Cancer in Dogs and Cats: The Later Stages

Hearing the word cancer changes everything. In our webinar “When Is It Time? Understanding the Euthanasia Window”, we describe how that moment in the exam room often feels: “You’re in a daze. You’re in shock as you’re trying to reel with this new reality that has literally just been thrust upon you.”

It’s a reality many families share. Cancer is one of the most common reasons families say goodbye to a dog. In a US survey of dog owners, it was the cause of death they named most often, just ahead of old age¹. It’s among the most common causes of death in cats, too².

When researchers asked owners of dogs with life-limiting cancer what they needed from their veterinary team, the answer at the center of it all was the truth. Honest information helped them make decisions and be prepared for what was ahead³. That’s what this article sets out to give you: what the later stages of cancer usually look like, including four common cancers, how to keep your pet comfortable, the signs that comfort is getting harder to keep, and what families weigh as they think about the decision ahead. It’s part of our series on common end-of-life conditions.

Two big questions sit outside this article, though. How much time a pet may have is covered in our article on understanding survival times and life expectancy, and treatment choices belong with your regular veterinarian or oncologist.

Every cancer is different, but in the later stages many of them change a pet in similar ways. Weight and muscle are often the first to go, and not always in a way you can see. In a study of dogs seen by a cancer service, only a few looked very thin, yet almost a quarter of the dogs with an earlier weight on record had lost more than a tenth of their body weight⁴. Cats lose muscle even more consistently. In a study of cats with cancer, nine in ten had lost muscle, and cats who had become too thin tended to have much less time⁵. So the scale, or running your hand along your pet’s spine and hips, often tells you more than looking does.

Along with weight loss, pain is part of the picture for many pets with cancer, and it rarely comes alone. Tiredness, weakness, trouble breathing and stomach or bladder problems all feed into how a pet with cancer feels⁶. Those signs aren’t always obvious, and cats can be especially hard to read. As we put it in our webinar, “if you have a cat that’s been hiding illness from you for the last 15 years of its life, the odds of it all of a sudden being open and transparent about what it’s going through are probably not very high.” Our article on recognizing pain in cats and dogs covers the subtle signs to watch for.

Beyond these shared changes, each cancer follows its own pattern, and the four below are among the most common.

The first of those, lymphoma, is one of the most common cancers in dogs. It usually shows up as swollen lymph nodes, often in several places at once, such as under the jaw⁷. Treatment can bring it into remission, often for months, but in most dogs it eventually comes back and stops responding⁷. So even when treatment goes well, most families face the later stages at some point.

Many families want to know whether a dog with lymphoma will die naturally. Without chemotherapy, lymphoma keeps progressing, and the time is usually short. In a study of 109 dogs with a common form of lymphoma who were given steroids alone, without chemotherapy, half had died by about 50 days and half lived longer⁸. Dogs who were already unwell when they were diagnosed, and dogs with a type called T-cell lymphoma, tended to have less time. The same study found that how a dog was doing in the first two weeks, as their owner scored it on a quality of life form, said a lot about how much time they had⁸.

That study looked at one form of lymphoma, in dogs, all given steroids, so it isn’t a prediction for your dog and doesn’t apply to cats. Our article on survival times explains what a number like this does and doesn’t tell you. And because lymphoma can end a dog’s life on its own, our article on letting a pet die naturally vs. hospice vs. euthanasia describes what a natural death involves and what to consider before choosing that path.

In cats, lymphoma often looks very different. One of the most common forms is a slow-growing lymphoma of the gut, now the most common cancer of the digestive tract in cats. It tends to come on gradually, over weeks or months, with weight loss, vomiting, a poor appetite and diarrhea⁹. Many cats with this slow-growing form do well with treatment for a long time, while faster-growing forms of lymphoma have a much harder outlook⁹. So a lymphoma diagnosis in a cat doesn’t always mean the later stages are close. When they do come, the same signs are the ones to watch: whether your cat is eating enough, keeping food down and holding their weight.

Where gut lymphoma in cats tends to come on slowly, hemangiosarcoma can change things in a moment. It’s a cancer of the blood vessels, mainly in dogs, and it often grows on the spleen. Its biggest danger is bleeding, because a tumor can bleed into the belly with little or no warning.

When an older dog suddenly bleeds into the belly without an injury, a tumor is the most likely cause. In one study, four in five of the dogs whose cause was found had a cancerous tumor, most often hemangiosarcoma, and almost all of the dogs in the study were anemic from the blood loss¹⁰. Not every bleeding mass is cancer, though. Across many studies of dogs with a ruptured mass on the spleen, about a quarter turned out to be benign, and some of those dogs may do well for a long time¹¹. So the diagnosis, and what it means for your dog, is a question for your veterinarian or the emergency team.

Whatever the cause, a bleed is an emergency, and these are the signs to know:

• Sudden weakness or collapse – A dog who suddenly can’t stand, or seems wobbly and flat.
• Pale gums – Gums that look white or much paler than usual, a sign of blood loss.
• A swollen belly – A belly that suddenly looks bigger or feels tight.

Any of these means an emergency veterinarian, right away. And because a bleed can come without warning, it helps to plan in two ways. The first is knowing these signs and having an emergency plan ready, which the section on planning for a sudden turn below covers. The second is a choice some families make: to say goodbye earlier, while their dog is still comfortable, rather than risk an emergency. Either choice is a loving one, and it’s yours to make based on what’s best for you and your family.

Bone cancer, or osteosarcoma, is another cancer seen mostly in dogs, where it often starts in a leg. Most of the research on it is in dogs, and this section is about them. Its later stages are shaped by two things: pain and spread.

Pain comes first. Bone cancer is painful, and the pain can be hard to fully control. In a small study, dogs with bone cancer in a leg were more sensitive to pain well beyond the tumor itself. A step-by-step course of pain medication improved their activity and their restless nights, but the researchers concluded that this pain is mostly hard to control with medication alone¹². If your dog’s pain medication stops being enough, it doesn’t mean you or your veterinarian failed. It’s the nature of this cancer, and it’s worth telling your veterinarian, who may have other options to try.

Then there’s spread, because bone cancer rarely stays where it started. In a study of dogs examined after death, the cancer had reached 20 different parts of the body across the group, most often the lungs and then other bones¹³. So in the later stages, changes in breathing, or a new limp in another leg, can be signs it has spread.

There’s one more risk to know about: the weakened bone can break. In a study of dogs whose leg bone cancer was treated with comfort-focused radiation, about one in five broke the bone, and nothing on their X-rays predicted which dogs would¹⁴. If your dog suddenly won’t put any weight on the leg, call your veterinarian the same day. And because bone cancer is often found late, there can be less time to decide, as our article on the euthanasia window explains.

The last of the four is a cancer of the mouth. In cats, the most common one there is squamous cell carcinoma. It grows into the surrounding tissue wherever it starts, whether that’s the gums, the jaw or the tongue, and it can rarely be cured. Comfort care can help for a while, but the damage the tumor does to the mouth is often what leads to euthanasia¹⁵.

As it grows, it can make eating, drinking and grooming harder, and those everyday things become the ones to watch. Dogs get mouth tumors too, of several different kinds, and the same questions about eating and comfort apply. Your veterinarian can tell you how your dog’s tumor is likely to behave. Whatever the cancer, there’s a lot you can do to keep your pet comfortable along the way.

Most of what keeps a pet with cancer comfortable is the same care any sick or aging pet needs: soft bedding, good footing, help with eating and a calm home. Our article on keeping your pet comfortable at home covers those in detail. A few things matter especially with cancer.

Things to consider:

• Raise pain early – Pain is common in dogs and cats with cancer, families can learn to recognize it, and treating it early, often in more than one way, can work well¹⁶. Cancer pain in pets is often missed or undertreated, though⁶, so if you think your pet hurts, it’s better to err on the side of caution and tell your veterinarian sooner rather than later.
• Keep track of their weight – Because weight and muscle loss can be hard to see, weighing your pet regularly, or noticing how their spine and hips feel under your hand, helps you and your veterinarian spot changes^4,​5.
• Make eating easier – For a pet with a sore mouth or a poor appetite, the ideas in the Eating section of our comfort care article, like softer food and smaller meals, can help.
• Care for open growths – If a tumor has opened up or bleeds, the Licking and Wounds section of the same article covers keeping it clean. Check with your veterinarian before putting anything on it.
• Protect a leg with bone cancer – Because the bone can break, good footing and fewer jumps and stairs matter even more. The Helping Them Get Around section of our comfort care article has ideas. In a Wisconsin winter, keeping sidewalks and steps clear of ice and snow makes every trip outside safer.

Medical advice isn’t something you should get from the internet, so questions about medication, including pain medication, are for your regular veterinarian or oncologist. Comfort care helps a great deal, but with some cancers the hardest moments come suddenly, and that’s worth planning for.

Some cancers can change in an hour. A dog with hemangiosarcoma can bleed, a dog with bone cancer can break a leg, and a cat with cancer in the chest can suddenly struggle to breathe. In cats with fluid around the lungs, cancer is one of the most common causes, and it often turns serious quickly¹⁷. A cat breathing fast or with their mouth open needs an emergency veterinarian right away.

None of this is easy to think about. But as we say in our webinar, “hope is not a plan. And so it’s better to get a plan in place so that if you are in a crisis situation, you’re not making these intense, scary decisions under that duress.” Veterinary end-of-life guidelines recommend a written care plan, made together with your veterinary team¹⁸. For a cancer that can turn suddenly, that plan is worth having before you need it.

Things to consider:

• Know your nearest emergency hospital – Find the closest 24-hour emergency veterinarian, and how long it takes to get there, day and night.
• Ask now what to give on the way – Ask your regular veterinarian now what you can give your pet for pain on the way to the emergency room if a crisis happens.
• Talk through what you’d choose – Decide with your family ahead of time what you would want if a crisis came, so you aren’t deciding under pressure.
• Pack a go bag – Our article on embracing the emergency room walks through what to bring and how to plan.

As that article says, ending up in the emergency room isn’t a failure. A medical crisis sets its own schedule, and a plan simply means you’ll be ready to make the choice that’s right for your pet. More often, though, cancer’s later stages come on gradually, and the signs are easier to miss.

In the later stages, the question shifts from what the cancer is doing to how well your pet can still be kept comfortable. When researchers built a quality of life questionnaire from the everyday things owners of dogs with cancer notice, those things fell into four areas: energy, closeness to the family, pain and getting around¹⁹. Those make a good frame for the changes to watch:

• Less energy – Sleeping more, tiring sooner, or losing interest in walks, play and the things they used to enjoy.
• Pulling away – Seeking less contact, hiding, or no longer coming to greet you.
• Pain that’s harder to control – Restlessness or trouble settling at night, or pain medication that no longer seems to be enough¹².
• Trouble getting around – Struggling to get up, lie down or manage the stairs.
• Eating and weight – Eating less, turning away from food, or losing weight and muscle⁵.
• Breathing changes – Breathing faster or harder while resting.

Sometimes the decision comes not from the cancer spreading but from what the tumor does where it is. In a study of dogs treated for bladder cancer, most were euthanized because of the tumor’s growth in place, often because it blocked the flow of urine, rather than because it had spread²⁰. A mouth tumor that stops a cat from eating, or bone cancer pain that can’t be controlled, can bring families to the same point.

Good days can make all of this harder to see. In our webinar, we describe the point at which many owners decide: “yes, they’re having a good day today, but boy, what did they have to go through last night to get to that good day?” A pet who is still eating can still be struggling, too. Our euthanasia timing page answers the question “My pet is still eating and drinking. Does this mean it’s not time?”

No single sign means it’s time. What matters is the overall picture and how it changes over weeks and days, which tracking makes easier to see. In the steroid study of dogs with lymphoma, the quality of life scores owners gave early on said a lot about how much time their dogs had⁸. A quality of life scale or the free Hospet app lets you record good days and bad days and see how things are trending. Our article on tracking your pet’s quality of life explains how, and the section of our comfort care article on when comfort isn’t enough anymore describes more of the general signs.

When the signs are hard to read, our veterinarians offer an in-home quality of life consultation. In a full, unrushed hour in your home, we can look at your pet’s comfort and pain away from the stress of a clinic, and you’ll leave with comfort care ideas and clear next steps. Seeing the picture clearly is one part, though. Deciding what to do with it is another.

There’s rarely a single right moment to say goodbye to a pet with cancer. Our article on the euthanasia window describes timing as a window rather than a moment, and walks through the medical, practical and emotional factors that shape where you fall in it. A few of those come up especially often with cancer.

The first is treatment. Many families are offered surgery, chemotherapy or radiation, and some choose them. Others choose to focus on comfort, and that’s a valid choice. It’s OK to say that, whether because of cost, worries about side effects or any other reason, you’ve decided not to pursue treatment.

Hope can also make the time ahead look longer than it is. In a study of owners of cats with cancer, almost a third of those who chose treatment overestimated how long their cat would live, and how owners felt about choosing euthanasia was tied to whether their cat could still groom, eat and play²¹. Asking your veterinarian what the later stages of your pet’s cancer usually look like can help you plan with a clearer picture. With a cancer that can turn suddenly, families also weigh more time against the risk of an emergency, as the section on planning for a sudden turn describes.

Your own wellbeing matters too, because caring for a pet with a serious illness is hard on the whole family. In one study, owners of a dog or cat with a chronic or terminal illness reported more burden, stress, anxiety and symptoms of depression than owners of healthy pets²². So how you’re holding up is a fair part of the decision.

As hard as those months of caring can be, they also bring something a sudden loss doesn’t: time to prepare. In a study of owners whose pets were euthanized, those whose pet had cancer reported less anger and less guilt, while a sudden death was linked to more anger²³. Our article on anticipatory grief talks about grieving a pet who’s still here.

That time can also go toward planning the goodbye itself, whenever it comes. If you’re considering a goodbye at home, What Happens During In-Home Euthanasia walks through the visit step by step, and our guide to preparing for an in-home euthanasia covers the practical side.

Cancer’s later stages look different for every pet and every cancer. Knowing what to expect, keeping your pet comfortable, having a plan for a sudden turn and watching how things change over time can all help you make the decision that’s right for your pet and your family.

Lean on your regular veterinarian or oncologist for the medical side. And please be gentle with yourself. However much time you have left together, you don’t have to face it alone.

→ Understanding Survival Times & Life Expectancy
What “6-12 months” really means, and why median survival times aren’t countdown timers.

→ Keeping Your Pet Comfortable at Home
Practical comfort care for a sick or aging pet, from footing and bedding to eating, breathing and medications.

→ The Euthanasia Window: Understanding When It’s Time
Why there’s no single “perfect moment,” and how to think about timing as a window rather than a moment.

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