What’s My Dog’s Prognosis? When to “Believe” Numbers You Hear from Your Vet

Once you have heard the horrible news that your dog has been diagnosed with cancer, a lot of thoughts start racing around your head. One of the common questions I get is, “How much time do we have?”

Reasonable question. And yet, despite all my training and experience as an oncologist, it is so hard to answer.

During my residency, I had to learn lots of numbers and statistics related to cancer. I’ve discussed some of the common terms in a previous article, The Oncologist’s Perspective on Statistics: Part Three, where I discuss median survival times, response rates, and metastasis rates. No matter what type of cancer your dog has, knowing these numbers are helpful to know.

But should you “believe” them? If the numbers are in your dog’s favor, does that mean your dog will be in the group of dogs that is “good?” Or if the stats are pretty bad, does that mean your dog will be in the “bad” group? Nope.

Don’t believe statistics. Instead, use them to navigate.

Numbers Are Just Numbers. They Are Not Your Dog!

In that series on statistics, I wrote about how stats are helpful but don’t predict the way individual cases turn out. I bring it up again because two cases this week drove this home, both good and bad.

Today, I’m going to go over a brief overview of stats and then discuss two recent cases of bone cancer (osteosarcoma), to show you just how different statistics can play out in real life.

 

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There are two questions we want to answer by looking at statistics in dog cancer.

• How does this treatment impact how long your dog will live, and how likely is he to respond to that treatment?
• What is the metastasis rate for this cancer? In other words, how likely is this cancer to spread?

These are very complicated questions for an oncologist to answer, even when we have “good” data to look at.

Statistics in Prognosis: a Super-Short Overview

Let’s say a dog has a soft tissue sarcoma (STS) and the parents opt for surgery. Afterward, a tumor sample is sent to a pathology lab for examination.

Ten days later, the report says it’s a “low or intermediate” grade soft tissue sarcoma with “incomplete surgical margins.”

It’s a less aggressive tumor, but there were cancer cells on the edge of the sample. This is also called “dirty margins.”

There are probably cancer cells left in the body. This happens because surgeons can’t know they are “getting it all out” just by looking, which is why we want large margins around cancer surgeries.

Unfortunately, in the case of STS, dirty margins make recurrence 10 times more likely.

That statistic is helpful, if grim. If we leave those cells alone, the tumor is almost certainly coming back. That’s why the standard recommendation for low or intermediate grade STS with dirty margins is additional surgery and post-operative radiation if there is a recurrence.

Statistics help us make decisions, but they aren’t laws.

Here’s another statistic about STS that helps us make decisions: the metastasis rate for low or intermediate STS is 10-15%.

In other words, cancer will spread to other areas in one or two dogs out of ten with low or intermediate STS. On the other hand, eight to nine dogs with low or intermediate STS will never see metastasis!

We treat likely metastasis with chemotherapy because it circulates medicine around the whole body, not just where the tumor is. Since metastasis is pretty uncommon in low or intermediate STS, it’s unnecessary in 80-85% of dogs, and we don’t recommend it.

What about high-grade STS? Well, if the pathology report shows a high-grade STS with dirty margins, the situation is different.

The metastasis rate for high-grade STS is much higher: 40%. With four out of ten dogs getting cancer spread, we use chemotherapy to delay that metastasis, plus the extra surgery and/or post-operative radiation.

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Statistics are Confusing

That’s a lot of numbers to juggle, and if you’re a little confused, it’s because it’s confusing. It might take you a while to understand your dog’s case stats. That’s totally normal.

Make sure to ask your veterinarian or oncologist to explain the numbers to you. It really helps you to make confident decisions if you know the “why” behind the recommendations.

Let’s look at a couple of real-life cases now. These two cases that involve bone cancer, specifically osteosarcoma. (Osteosarcoma is the most common bone tumor — 85% of dogs with cancer in the bone will be diagnosed with osteosarcoma.)

By looking at these cases, we can see how statistics help, but don’t define, decisions about dog cancer treatment.

Case: Osteosarcoma in Blackie’s Wrist

Blackie is a mixed breed dog with osteosarcoma (OSA) of his left metacarpal bone. This is one of the bones in the paw just, below the wrist or carpus.

After Blackie had his front leg amputation, I met with him and his parents to review all the stats for OSA.

Statistics tell us that 75% of these tumors develop in the limbs (legs), and the front legs are twice as likely to develop this tumor.

Here’s another stat: the most common locations are “towards the knee and away from the elbow” or at the top of the shoulder, the wrist, and the knee.

Blackie’s metacarpal location is less common, but it is reported, as well.

Here are more numbers, or stats:

• Median survival times for OSA cases with amputation and no other treatment is about four to five months. Dogs with amputation only almost all die within in one year; the statistic is 90-100%. Only 2% of dogs with only amputation are still alive at two years.
• Median survival times for OSA cases with amputation and chemotherapy increase to ten to twelve months. If a dog gets both amputation and chemotherapy, 20-25% are still alive at two years.

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Adding chemotherapy to Blackie’s treatment plan might help prolong his life. With amputation only, four to five months. Add chemo, and the stats say Blackie could live ten to twelve months and has a 20-25% chance of living two years. Was there a guarantee that he would be in the “good” half of the longer median survival time? No, but it was certainly possible.

I also told Blackie’s dad that chemotherapy is well tolerated, with 80% of dogs having no side effects at all. About 15-20% have side effects that are mild and self-limiting, meaning dogs recover with little intervention. This includes some nausea and/or diarrhea, which is treated at home with meds or antibiotics. The rest, 5% or less, experience severe side effects like the ones we expect in human cancer chemo treatments. Those dogs might have to be hospitalized as a result.

Blackie’s Treatments

So back to Blackie. He had three doses of chemo and experienced very few side effects at first.

But when he came in for his fourth treatment, he was not doing well. His legs were swollen and he was reluctant to walk. His right front leg was most swollen, and since he only has three legs, I was very concerned.

My training and my experience with OSA made me suspicious: had he had developed lung mets (metastasis)?

And did he have a very uncommon condition related to that called hypertrophic osteopathy (HO)? We see HO most commonly with metastatic disease to the lungs, especially OSA.

HO is when new bone develops along the shafts of long bones. But it doesn’t develop the way it should: it sort of juts out and builds out into the tissue. On X-rays, it looks jagged and spikey and painful. It causes a lot of pain and swelling because the bones are sort of poking out into other tissues.

Blackie’s chest X-rays confirmed metastasis (met). Plus, he had a skin met near his thigh and another deeper under his skin by his right humerus. It wasn’t good news, especially only three months after his amputation!

I took a few deep breaths before I went in the room to tell his dad that the statistics I gave him at the initial appointment had not predicted poor Blackie at all. He probably wasn’t going to be in the half of OSA dogs that live ten to twelve months with amputation and chemotherapy.

His tumor has turned out to be way more aggressive than the statistics led us to believe. This week, we are shifting gears to make him more comfortable and see if we can slow the progression of the mets.

Case: Daisy Mae’s Splenic Osteosarcoma

I saw another case this week where the stats were less than helpful, but the case itself is more hopeful. You can see Dasy Mae’s picture on my Dr. Sue Facebook page – she’s pretty darn cute!

Daisy Mae has OSA of her spleen, called extraskeletal OSA because it is outside the skeleton. It’s an uncommon tumor in the spleen; hemangiosarcoma and benign hemangiomas are more common. Since it’s less common, there is limited data in the literature.

Here are the stats from one report:

• Median survival time for splenic OSA with only surgery was one month. That means half the dogs in the study with splenic OSA were alive one month later, and the other half had passed.
• Median survival time for splenic OSA with surgery and chemotherapy was five months. That means half the dogs were alive five months later, and the other half had passed.

Now, I am certain I speak for all us dog lovers when I say that even the longer time frame, five months, doesn’t sound like a long time.

Even so, her parents decided to give Daisy Mae surgery and then conventional IV chemo followed by anti-angiogenic oral chemo.

When I saw her this week for a check-up 18 months AFTER her surgery, she was tumor-free!

So in Daisy Mae’s case, the stats were wrong, too. But in her case, they might have led us not to treat in the first place.

I always say I love when I am “wrong” because my patients outlive the stats.

Bottom Line: Use the Stats, Don’t Live By Them

So should you ignore statistics and published studies? Of course not! They can help you to make treatment decisions.

But you must realize that stats will never predict the individual. I personally hope your pet outlives the stats, but my advice — after all my training and years of clinical experience — is this:

Learn the facts, and then be hopeful.

Live longer, live well,

Dr Sue

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