CONDITION
Soft Tissue Sarcomas
Soft tissue sarcomas are a family of tumours that arise from the connective tissues beneath the skin — the structures that support, cushion and bind the body together. They can develop from fat, muscle, the sheaths around nerves, blood vessel walls or the fibrous tissue that separates organs. Most are noticed as a lump under the skin, often on a limb or the body wall, that may have been present for weeks or months before growing noticeably. These tumours tend to grow locally, expanding into surrounding tissue rather than spreading widely to other organs in the early stages. The lump may feel firm or soft, fixed or mobile, and size alone does not reliably indicate behaviour. Many owners arrive here wondering whether a lump that has appeared or changed is something to monitor, investigate or address, and what the timeframe for that decision looks like. This page explores what soft tissue sarcomas look like when they first appear, the signals that prompt investigation, how they are identified and assessed, and the range of approaches used to manage them. It also considers factors that influence outcome, including tumour type, location and grade, and what follow-up typically involves.
Why this matters now
Soft tissue sarcomas most commonly develop in middle-aged to older animals, though they can occur at any age. In cats, there may be associations with previous injections at certain sites (injection-site sarcomas), which has influenced vaccination and injection practices. The tumours typically grow slowly at first, which may delay recognition.
These tumours often begin as small, firm lumps beneath the skin that gradually enlarge over weeks to months. The growth rate can vary considerably between tumour types and individual cases. Soft tissue sarcomas characteristically send microscopic extensions into surrounding normal tissue, making them difficult to remove completely. Without adequate treatment, local recurrence is common. Spread to other organs occurs less frequently than with some other malignancies but becomes more likely with higher-grade tumours.
Signals & patterns
Early signals
Firm, subcutaneous lump
A mass felt beneath the skin that may be attached to underlying structures or relatively mobile.
Gradual increase in size
Slow but progressive growth over weeks to months, though rate varies considerably.
Lump appears in limb, trunk, or head/neck
Common locations include legs, flank, chest wall, and head regions.
Mass may feel smooth or irregular
Surface characteristics vary depending on tumour type and location.
Later signals
More rapid growth
Acceleration in growth rate may occur, with the mass becoming more obvious.
Ulceration or surface changes
Larger tumours may outgrow their blood supply, leading to skin breakdown.
Reduced mobility or lameness
Masses affecting limbs may interfere with movement as they enlarge.
Signs of systemic illness
Advanced or metastatic disease may cause weight loss, reduced appetite, or lethargy.
Click to read about the biological mechanisms
How this is usually investigated
Investigation aims to characterise the tumour type, assess its extent, and determine whether spread has occurred. This information guides treatment planning.
Fine needle aspirate
Tissue biopsy
Advanced imaging (CT or MRI)
Chest radiographs or CT
Regional lymph node assessment
Options & trade-offs
Management typically involves surgery as the primary treatment, potentially combined with radiation therapy. The approach depends on tumour location, size, grade, and individual circumstances.
Wide surgical excision
Removal of the tumour with substantial margins of normal tissue in all directions.
Trade-offs: Aims to remove microscopic extensions. May require significant tissue removal. Location affects what margins are achievable. Amputation may be considered for limb tumours.
Radiation therapy
Use of targeted radiation to kill tumour cells, often used before or after surgery.
Trade-offs: Can improve local control, particularly when surgical margins are incomplete. Requires multiple treatment sessions. May cause localised side effects.
Chemotherapy
Systemic treatment that may be considered for high-grade tumours or metastatic disease.
Trade-offs: Role in soft tissue sarcomas is less established than for some other cancers. May be recommended in specific situations.
Palliative management
Focus on maintaining comfort and quality of life when curative treatment is not pursued.
Trade-offs: May be appropriate for extensive tumours, elderly animals, or when owners choose this approach. Can include pain management and supportive care.
Monitoring approach
Observation with regular reassessment for slow-growing, low-grade tumours in older animals.
Trade-offs: May be reasonable when the tumour is small and slow-growing, particularly if surgery would be challenging. Requires commitment to regular monitoring.
Common misconceptions
"A moveable lump cannot be a sarcoma"
Whilst some sarcomas feel fixed to underlying structures, others can be relatively mobile on examination, particularly when small. The feel of a mass does not reliably predict its nature.
"If a lump has been present unchanged for a long time, it must be benign"
Soft tissue sarcomas can grow slowly, particularly initially. A long duration does not exclude malignancy, and any growing mass warrants investigation.
"Simple removal is sufficient treatment"
The invasive nature of soft tissue sarcomas means that surgical removal without adequate margins often leaves microscopic tumour cells behind, leading to regrowth.
Noting when the lump was first observed and whether it has changed in size provides useful timeline information. Photographing the mass and measuring it periodically can help track changes objectively. Considering the mass's exact location and whether it affects the animal's movement or comfort helps inform discussions about investigation and management options.
Last reviewed: 24 April 2026 · Dr Alastair Greenway MRCVS