TL;DR: A hard, fast-growing mass on your dog's leg should be evaluated by a veterinarian within 24 to 48 hours, but requires immediate attention if your dog is in severe pain or unable to walk.
What could a hard, fast-growing mass on my dog's leg be?
Finding a firm lump on your dog's leg that seems to have appeared out of nowhere can be very concerning. A hard, rapidly enlarging mass may represent several different conditions, which require a physical examination to identify:
- An inflammatory reaction
- A deep-seated abscess
- A bone tumor (osteosarcoma)
- A soft tissue sarcoma
Is a fast-growing mass on my dog's leg a veterinary emergency?
The urgency level for this symptom is categorized as Medium. While it is rarely a life-threatening emergency in the immediate hours, you should take action based on the following:
- Schedule a veterinary appointment within 24 to 48 hours if your dog is still eating, drinking, and walking normally.
- Seek urgent care immediately if your dog is non-weight-bearing on the leg, vocalizing in pain, or if the skin over the mass is broken and bleeding.
Why should I take a photo of the mass to help with triage?
Providing your veterinarian or a triage professional with a clear, high-resolution photo of the mass is incredibly helpful. Visual evidence helps the medical team in the following ways:
- Identifies the exact location of the growth.
- Assesses the color and condition of the skin.
- Provides scale by comparing the mass to a household object.
- Helps the clinic determine the priority of your appointment.
- Allows the veterinary team to track the rate of growth from the time you first noticed it.
Clinical Context (Merck Veterinary Manual)
A hard, fast-growing mass on a dog's leg may indicate various underlying conditions, including osteosarcoma, chondrosarcoma, fibrosarcoma, or hemangiosarcoma. Clinical signs of osteosarcoma include lameness, bone swelling, and potential pathologic fracture. Radiography reveals osteolysis, proliferation, and soft-tissue swelling; thoracic radiographs should be performed to check for metastatic masses. Bone biopsy is imperative to confirm the diagnosis. Cutaneous smooth muscle tumors (leiomyomas or leiomyosarcomas) can also present as firm cutaneous masses, with leiomyosarcomas tending to be larger and extend into the subcutaneous fat. Undifferentiated and anaplastic sarcomas should also be considered, though they are difficult to characterize microscopically.
Chapter: Oncology, Surgery
Source: The Merck Veterinary Manual, 11th Edition (Page 1205)