Diagnosing Cutaneous & Subcutaneous Masses in Dogs: Best Practices for Accurate Evaluation and Sampling
Cutaneous and subcutaneous masses—often described as lumps and bumps by pet owners—are among the most common findings in dogs. Physical exams, as part of a screening study of 100 senior and geriatric dogs, found 151 different masses in 56 dogs—nearly 3 per dog. Masses included follicular cysts and benign tumors, and more concerning tumors like mast cell tumors and sarcomas.1 Determining which masses require diagnostics and which can be safely monitored is a core clinical skill.
This article will guide veterinarians through best practices for evaluating, sampling, and submitting samples from cutaneous and subcutaneous masses for accurate diagnosis and improved patient outcomes.
Common Cutaneous and Subcutaneous Masses in Dogs
A simple way to remember the major categories of these lesions in dogs is the acronym CHANGE:
- Cysts — including follicular cysts, which may rupture or become inflamed
- Hyperplasia or Hypersensitivity reactions — including insect bites and hives/urticaria, which may look like cutaneous masses
- Abscesses — often associated with bite wounds, foreign bodies, or bacterial infections
- Neoplasia — both benign (lipomas, sebaceous adenomas, histiocytomas) and potentially malignant (mast cell tumors, soft tissue sarcomas, cutaneous lymphoma)
- Granulomas — may be sterile, infectious, or secondary to foreign bodies
- Everything else — including injection site reactions and viral papillomas
Benign tumors like basal cell tumors, histiocytomas, plasma cell tumors, sebaceous gland tumors, and many melanomas of the haired skin tend to be slow-growing and have limited metastatic potential. Surgery alone is often curative. Lipomas are one of the most common benign subcutaneous tumors in dogs and typically don’t require treatment unless they are impacting mobility. In contrast, mast cell tumors and soft tissue sarcomas may behave more aggressively and require extensive surgery or other treatments. Unfortunately, appearance alone rarely distinguishes benign from malignant, so sampling is essential.
When to Collect a Sample
A fine needle aspirate (FNA) is a reasonable first step for any new mass. Early sampling can provide owners with peace of mind if a benign condition is diagnosed, or avoid treatment delays if a malignant tumor is diagnosed.
Features of a mass that may raise your level of concern include:
- Rapid growth or changes
- Non-healing wounds or ulcers
- Pruritus or erythema
- Firm or fixed swellings
- Multiple masses
- Enlargement of nearby lymph nodes
Fine Needle Aspiration Technique
Two FNA techniques are commonly used:
Needle-on (“mosquito”) technique: A needle attached to a syringe is inserted into the mass, and gentle suction is applied.
This technique is best for especially firm masses, which may require suction to obtain a sufficient sample.
Needle-off (“woodpecker”) technique: A needle alone is inserted into the mass and redirected several times without suction.
This technique is best for smaller masses or when you're particularly concerned about blood contamination.
Avoid sampling the necrotic center of a mass, as this may yield a nondiagnostic sample. After collecting your sample, attach a syringe containing a small amount of air, gently expel the sample onto a slide, and then spread it by sliding or touching a second slide onto your sample. Remember: Too much pressure ruptures cells; too little results in a sample that's too thick to be useful.
While needle size does not impact the ability to make a diagnosis, smaller 25-gauge needles tend to result in more cellular debris and cellular trauma, and larger 22-gauge needles result in more blood contamination.2 When using a 25-gauge, I often need more pokes (and more pain) to obtain a diagnostic sample, so I prefer a 22-gauge “not-so-fine-needle.”
In‑clinic tools such as digital cytology and remote slide review enhance cytology's utility and may rapidly help guide the next steps for a patient. Greasy non-drying lipid and adipocytes may suggest a lipoma, while large round cells full of basophilic granules suggest a mast cell tumor that warrants further testing (and a dose of diphenhydramine, if warranted).
Indications for a Biopsy
Biopsy is indicated when cytology is nondiagnostic or when a grade is needed. Soft tissue sarcomas exfoliate poorly with FNA, and a biopsy may be needed to obtain a diagnosis.
There are two broad types of biopsies:
Excisional biopsy: This approach involves removing the entire tumor along with a surrounding area of normal tissue. This approach may be indicated for small tumors that can be readily removed. Recommendations for the amount of normal tissue to remove surrounding a mass vary depending on the tumor type, but typically extend up to 2 to 3 cm from the visible edge of the tumor, and 1 fascial plane deep.
Incisional biopsy: This approach involves removing just a small piece of a tumor. This approach may be indicated for larger tumors or tumors that can’t be easily removed. I tend to avoid sampling the center of most cutaneous and subcutaneous masses since this area may be necrotic and non-diagnostic.
Remember: Biopsy tracts need to be removed at the time of definitive surgery to reduce the risk of tumor recurrence, so take care not to extend biopsy tracts into normal tissues.
Fine Needle Aspirate vs. Biopsy
Histopathology remains the gold standard for diagnosing cutaneous and subcutaneous masses. Agreement between cytology and histopathology for cutaneous and subcutaneous masses in dogs and cats was 91% in one study, but cytology samples were more likely to be nondiagnostic, with 17% insufficient for diagnosis.3 Importantly, slides with cytology samples should always be shipped separately from formalin jars, as exposure to formalin fumes will interfere with routine cytological stains.
Beyond In-House Cytology and Histopathology
Future technologies promise in-clinic artificial intelligence-assisted review of samples without even needing to make a slide. These technologies are already available for ear cytology and hematology, and ongoing work is applying the approach to cytology. By understanding when and how to employ tried-and-true techniques such as FNA and biopsy, and leveraging new and emerging in-clinic decision-making tools, veterinarians will soon have even more tools to help clients make good, rapid decisions. Together, these tools can aid in early and accurate evaluation of cutaneous and subcutaneous masses and in directing interventions that can lead to improved patient outcomes.
References:
1. Willems et al. Results of screening of apparently healthy senior and geriatric dogs. J Vet Intern Med. 2017 Jan;31(1):81-92. doi: 10.1111/jvim.14587. Epub 2016 Oct 17.
2. Arai S et al. Fine-needle aspiration of cutaneous, subcutaneous, and intracavitary masses in dogs and cats using 22- vs 25-gauge needles. Vet Clin Pathol. 2019 Jun;48(2):287-292. doi: 10.1111/vcp.12751. Epub 2019 Jun 18.
3. Ghisleni G et al. Correlation between fine-needle aspiration cytology and histopathology in the evaluation of cutaneous and subcutaneous masses from dogs and cats. Vet Clin Pathol. 2006 Mar;35(1):24-30. doi: 10.1111/j.1939-165x.2006.tb00084.x.
