SYMPTOM
Skin lumps or bumps
A new or changing lump, bump, or raised area on or under the skin that may vary in size, firmness, or appearance.
Lipoma (benign fatty tumour)
Lipomas are the most common skin masses in dogs, composed of mature fat cells that form soft, well-defined lumps beneath the skin. They are typically slow-growing, painless, and movable under the skin when manipulated. While generally harmless, they can grow quite large and may occasionally impede movement depending on their location. Lipomas are most common in middle-aged to older dogs and certain breeds may be predisposed.
Mast cell tumour
Mast cell tumours are the most common malignant skin tumours in dogs and can present as lumps of virtually any appearance — soft or firm, raised or flat, small or large, with or without hair covering. They are sometimes referred to as 'great pretenders' because they can mimic the appearance of benign masses. Some mast cell tumours show characteristic size fluctuation, swelling and shrinking over days due to the release of histamine and other chemicals from the tumour cells. The surrounding skin may also become red or irritated.
Sebaceous or epidermal cyst
Cysts arising from blocked oil glands or hair follicles can produce firm, round lumps within or just beneath the skin surface. They may be skin-coloured, slightly raised, and may have a small opening (pore) on their surface. Some cysts may become inflamed or infected, producing redness, pain, and discharge. While generally benign, they can be difficult to distinguish from other mass types based on appearance alone.
Histiocytoma
Histiocytomas are benign tumours that occur most commonly in young dogs, typically under three years of age. They often appear as rapidly growing, dome-shaped, hairless, and sometimes ulcerated pink or red nodules, most commonly on the head, ears, and limbs. Despite their sometimes alarming appearance and rapid growth, most histiocytomas regress spontaneously within two to three months as the immune system recognises and eliminates the abnormal cells.
Soft tissue sarcoma
Soft tissue sarcomas arise from connective tissue cells and typically present as firm, well-defined masses beneath the skin that may feel attached to deeper structures. They tend to grow slowly but can be locally invasive, extending microscopic tentacles beyond their palpable borders into surrounding tissues. While they have a lower tendency for distant metastasis compared with some other tumour types, their local invasiveness can make complete surgical removal challenging.
Abscess or inflammatory mass
Infections, foreign bodies, or inflammatory reactions can produce masses that mimic tumours. Abscesses may develop following bite wounds, foreign body penetration, or bacterial infection, producing warm, painful, fluctuant swellings that may develop and grow over days. Inflammatory reactions to injections, foreign materials, or insect stings can similarly produce lumps that may be confused with neoplastic growths. These masses may be distinguishable by their relatively rapid onset, associated pain, warmth, and the possible presence of discharge.
Why timing matters
Early observation
When a new skin lump is first noticed, its characteristics at presentation — size, location, texture, rate of appearance, and any associated skin changes — provide initial clues about its nature. Masses that have appeared rapidly (over days) carry different implications than those that seem to have been growing slowly over weeks or months. A mass that is noticed by chance during grooming may have been present for some time before detection, making it difficult to determine the true duration. Early assessment through palpation and, when appropriate, fine needle aspiration can help characterise the mass before making assumptions about its nature.
Later presentation
Masses that are growing, changing in character, or developing new features over time warrant closer attention than those that remain stable. Increasing size, changes in texture (from soft to firm, or vice versa), development of surface changes (ulceration, hair loss, colour change), or the appearance of redness in the surrounding skin may all represent meaningful changes. Some masses remain benign and stable indefinitely, while others undergo changes that reflect their underlying biology. A mass that has been present and unchanged for months may carry different implications than one that has doubled in size over the same period.
The trajectory of a skin mass depends entirely on its nature. Benign lipomas may grow slowly over months to years but maintain their soft, well-defined character. Mast cell tumours may fluctuate in size or grow steadily depending on their grade. Histiocytomas in young dogs typically appear rapidly, reach a certain size, then regress over weeks. Abscesses develop and resolve over days to weeks. Soft tissue sarcomas tend to show slow but relentless growth. Understanding the growth pattern — whether rapid, slow, stable, fluctuating, or regressing — provides valuable context about the likely nature of the mass, though definitive characterisation ultimately relies on cellular examination rather than clinical observation alone.
Conditions commonly associated
Mast Cell Tumours in Dogs
Mast cell tumours are the most common malignant skin tumours in dogs and can present as lumps of virtually any appearance. Their ability to fluctuate in size due to histamine-mediated degranulation is a distinctive feature, though many present as stable, unremarkable masses indistinguishable from benign growths on visual inspection.
Lymphoma in Dogs
Multicentric lymphoma in dogs commonly presents with enlarged lymph nodes that may be noticed as lumps beneath the skin, particularly around the jaw, shoulders, and hind legs.
Mammary Tumours in Dogs
Mammary tumours typically present as palpable lumps along the mammary chain, which may be single or multiple.
Lipomas
Lipomas present as soft, mobile subcutaneous masses that are typically discovered during routine handling or grooming.
Histiocytoma
Histiocytomas typically present as solitary, dome-shaped, button-like skin lumps in young dogs. These benign tumours appear rapidly and characteristically regress spontaneously over several weeks to months.
Sebaceous Cysts
Sebaceous cysts present as firm, round, skin-attached masses that are typically discovered during routine grooming or petting.
Soft Tissue Sarcomas
Soft tissue sarcomas typically present as firm masses beneath the skin that gradually enlarge, often initially mistaken for benign lumps.
Mast Cell Tumours
Mast cell tumours most commonly present as raised skin masses that may resemble other benign lumps.
Cutaneous Lymphoma
This cancer forms nodules or thickened plaques within the skin as abnormal lymphocytes accumulate in affected areas.
Fibrosarcoma in Dogs
A fibrosarcoma grows as a distinct lump beneath the skin that gradually enlarges over time.
Injection-Site Sarcoma in Cats
This tumour develops as a firm mass beneath the skin at a previous injection site, felt as a lump that continues to grow rather than shrink over the following weeks.
Mammary Adenocarcinoma in Cats
The tumour presents as a discrete, firm lump beneath the skin of the lower abdomen, often growing steadily once first noticed.
Mast Cell Tumours in Cats
The cutaneous form of this tumour presents as a raised, firm nodule in the skin, most often found on the head or neck.
Plasmacytoma
Plasmacytoma typically appears as a distinct, raised nodule on the skin, most often on an ear, lip or toe.
Atypical Mycobacterial Infection
Infection settling in the fat beneath the skin forms firm to soft nodules, often over the abdomen or flanks.
Calcinosis Circumscripta
Calcinosis circumscripta forms a firm, chalky-feeling nodule under the skin, most often over a pressure point such as an elbow or paw.
Calcinosis Cutis
Mineral deposits within the dermis create firm, whitish plaques or nodules that can be felt as distinct lumps under the skin.
Cryptococcosis
Cryptococcus can spread to the skin, forming firm nodules under the coat.
Cutaneous Histiocytosis
Clusters of immune cells gather within the skin, forming multiple firm nodules that often appear on the trunk, face or limbs.
Eosinophilic Granuloma Complex in Cats
The granuloma form of this complex raises firm, well-defined nodules, most often on the thighs or in the mouth.
Eosinophilic Plaque in Cats
The lesion itself presents as a raised, well-defined plaque that can be felt as a distinct lump under the coat.
Erythema Multiforme
Early lesions of erythema multiforme often appear as raised, well-defined patches before they develop their characteristic ring pattern.
Hyperlipidaemia
Small yellowish skin lumps, known as xanthomas, can form where excess circulating fat deposits beneath the skin.
Sterile Nodular Panniculitis
Inflammation of the fat layer beneath the skin forms firm nodules that owners feel scattered over the trunk or limbs.
When to explore further
A skin mass that is growing noticeably over weeks, particularly if the growth appears to be accelerating rather than remaining steady, may warrant evaluation to determine its cell type before it reaches a size that complicates management options.
Any mass that shows characteristic features of mast cell tumours — size fluctuation, surrounding redness or wheals, or a flare response when manipulated — may benefit from prompt fine needle aspiration to confirm or rule out this tumour type, as mast cell tumours carry specific management implications.
Multiple new masses appearing over a relatively short time period, or a new mass developing in a dog with a history of previous mast cell tumours or other skin cancers, may warrant systematic evaluation of all masses to characterise each one individually.
A mass that is firm, fixed to deeper structures, or that has poorly defined borders may have different implications than a soft, freely movable mass, though appearances can be misleading and sampling remains the most reliable way to characterise any skin mass.
Any skin mass in a breed known to be predisposed to specific tumour types — such as Boxers, Labrador Retrievers, or Golden Retrievers for mast cell tumours — may warrant earlier evaluation than might be considered in a lower-risk breed.
Developing a habit of regular, systematic skin checks — running your hands over the entire body including the legs, belly, and areas normally covered by the coat — helps establish awareness of what is normal for the individual dog and makes new masses easier to detect early. When a new lump is found, measuring it (using a ruler, or comparing it to everyday objects like a pea, marble, or golf ball) and noting its location, texture, and appearance creates a baseline record. Checking the mass again after one to two weeks allows assessment of whether it is stable, growing, or changing. Photographing the mass alongside a ruler provides a visual record that can be compared over time and shared with veterinary professionals.
Last reviewed: 24 April 2026 · Dr Alastair Greenway MRCVS