TL;DR: Most soft, painless lumps on a cat's side are benign lipomas or cysts, but they require a veterinary exam to rule out serious conditions. Schedule a routine visit within a week unless the lump grows rapidly or your cat appears ill, which requires immediate attention.
What are the common causes of a soft, painless lump on my cat's side?
Finding a large, soft mass on your cat's side can be worrying, but many of these growths are benign. Common causes include lipomas, which are non-cancerous fatty tumors that feel squishy and move easily under the skin. Other possibilities include sebaceous cysts or fluid-filled pockets called seromas. While these are often harmless, a veterinarian must perform a physical exam to rule out more serious issues like a localized infection or a soft tissue sarcoma.
Is a soft lump on my cat's flank considered an emergency?
- Schedule a routine appointment within the next week if your cat is behaving normally, including eating, drinking, and playing.
- Increase priority to a same-day visit if the lump is rapidly growing or feels hot to the touch.
- Seek immediate care if the lump is accompanied by lethargy or a loss of appetite.
How can I take helpful photos of the lump for my veterinarian?
- Use a well-lit area to take a clear, high-quality photo of the mass.
- Place a common object, such as a coin or a ruler, next to the lump to provide a sense of scale.
- Send the photo to your vet to help them determine the urgency of the visit and establish a baseline to monitor for changes in size or shape.
Clinical Context (Merck Veterinary Manual)
A large, soft lump on the flank of a cat could be caused by several conditions. Perirenal pseudocysts, accumulations of fluid external to the renal parenchyma, may manifest as a large, firm, non-painful abdominal mass located in the area of the kidneys, although renal function tests and urinalysis are typically normal. Lipomas, especially in older, neutered male Siamese cats, can appear as soft, occasionally pedunculated, discrete nodular masses, most commonly on the ventral abdomen, and are freely movable; more than 5% may be multiple. Renal lymphosarcoma, often associated with feline leukemia virus, can result in large, irregular kidneys and may be noticed by owners as "lumps" in the abdomen or abdominal enlargement. Diagnosis may require ultrasonography or radiography, and potentially an excretory urogram or renal arteriogram.
Chapter: Urology, Diagnostic Imaging
Source: The Merck Veterinary Manual, 11th Edition (Page 1497)