Some skin lumps in dogs and cats grow from blood vessels. A hemangioma is a benign (non-cancerous) growth of this kind, while a hemangiosarcoma (also called angiosarcoma) is a malignant one [1][2][3]. Both are seen in dogs and in cats [3], and on the skin they often show up as red to dark, sometimes bleeding lumps [1].
You cannot tell these tumors apart by look or feel, and even a needle sample can confuse the two [1]. Only a tissue sample (biopsy) examined in the lab gives the answer [1]. If you find a new red, dark or blood-filled lump on your pet, have your vet check it.
For other kinds of lumps, see our posts on lumps and bumps on dogs and lumps and bumps on cats.
What these tumors are
Hemangiomas
Hemangiomas are benign tumors made of blood vessel tissue. In a Norwegian population study of 439 blood vessel tumors and similar growths in 420 dogs, more than half were in the skin, and nearly all of the skin growths were benign. Most of the vascular lesions in that study were found in dogs older than 6 years [2].
- Dogs: in the same study, many skin hemangiomas sat on the limbs, with a large share of one type found on the skin of the hind legs [2].
- Cats: a 2025 review of tumor biopsies from Lithuania found skin hemangiomas in both dogs and cats, though far fewer cats were affected [3].
Hemangiosarcomas (angiosarcomas)
Hemangiosarcoma is a cancer of the cells that line blood vessels. A 2023 consensus review by a veterinary oncology group explains that it can be non-visceral, growing in the skin, the tissue under the skin or muscle, or visceral, growing in internal organs such as the spleen, liver and heart [1]. This post covers the skin and under-the-skin forms.
- Dogs: among pets, dogs are the species most often affected by the skin form. It is usually found in middle-aged and older dogs, roughly 8 to 15 years of age, and males and females are affected about equally [1].
- Cats: skin hemangiosarcoma is less common in cats. Recent case reports link it to a white coat and older age [7][8].
The review splits the canine non-visceral form into four groups: skin tumors linked to sun damage, skin tumors without sun damage, tumors under the skin, and tumors in muscle. Tumors under the skin and in muscle tend to be more aggressive than those confined to the skin, because they grow into deeper tissue and spread more readily [1].
The sun link
- Dogs: the review says dogs with lightly pigmented skin and a thin or short coat are at higher risk of the sun-related skin form, and lists breeds such as Pit Bulls, Whippets, Greyhounds, Boxers, Beagles and Dalmatians. Pale skin with little hair gets more sun, and signs of sun damage are often found in the skin around these tumors [1].
- Skin tumors in dogs with dark skin and thick coats, and tumors under the skin or in muscle (reported more often in Golden and Labrador Retrievers and mixed breeds), do not seem to be linked to sun exposure [1].
- Cats: a 2022 study of five cats with ear tumors from a university tumor registry, one of them a hemangiosarcoma, found signs of long-term sun damage in the skin next to every tumor. The authors suggest that UV light may play a part in these tumors in cats [7]. A 2026 case report describes an ear hemangiosarcoma in an older white cat and notes that light, unpigmented cats with long sun exposure are prone to ear tumors [8].
Sun damage and sun-related skin cancer in white cats are covered in our post on lumps and bumps on cats (section “Squamous cell carcinoma and sun damage”), and sun-related nose problems in dogs in dry, crusty nose in dogs.
What you may notice
- Skin hemangiosarcoma in dogs: one or several small bumps or raised spots on the skin surface, red to almost black in color. Often the only sign is a little bleeding from the lump now and then [1].
- Where sun-related tumors appear in dogs: mostly on the belly, around the sheath of the penis and on the hind legs, in short-haired dogs with thin hair cover that have spent a lot of time in the sun. Up to about a third of these dogs develop more than one lump [1].
- Tumors under the skin or in muscle: larger lumps that may be firm or soft, fixed or movable, and may form an open sore. They can appear anywhere on the body, and they often are not red or purple, so they are easily mistaken for other lumps [1].
- Hemangiomas: also red to dark lumps, which is why the two are hard to tell apart without testing [1].
If a lump bleeds
Cover it gently with a clean cloth or dressing, press gently, and call your vet. Do not try to remove, squeeze or treat the lump at home.
How it is diagnosed
- Biopsy: the review says the diagnosis is confirmed by examining tissue from a biopsy under the microscope [1]. A biopsy can take part of the lump (incisional) or all of it (excisional).
- Needle samples: a needle sample (cytology) can help as a first check before surgery, but it is of limited use for these tumors, because the sample is often diluted with blood. On a needle sample, hemangiosarcoma can easily be mistaken for a benign hemangioma [1].
- A benign result is not the end of the story: in the Norwegian study, the growth came back in 17 dogs, including some whose first diagnosis had been benign [2]. Follow your vet’s plan for rechecks.
- Checking for spread: a skin tumor can also be a spread from a hemangiosarcoma inside the body, so vets usually check the chest and belly with imaging, especially for tumors under the skin, in muscle, or without sun damage [1]. Your vet will explain which tests fit your pet.
Treatment
Your vet or a veterinary oncologist will plan treatment for your pet’s tumor. The published studies describe these general approaches:
- Surgery first: the review names surgery as the usual first choice for dogs whose tumor has not spread [1]. For sun-related tumors confined to the skin that have not spread, removing the tumor completely is considered curative [1].
- Deeper tumors: tumors under the skin or in muscle tend to need wider surgery, and additional treatment such as chemotherapy is more often advised for them [1][5].
- Cats: in a study of 18 cats with skin hemangiosarcoma, cats that had surgery lived longer than cats that did not, and the authors felt that cats whose tumors were removed with wide surgery could do well over the long term [6]. A 2026 case report notes that the tumor often comes back after incomplete removal and recommends wide, clear margins [8].
- Sun protection: for dogs at risk of the sun-related form, the review advises limiting sun exposure, protecting the skin, and checking regularly for new lumps. It adds that in dogs with years of sun damage, cutting back on sun may no longer prevent new tumors, so these steps work best when started early in life [1]. Ask your vet which sun protection suits your dog.
Outlook
Outcomes vary between studies and between tumor types:
- In one study summarized by the review, dogs with skin hemangiosarcoma treated with surgery alone had a median survival of more than two years. Dogs with skin tumors that showed no sun damage had shorter survival and more spread [1].
- Sun-related skin tumors can keep coming back in the same sun-damaged area over the years, even after complete removal, but in that study this did not shorten survival [1].
- A 2011 multi-center study of 71 dogs with tumors under the skin or in muscle found a median survival of 172 days, with one in four dogs alive after a year. The authors concluded that the outlook for these tumors is generally poor and varies a lot, and that dogs with smaller tumors, no signs of illness and no spread, whose tumor can be well controlled locally, have the best chance of long survival [4].
- A 2026 study of 53 dogs describes tumors under the skin and in muscle as aggressive cancers that often come back and spread [5].
- In the 18-cat study, cats treated with surgery survived longer than cats that were not [6].
These are figures and statements about groups of animals in particular studies. They do not prove that a treatment caused an outcome, and they cannot predict how one pet will do. Your vet can explain what the biopsy and test results mean for your pet.
When to see your vet
See your vet if your dog or cat has:
- any new lump on or under the skin [1]
- a red, dark or blood-blister-like lump [1]
- a lump that bleeds, grows or develops an open sore [1]
- new red or dark spots on pale, thin-haired, sun-exposed skin [1][7]
FAQ
Is a hemangioma cancer?
No, a hemangioma is benign [2]. But it can look just like a hemangiosarcoma, and even a needle sample can confuse the two [1]. Only a biopsy can tell, so have every new lump checked.
Does sun exposure cause these tumors?
In dogs with pale skin and thin coats, long sun exposure is linked to one form of skin hemangiosarcoma [1]. In cats, studies of ear tumors in white cats point the same way [7][8]. Other forms are not linked to sun [1].
Can my vet tell from a needle sample?
Not always. Needle samples from these tumors are often diluted with blood, so the diagnosis is made by biopsy [1].
Does sun protection help?
For dogs at risk, the review advises limiting sun, protecting the skin and watching for new lumps, ideally from early in life [1].
What should I do if the lump bleeds?
Cover it gently, press gently and call your vet.
The bottom line
Hemangiomas are benign blood vessel tumors, while hemangiosarcomas (angiosarcomas) are malignant [1][2]. Both can look like red to dark lumps, so a biopsy is needed to tell them apart [1]. Sun exposure is linked to some of these tumors in pale, thin-coated dogs and in white cats [1][7]. Have any new lump checked by your vet; if a lump bleeds, cover it gently, press gently and call your vet.
This article is for general information and does not replace a visit to your veterinarian. If you find a lump on your pet, contact your vet.
Sources
- De Nardi AB, de Oliveira Massoco Salles Gomes C, Fonseca-Alves CE, et al. Diagnosis, Prognosis, and Treatment of Canine Hemangiosarcoma: A Review Based on a Consensus Organized by the Brazilian Association of Veterinary Oncology, ABROVET. Cancers (Basel). 2023;15(7):2025. Open access, CC BY 4.0. https://pmc.ncbi.nlm.nih.gov/articles/PMC10093745/
- Gamlem H, Nordstoga K, Arnesen K. Canine vascular neoplasia: a population-based clinicopathologic study of 439 tumours and tumour-like lesions in 420 dogs. APMIS Suppl. 2008;(125):41-54. https://pubmed.ncbi.nlm.nih.gov/19385280/
- Juodžiukynienė N. Retrospective Analysis of Prevalence of Tumors in Dogs and Cats in Lithuania. Vet Sci. 2025;12(11):1038. Open access, CC BY 4.0. https://pmc.ncbi.nlm.nih.gov/articles/PMC12656924/
- Shiu KB, Flory AB, Anderson CL, et al. Predictors of outcome in dogs with subcutaneous or intramuscular hemangiosarcoma. J Am Vet Med Assoc. 2011;238(4):472-479. https://pubmed.ncbi.nlm.nih.gov/21320017/
- Rebelo AP, Furtado A, Sellon R, et al. Outcomes and prognostic factors in dogs with subcutaneous and intramuscular hemangiosarcoma treated with hypofractionated radiation therapy and chemotherapy. Front Vet Sci. 2026;13:1925779. Open access, CC BY. https://pubmed.ncbi.nlm.nih.gov/42903052/
- McAbee KP, Ludwig LL, Bergman PJ, Newman SJ. Feline cutaneous hemangiosarcoma: a retrospective study of 18 cases (1998-2003). J Am Anim Hosp Assoc. 2005;41(2):110-116. https://pubmed.ncbi.nlm.nih.gov/15767654/
- Millanta F, Parisi F, Poli A, et al. Auricular Non-Epithelial Tumors with Solar Elastosis in Cats: A Possible UV-Induced Pathogenesis. Vet Sci. 2022;9(2):34. Open access, CC BY 4.0. https://pubmed.ncbi.nlm.nih.gov/35202288/
- Feio JV, de Carvalho EL. Total ear canal ablation for the treatment of auricular hemangiosarcoma in a cat: case report. Braz J Vet Med. 2026;48:e012626. Open access, CC BY. https://pubmed.ncbi.nlm.nih.gov/42807950/