The Dog Cancer Survival Guide (96 page)

MCT is a complicated cancer and there are many ways it can spread in the body. Some owners choose to stage the cancer without knowing the grade of the primary tumor, because if spread is found, they might not attempt to treat the primary tumor with surgery.

There is no one right way to handle MCT, and I recommend that you carefully and closely consult with your veterinarian or oncologist to decide what is right for you and your dog.

There are a couple of screening tests that were historically used for MCT diagnosis which have proven unreliable. A buffy coat evaluation (a blood sample is spun in a centrifuge so that the white and red blood cells are separated) can reveal mast cells circulating in the blood. This is not evidence of MCT, however; other conditions that can cause the same result include inflammatory skin disease, parvo, regenerative anemias, traumas and even other cancers. Because of all these possibilities of false positives with buffy coat evaluations, I do not use them to screen for MCT (rarely, I use them to monitor MCT patients over time).

Another test sometimes used to screen for MCT involves an examination of the eosinophils in the blood. Eosinophils are a type of white blood cells often associated with mast cells, and sometimes MCT can cause an elevation of their concentration, as can allergies, parasites, and other cancers, including lymphoma. Therefore, I do not consider eosinophil levels to be a useful test for MCT in general, but if, in your dog’s case, eosinophil levels rise and fall along with their MCT, I may use that information to monitor him, over time.

It is very difficult to definitively offer a prognosis for MCT, because there are so many variables to consider. Each dog’s case should be considered individually, and several factors should be weighed.

Grade is the most consistent predictor of outcome. Here is the current grading system:

• Grade I
  tumors are usually confined to one tumor in the skin and do not recur once they are surgically removed, with clean margins. These account for approximately 33-50% of all MCT.
  
• Grade II
  tumors account for approximately 25-45% of all MCT, are locally invasive (have gone into the deeper layers below the skin) and are more likely to disseminate to other parts of the body. They are also more likely to recur, especially if they have incomplete or narrow margins.
  
• Grade III
  tumors are usually very malignant, have invaded deep into the body, and are highly aggressive, with a 55% to 95% rate of metastasis. Grade III tumors are also extremely likely to recur and account for approximately 20-40% of all MCT cases.
  

When I say that MCT is unpredictable, I am particularly thinking of Grade II tumors. Some act like Grade I tumors and are highly treatable. Others, however, act more like Grade III tumors and have a greater likelihood of spreading. Plus, some dogs present multiple, new tumors over time. This unpredictability requires us to look at as many prognostic factors as possible, beyond grade. When the lymph nodes are involved, if there has already been metastasis, if the proliferation scores are high, or if the mitotic index is five or higher, the prognosis worsens.

Another factor that affects the prognosis includes stage – the location(s) where MCT actually appears in the body. Here is the World Health Organization’s (WHO’s) Clinical Staging System for MCT:

Stage 0:
One tumor, which has been removed for a biopsy, but still has microscopic cells left at the surgical scar (this is also called an incomplete removal),
without
regional lymph node spread.

Stage I:
One tumor, completely confined to the skin (no spreading into deeper subcutaneous layers)
without
regional lymph node spread.

Stage II:
One tumor, completely confined to the skin (no spreading into deeper subcutaneous layers)
with
regional lymph node spread.

Stage III:
Multiple skin tumors, or large tumors which infiltrate deeper layers,
with
or
without
regional lymph node spread.

Stage IV:
Any tumor with distant spread, including blood and bone marrow involvement.

In general, the higher the Stage number, the worse the prognosis.

Please note that, although multiple tumors do indicate a higher stage, according to this official staging system, many oncologists – including myself – believe the Stage III classification is outdated. In our experience, multiple MCT tumors do not indicate a more advanced disease. Although counterintuitive, it is our experience that MCT presenting with multiple tumors are not skin metastases, but new skin MCT. Therefore, it is not necessarily more aggressive and does not have a worse prognosis than a Stage II single tumor. WHO’s official staging has not yet been modified to reflect this new understanding.

Another factor we consider, when offering a prognosis, is whether your dog is feeling sick.

Substage (a)
means that there are no systemic clinical signs related to MCT – in other words, your dog does not feel sick at time of diagnosis. In that case, the prognosis is better.

Substage (b)
means there are systemic clinical signs related to MCT – such as decreased appetite, vomiting, bloody stools, and swelling or edema associated with MCT degranulation. If your dog feels sick at the time of diagnosis, the prognosis worsens.

There are several other predictive factors that can help form a prognosis. The most reliable are:

• Proliferation Rate:
  If mast cell tumors are rapidly multiplying, the prognosis worsens.
  
• Recent Rapid Growth:
  If there has been recent, rapid growth of a tumor, the prognosis worsens.
  
• Tumor Size:
  The larger the tumor, the worse the prognosis.
  
• Recurrence:
  If tumors have been removed and recur, the prognosis worsens.
  
• C-Kit Mutation:
  If a c-Kit genetic mutation is detected, the prognosis worsens.
  
• Location in the Body:
  Certain locations have been associated with a higher rate of metastasis. Mast cell tumors located in the internal organs, on the genitals, scrotum, muzzle, ear and gums all have a greater tendency to metastasize. While this makes these MCT tumors more aggressive, it does not necessarily mean that they are untreatable. For example, a recent study of muzzle MCT showed that while metastasis was more likely, it occurred later in dogs with lower grades of cancer. The overall mean survival time was still two and a half years with aggressive (not palliative) treatment.
  

Let me remind you, here, that every dog is different from every other dog, and that every cancer case is different from every other cancer case – especially if your dog has MCT. As you can tell, it’s a tremendous task to pull all of this information together and formulate a prognosis and treatment plan. There is a lot to consider, and if you can’t absorb all of this right now, that’s OK. Your vet or oncologist can help you understand your own dog’s case.

Surgery is the treatment of choice for MCT, and often the only treatment needed. The goal is to completely excise (remove) it and prevent its recurrence. The surgeon should aim for a minimum margin of two centimeters all the way around the tumor, including at least one deeper tissue layer. A surgical biopsy is necessary to determine whether the margins are complete. When they aren’t – or if they’re dirty – the surgeon will consider a second surgery (a scar revision) to remove more tissue. The tissue from the second surgery will also be biopsied to check for clean margins, because a completely removed tumor is less likely to recur or metastasize.

If your dog has more than one MCT, both should be completely removed. Although, in my experience, more tumors do not indicate more aggression or a worse prognosis, I still advocate removing as many as possible. For the uncommon case of many MCT all at the same time, this surgery may not be realistic and, in that case, I recommend a systemic chemotherapy protocol (see below). Even then, I recommend removing at least one tumor for biopsy, to obtain the grade and other prognostics from the lab.

Additionally, if a tumor is really huge, I may try to shrink it before surgery, so that we can remove it more easily. For this, I use chemotherapy or radiation treatments (see below).

For some MCT, especially those located on the lower leg, surgery cannot achieve the necessary clean and wide margins because there isn’t sufficient tissue surrounding the tumor. If wide excision is not possible, or if the margins are contaminated with tumor cells, radiation therapy will be considered.

Post-surgical radiation provides excellent longterm MCT tumor control for dogs with microscopic cells remaining after surgery. The vast majority (85% to 95%) of dogs with low- or intermediate-grade MCT remain tumor-free two to five years after treatment. Even grade III MCT cases can benefit: in a recent study, 70% of these dogs were still alive one year after radiation treatment. Radiation treatment protocols usually involve fifteen daily treatments (Monday through Friday for three weeks). These will be scheduled two to three weeks after surgery, before MCT has a chance to recur and after the surgical scar has healed. The radiation should be directed to the area three centimeters around the scar.

While radiation therapy can be helpful for treating microscopic cells post-surgery, it is less successful as a primary treatment. Statistics show that treating measurable tumors (tumors large enough to see and measure, also called macroscopic) with radiation therapy alone leaves only 50% of dogs still alive after one year. If radiation is being considered as a primary treatment, an increase in the amount of radiation and the addition of chemotherapy and steroids may improve the response rate and duration.

Radiation can also be used for non-resectable, or inoperable, tumors. In these cases, a palliative approach of four weekly treatments, combined with steroids, has produced a response in an impressive 88% of all dogs with MCT. In this study, the MCT tumors did not progress (grow) for a median time of thirty-four months – almost three years! This is an extremely positive outcome for an inoperable tumor. It’s also worth noting that dogs with non-resectable MCT on the leg did better than those who had nonresectable MCT on the head.

Radiation therapy can also be helpful when it is started prior to surgery, with the goal of shrinking very large tumors. This approach may allow more complete removal of the remaining tumor and also slow tumor progression (growth).

In many MCT cases, I do not recommend chemotherapy at all, because it is not as effective as surgery and radiation for a primary tumor in the skin, especially if the cancer is of a lower grade and/or confined to one local area. Sometimes, I use chemotherapy instead of radiation for large MCT tumors which need to be shrunk prior to surgical removal, and I would also consider palliative chemotherapy for nonresectable tumors, if radiation therapy is not available in your area or if you have decided against radiation.