool for cats with nonresectable tumors. In combination with radiotherapy, these treatments may be able to significantly downstage the disease and mitigate symptoms. In these cases, it is still consider a palliative option and is unlikely to cure the cat of cancer.

Ultimately, each case is so unique. However, the anticipated side effects and little research showing that administration of chemotherapy drugs alongside radiotherapy treatments can significantly improve disease-free and/or survival time in cats leads me to believe that overall, chemotherapy is not worth pursuing when treating FISS.

Immunotherapy

Immunotherapy is treatment that uses a cat’s own immune system to fight cancer. Immunotherapy can boost or change how the immune system works so it can find and attack cancer cells. 

The most common immunotherapy drug used when treating FISS is recombinant canarypox virus (ALVAC) expressing feline interleukin-2 (IL-2), also known as Recombinant Feline IL-2/ALVAC IL-2/Oncept IL-2. The treatment typically consists of six doses administered over the course of several months, with the first dose given approximately one week prior to tumor removal surgery. The drug is injected subcutaneously at the site of the tumor. Reported side effects are limited to acute irritation where the drug is administered, and no significant long-term side effects have been reported in the research available. While studies on the efficacy of immunotherapy in combination with radiotherapy in treating FISS is limited, the available research suggests that the use of IL-2 immunotherapy concurrent with surgery and radiotherapy could significantly extend disease-free intervals and survival times in cats with FISS.

I’m curious to hear more about this treatment option. It seems like it could be a valuable addition to surgery and radiotherapy, especially considering its minimally-invasive nature and limited report side effects. Boehringer Ingelheim’s Oncept IL-2 vaccine is the only drug of its kind, specifically designed to fight FISS. It was only approved by the USDA in 2019 after conditional approval in 2015 and it does not yet appear to be widely used in the U.S.