# Dog Cyst Getting Bigger or Changing Size: What It Means A dog cyst getting bigger is often a sign that fluid, tissue, or cells inside the lump are changing, and the meaning depends on what the lump actually is. Some cysts grow slowly for harmless reasons, while others enlarge because of infection, bleeding, or cancer. Size alone does not tell you which one you are dealing with, so a veterinarian usually needs to look at the lump directly. This article is educational and is not a substitute for veterinary diagnosis or treatment. ## Quick Answer - A lump that grows over days to weeks deserves a veterinary visit, even if your dog seems fine. - A lump that gets bigger and smaller can be a cyst, an abscess, or a fluid pocket, but some tumors also fluctuate. - Fast growth (noticeable change within days), redness, heat, pain, or drainage means call your veterinarian promptly. - A lump that suddenly appears and grows quickly, or one that bleeds or ulcerates, may need same-day evaluation. - Do not squeeze, lance, or apply home remedies to a lump. Let your veterinarian identify it first. ## What a Changing Lump Looks Like and What Is Normal Lumps on dogs are common, and most owners find them by petting their dog during grooming or cuddling. A cyst is a sac filled with fluid, keratin, or other material. It can feel soft, squishy, or firm depending on what is inside. Size changes are the part that worries owners most. A cyst may stay the same for months, then seem larger after your dog scratches or bumps it. Some cysts drain and shrink, then refill and grow again. That up-and-down pattern is common with fluid-filled structures. What is not normal is steady, rapid growth. A lump that doubles in size in a week, or one that keeps expanding without shrinking, needs a closer look. So does any lump that changes texture, becomes fixed to the tissue underneath, or starts to hurt. Keep in mind that you cannot tell a cyst from a tumor by touch. Many benign and malignant lumps feel similar. A veterinarian can often tell more with a needle sample than with hands alone. If you are comparing lump types at home, this guide on [lipoma vs mast cell vs cyst diagnosis](/knowledge/veterinary-medicine/symptom-guides/why-is-my-dog-having-lumps-on-skin-lipoma-vs-mast-cell-vs-cyst-diagnosis) explains how those common lumps differ. ## Why a Dog Cyst Getting Bigger Matters A growing lump means something inside it is accumulating. That could be fluid, keratin, blood, inflammatory cells, or cancer cells. The rate of growth and the other signs around it help your veterinarian narrow the list. ### Common Causes of a Growing or Fluctuating Lump | Cause | Typical clues | How urgent | |---|---|---| | Follicular cyst | Round, smooth, often on the back or limbs, may drain a thick material and refill | Routine visit | | Sebaceous cyst | Attached to skin, often has a pore or dark dot, slow growth | Routine visit | | Abscess | Warm, painful, may drain pus, often after a bite or scratch | Same day | | Seroma or hematoma | Soft, fluid-filled, appears after trauma or surgery | Same day to routine | | Lipoma | Soft, movable, under the skin, grows slowly | Routine visit | | Mast cell tumor | Firm or soft, may wax and wane, can redden or ulcerate | Prompt visit | | Other skin tumors | Firm, deep, irregular, may grow steadily | Prompt visit | | Paraprostatic cyst | Male dog, related to the prostate, can cause abdominal distention | Prompt visit | Paraprostatic cysts form next to the prostate gland from embryological remnants and become fluid-filled. Signs appear only when the cyst grows large enough to press on nearby organs like the bladder or colon, and large ones can cause abdominal distention [1]. Medical treatment and castration alone do not resolve them, and surgical removal is the treatment of choice [1]. Some tumors can change size too. A Merkel cell carcinoma, a rare skin cancer, appeared as a solitary, rapidly growing mass on the chest wall of a young dog in one case report [2]. That is an uncommon tumor, but it shows why rapid growth should never be dismissed as harmless. ### When Growth Is More Likely Benign Slow growth over months, a soft and movable feel, and no changes to the skin surface are somewhat reassuring. Benign tumors that are not ulcerated and do not interfere with your dog's normal routine may not need treatment at all, especially in older dogs [3]. Your veterinarian may recommend monitoring instead of surgery in those cases. That said, monitoring means actual measurement and regular rechecks, not just a glance. Take a photo with a ruler next to the lump every two to four weeks. Bring those photos to your appointments. ## Red Flags That Mean Emergency or Same-Day Care Call your veterinarian right away, or go to an emergency clinic, if you see any of these: - The lump grows visibly within hours to a few days. - The lump is hot, red, painful, or your dog cries when you touch it. - The lump opens, bleeds, or drains foul-smelling fluid. - Your dog has trouble breathing, eating, urinating, or defecating. - Your dog is lethargic, vomiting, or losing weight. - The lump is near the eye, mouth, throat, or rectum. - You find many new lumps at once. A rapidly growing mass on the skin can be malignant, and some tumors spread to lymph nodes and other organs [3]. Early evaluation gives you more options. Behavior changes can also be a clue. If your dog seems painful, withdrawn, or restless along with the lump, read [what it means when your dog's behavior changes](/knowledge/veterinary-medicine/behavior/what-does-it-mean-when-your-dog-s-behavior-changes). Pain and discomfort often show up as behavior first. ## What You Can Safely Do at Home While Waiting You cannot treat a cyst at home, but you can protect it and gather useful information. - Do not squeeze, poke, or try to drain the lump. This can cause infection or spread cancer cells. - Do not apply human creams, essential oils, or hydrogen peroxide. - Take clear photos with a ruler or coin next to the lump for scale. - Note the date you first saw it and how it has changed. - Measure the lump with a soft tape measure and write down the number. - Keep a simple log of your dog's appetite, energy, and bathroom habits. - Prevent your dog from licking or scratching the area. A soft cone or a light bandage can help. - Call your veterinarian to schedule an exam. Ask whether the lump should be seen sooner based on how it looks. If the lump starts bleeding and will not stop, or your dog seems weak or pale, treat it as an emergency. ## What Your Veterinarian Will Check and Test Your veterinarian will start with a full physical exam, including feeling the lump and checking nearby lymph nodes. They will ask when you first noticed it, how fast it has changed, and whether your dog has other symptoms. Common next steps include: - Fine needle aspiration. A thin needle collects cells from the lump. This is quick, usually well tolerated, and often gives a diagnosis or narrows the list. - Impression smear. If the lump is draining, a sample of the fluid can be examined under a microscope. - Biopsy. A tissue sample gives the most complete answer, especially if cancer is possible. - Imaging. Ultrasound or X-rays help if the lump is deep or internal. Transabdominal ultrasonography with physical exam and blood work can tell a paraprostatic cyst apart from a prostatic abscess [1]. - Blood work. This checks overall health and can reveal problems linked to certain tumors. Imaging does not always give a clear answer on its own. In one case, ectopic splenic tissue (small pieces of spleen tissue growing outside the spleen) looked like aggressive cancer on ultrasound, and only histopathology confirmed it was benign [4]. A similar report described peritoneal nodules that turned out to be splenosis, which is ectopic splenic tissue that can enlarge or rupture and cause problems in some dogs [5]. These cases show why a tissue sample often matters more than a picture. If your dog has a history of spleen surgery or abdominal trauma, tell your veterinarian. It changes what they consider. ## Prevention and Monitoring You cannot prevent most cysts or tumors. What you can do is catch changes early. - Do a monthly lump check from nose to tail. Run your hands slowly over your dog's body, including the belly, legs, and around the tail base. - Record every lump you find with a location, size, and date. - Keep recheck appointments even if the lump seems stable. - Ask your veterinarian how often to recheck each lump. The schedule depends on the type and location. - Watch for weight changes. Unexplained weight loss or gain can point to a deeper problem. See [why is my dog losing weight](/knowledge/veterinary-medicine/symptom-guides/why-is-my-dog-losing-weight) or [why is my dog gaining weight](/knowledge/veterinary-medicine/nutrition/why-is-my-dog-gaining-weight) if you notice a change. - If your dog has a follicular cyst, this [veterinary reference guide on dog follicular cysts](/knowledge/veterinary-medicine/dermatology/dog-follicular-cyst) covers what to expect over time. For benign lumps that are not causing problems, your veterinarian may choose to monitor instead of remove them, especially in older dogs [3]. That is a reasonable plan when the diagnosis is confirmed and the lump is not bothering your dog. ## Limitations and When to Contact a Veterinarian No article can tell you what your dog's lump is. Only an exam and testing can do that. Individual cases need a veterinarian who can see, feel, and sample the lump. Contact your veterinarian the same day if the lump is warm, painful, red, or draining, or if your dog seems unwell. Go to an emergency clinic now if your dog has trouble breathing, collapses, has a lump that bleeds heavily, or cannot urinate or defecate. Schedule a routine visit within a week or two if the lump is growing slowly, is not painful, and your dog is acting normal. Do not wait months just because your dog seems fine. ## Frequently Asked Questions ### Can a cyst on my dog get bigger and then shrink on its own? Yes, this happens with fluid-filled cysts that drain and refill. The up-and-down pattern is common and often harmless, but some tumors also change size. Any lump that fluctuates should still be examined once so your veterinarian can identify it. ### Does a growing lump always mean cancer? No. Cysts, abscesses, seromas, and benign tumors can all grow. However, rapid growth is a warning sign that needs prompt evaluation because some skin cancers grow quickly and can spread. ### How fast is too fast for a lump to grow? There is no single cutoff, but growth you can see within days is concerning. A lump that doubles in size in a week or less should be seen right away. Slow growth over months is less alarming but still worth a visit. ### Should I try to drain my dog's cyst at home? No. Never squeeze or lance a lump yourself. You could cause an infection, make the lump worse, or spread cancer cells if the lump is a tumor. Let your veterinarian handle any draining or removal. ### Will my veterinarian remove every lump they find? No. Benign lumps that are not ulcerated and do not affect your dog's daily life may not need treatment, especially in older dogs. Your veterinarian will base the decision on the diagnosis, the lump's location, and your dog's overall health. ## Related Articles - [Why Is My Dog having lumps on skin? Lipoma vs Mast Cell vs Cyst Diagnosis](/knowledge/veterinary-medicine/symptom-guides/why-is-my-dog-having-lumps-on-skin-lipoma-vs-mast-cell-vs-cyst-diagnosis) - [What Does It Mean When Your Dog's Behavior Changes](/knowledge/veterinary-medicine/behavior/what-does-it-mean-when-your-dog-s-behavior-changes) - [Dog Follicular Cyst: Comprehensive Veterinary Reference Guide](/knowledge/veterinary-medicine/dermatology/dog-follicular-cyst) - [Why Is My Dog Gaining Weight?](/knowledge/veterinary-medicine/nutrition/why-is-my-dog-gaining-weight) - [Why Is My Dog Losing Weight?](/knowledge/veterinary-medicine/symptom-guides/why-is-my-dog-losing-weight) - [Dog Hair Loss with Scabs: What Does It Mean?](/knowledge/veterinary-medicine/skin-allergy-and-ear-care/dog-hair-loss-scabs) - [Bump on My Dog: What It Means and When to Worry](/knowledge/veterinary-medicine/skin-allergy-and-ear-care/bump-on-dog-meaning) - [Bump on a Young Dog That Keeps Growing: What It Means](/knowledge/veterinary-medicine/skin-allergy-and-ear-care/growing-bump-on-young-dog) - [Fast-Growing Lump on Your Dog's Neck: When to Worry](/knowledge/veterinary-medicine/symptom-guides/dog-neck-lump-growing-fast) ## References 1. [Paraprostatic Cysts in Dogs - Reproductive System](https://www.merckvetmanual.com/reproductive-system/prostatic-diseases-in-small-animals/paraprostatic-cysts-in-dogs). Merck Veterinary Manual. 2. [Diagnosis and treatment of Merkel cell carcinoma in a puppy: A comparative oncology approach](https://pubmed.ncbi.nlm.nih.gov/42266333/). The Canadian veterinary journal = La revue veterinaire canadienne, 2026. 3. [Tumors of the Skin in Dogs - Dog Owners](https://www.merckvetmanual.com/dog-owners/skin-disorders-of-dogs/tumors-of-the-skin-in-dogs). Merck Veterinary Manual. 4. [Ectopic Splenic Tissue Mimicking Malignant Neoplasia in a Dog](https://pubmed.ncbi.nlm.nih.gov/42433008/). Veterinary radiology & ultrasound : the official journal of the American College of Veterinary Radiology and the International Veterinary Radiology Association, 2026. 5. [Imaging Diagnosis-Peritoneal Splenosis in a Dog: Computed Tomographic Findings](https://pubmed.ncbi.nlm.nih.gov/41540971/). Veterinary radiology & ultrasound : the official journal of the American College of Veterinary Radiology and the International Veterinary Radiology Association, 2026. ## Further Reading - [Merck Veterinary Manual: Dog Owners](https://www.merckvetmanual.com/dog-owners) - [Cornell Richard P. Riney Canine Health Center: Canine Health Information](https://www.vet.cornell.edu/departments-centers-and-institutes/riney-canine-health-center/canine-health-information) - [American Veterinary Medical Association: Pet Care](https://www.avma.org/resources-tools/pet-owners/petcare)