Introduction: When a lump isn’t what it seems

Lipomas and liposarcomas are both tumours that start in fat cells, but they are very different in nature. On the surface, these lumps may appear similar, making it difficult to distinguish them without proper testing.  The most important difference is that lipomas are benign (non-cancerous), harmless fatty lumps. At the same time, liposarcomas are malignant (cancerous) tumours which can spread to other parts of the body, potentially affecting vital organs. Due to their cancerous nature, liposarcomas require prompt treatment, which often involves surgery, radiation and/or chemotherapy. Unfortunately, their resemblance to harmless lipomas can sometimes lead to misdiagnoses. This can delay the start of appropriate treatment and can negatively affect prognosis. On top of the medical consequences, finding out about a cancer diagnosis later than expected can also be emotionally distressing for patients and their loved ones.

This article aims to help the reader understand the similarities and key differences between lipomas and liposarcomas. It also sheds light on how these similarities can lead to misdiagnosis, potentially delaying correct treatment. By raising awareness, we aim to empower the reader to ask the right questions and seek appropriate medical advice when faced with a suspicious lump.

Understanding tumours

What is a tumour?

A tumour is a general term used to describe a lump or an abnormal mass of tissue that forms when cells divide and grow in an uncontrolled manner. Tumours are grouped according to the cells from which they originate from. A mesenchymal tumour is a mass that grows from the body’s support tissues (also known as connective tissues) such as fat, bone, muscle, cartilage or blood vessels. Both lipomas and liposarcomas originate from fat cells and are therefore classified as mesenchymal tumours. The World Health Organisation divides mesenchymal tumours into 3 classes¹ as shown in Table 1.

Table 1: Classification of soft tissue fat tumours 

Benign tumours may grow large, but they do not invade other tissues or metastasise (spread to other parts of the body).  A lipoma is an example of a benign tumour. Intermediate tumours are locally aggressive but do not metastasise. Atypical lipomatous tumour is an example. Lastly, malignant (cancerous) tumours invade nearby tissues and have the potential to metastasise. A liposarcoma is an example of a malignant tumour.

What is a lipoma?

A lipoma is the most common form of a mesenchymal tumour and occurs in about 1% of the population.² Lipomas arise from fat cells and are generally soft to the touch, easily movable and round or oval in shape. They most often develop just under the skin, but can occasionally be found on internal organs such as the stomach or intestines.

Most lipomas are painless unless they occur near a joint, a nerve or when they become very large. They are slow-growing and do not metastasise. Lipomas are sometimes surgically removed for cosmetic reasons or to relieve discomfort. The likelihood of a lipoma returning after surgical removal is low, with recurrence occurring in approximately 5% of cases.³

What is a liposarcoma?

A liposarcoma is a rare malignant mesenchymal tumour of fatty tissue that is fast-growing and has the potential to metastasise. They most commonly develop in the thighs, arms, or the retroperitoneum (space behind the abdominal organs), although they can occur anywhere in the body. Liposarcomas usually feel firm to the touch and may seem attached to the surrounding tissues. Liposarcomas require specialised treatment, including aggressive surgery to ensure that all the cancerous cells are removed.  Radiotherapy and chemotherapy may also be required in some cases.

Lipoma versus liposarcoma

Lipomas and liposarcomas both originate from fatty tissue and can look very similar on the surface. They can both appear as soft, slow-growing lumps and are often painless in the early stages. Table 2 below summarises the subtle differences between a lipoma and a liposarcoma. 

Table 2: Comparison between lipoma and liposarcoma

How doctors reach a diagnosis

Physical examination

This is the first step, but unfortunately, a lipoma and a liposarcoma may appear similar and can be difficult to differentiate. Further tests may therefore be required to distinguish between them.

Imaging tests

When there is doubt, magnetic resonance imaging (MRI), computed tomography (CT), or an ultrasound is required to examine the mass more closely. On imaging scans, a lipoma typically appears as a mass composed entirely of fat. A liposarcoma may also look like fat but there are usually unusual features such as thick septa or nodules.

Biopsy and further laboratory testing

A biopsy is a medical procedure where a small sample of tissue is removed from the tumour and examined in the laboratory.  A biopsy is essential for diagnosing a liposarcoma.  The following tests are usually performed:^1–5

• Histopathology – this involves looking at the cells under a microscope
• Immunohistochemistry – this is a method that uses special antibodies (type of protein) to detect specific markers in a tissue sample. These markers are used to determine the type of cells present in the sample and to identify whether they are normal or cancerous.
• Molecular genetics – this test looks at the genes to find out exactly what kind of tumour it is

Risks of misdiagnosis

Despite the advances in modern medicine, mesenchymal tumours can be challenging to diagnose, and unfortunately, up to 30% are misdiagnosed. A liposarcoma may be mistaken for a harmless lipoma, especially in the early stages.

A lipoma can usually be left in place, while a liposarcoma may continue to grow large and can cause pain, swelling or other problems such as weakness in the arms or legs. Furthermore, although a lipoma can be relatively easily removed through surgery, this is usually not enough to treat a liposarcoma. Since liposarcomas invade nearby tissue, they usually require more aggressive surgery to ensure the tumour is removed completely. Additional treatments, such as radiotherapy or chemotherapy, are also commonly needed for liposarcomas since they are malignant and may spread to other parts of the body if not treated early.

An incorrect diagnosis may result in a delay in getting the proper treatment. As a result, the prognosis may be negatively affected, with potentially more complicated treatment and reduced chances of full recovery. Beyond the medical risks, a delayed cancer diagnosis can cause significant emotional stress for both patients and their families.

Warning signs that a lump might not be harmless

There are a few subtle signs that a suspected lipoma may, in fact, be more dangerous. If a lump has any of the following characteristics, then further tests should be done:

• Size larger than 5 cm (about the size of a golf ball)
• Growing quickly
• Feels firm, deep or attached to other tissues
• Causes pain or discomfort
• Reappears after surgical removal
• Located deep in the body, such as the thigh, abdomen, or behind the knee

 What patients can do: Advocate for your health

• Always seek medical attention for a new lump
• Don’t ignore new, growing, or uncomfortable lumps
• Seek a second opinion or specialist referral if you are concerned about a lump 

Summary: Be informed, not alarmed

 While most fatty lumps turn out to be harmless lipomas, it’s important to be aware that some may be more serious, such as liposarcomas. Because these two types of tumours can look and feel alike, proper medical evaluation, including imaging and biopsy, is essential for an accurate diagnosis.

If you notice a new lump, or if an existing lump is growing, painful, or feels unusually firm or deep, don’t ignore it. Early diagnosis can significantly impact available treatment options and improve outcomes. By understanding the difference between a lipoma and a liposarcoma and knowing when to seek further investigation, you can take an active role in protecting your health and well-being. Being proactive can make a big difference.