Atypical Lipomatous Tumour (ALT): Understanding Your Pathology Report

by Bibianna Purgina, MD FRCPC
April 12, 2026

An atypical lipomatous tumor (ALT) is a slow-growing, low-grade type of sarcoma — a cancer that arises from fat cells. ALT is closely related to well-differentiated liposarcoma; in fact, both names refer to the same underlying tumor. The term ALT is used when the tumor arises in a location where complete surgical removal is achievable, such as the arms or legs. The term well-differentiated liposarcoma is used when the tumor arises in a deeper or less accessible location — most often the retroperitoneum, the space at the back of the abdominal cavity — where complete removal is more difficult. ALT most commonly affects adults in their fifth to seventh decades of life and is one of the most common subtypes of soft tissue sarcoma overall. Other liposarcoma subtypes include dedifferentiated liposarcoma, myxoid liposarcoma, and pleomorphic liposarcoma.

This article will help you understand the findings in your pathology report — what each term means and why it matters for your care.

What causes atypical lipomatous tumor?

ALT is caused by a specific genetic change in fat cells: the amplification of the MDM2 gene and, in most cases, the CDK4 gene as well. Amplification means that tumor cells contain far more copies of these genes than normal cells do. MDM2 normally helps regulate a protein called p53, which acts as a brake on cell growth. When MDM2 is amplified, this braking system is disrupted, and fat cells begin to grow and divide uncontrolled. This genetic change is not inherited — it occurs spontaneously in the tumor cells and is not passed down through families. The exact reason why this amplification occurs is not known. ALT is not associated with known environmental or lifestyle risk factors.

What are the symptoms?

ALT most often presents as a slow-growing, painless lump or mass. Because the tumor grows slowly, it may be present for months or years before it is noticed or investigated. Tumors in the arms or legs are typically felt as a soft, fatty lump beneath the skin. Tumors in the retroperitoneum — the deep abdominal space behind the organs — can grow very large before causing any symptoms, and may only come to attention when they cause abdominal fullness, discomfort, or are found incidentally on imaging performed for another reason. Rarely, a large retroperitoneal tumor can compress nearby structures such as the kidney, ureter, or large blood vessels, causing related symptoms.

How is the diagnosis made?

The diagnosis of ALT is made by examining a tissue sample under the microscope. The sample is usually obtained through a biopsy — typically a core needle biopsy, in which a needle is passed into the tumor under imaging guidance to collect small cylinders of tissue. For smaller, superficial tumors, the diagnosis is sometimes made after the entire tumor is removed surgically. The tissue is sent to a pathologist, who examines it under the microscope.

Under the microscope, ALT is composed of fat cells (adipocytes) that still resemble normal fat but exhibit important abnormalities. The fat cells vary in size more than normal, and scattered among them are cells with unusually large, dark nuclei called lipoblasts — immature fat cells with an abnormal appearance that are a hallmark of liposarcomatous tumors. The tumor may also contain thickened fibrous bands containing atypical-looking cells. Because ALT can closely resemble a benign fatty tumor called a lipoma, pathologists routinely use a laboratory test called fluorescence in situ hybridization (FISH) to count the number of MDM2 gene copies in the tumor cells. Finding more than two copies — called MDM2 amplification — confirms the diagnosis of ALT and distinguishes it from a lipoma, which has a normal MDM2 copy number. Once the diagnosis is established, imaging — typically CT or MRI — is used to determine the full extent of the tumor.

Histologic grade

ALT is always a grade 1 (low-grade) tumor by definition. Pathologists use the FNCLCC grading system (developed by the French Federation of Cancer Centers Sarcoma Group) to grade most soft tissue sarcomas. This system scores three microscopic features — tumor differentiation, mitotic count, and necrosis — and adds the scores together to produce a final grade. Because ALT cells still closely resemble normal fat cells, they always receive the lowest differentiation score (1 point), and the overall FNCLCC score for ALT is always 2 or 3 points, placing it firmly in grade 1.

• Tumor differentiation — Describes how closely the tumor cells resemble normal fat cells. ALT cells look very similar to normal fat, so they receive 1 point. This score is fixed for ALT.
• Mitotic count — Counts the number of actively dividing cells (called mitotic figures) seen in ten microscopic fields. Tumors with 0–9 mitotic figures per 10 fields receive 1 point; those with 10–19 mitotic figures per 10 fields receive 2 points; those with 20 or more mitotic figures per 10 fields receive 3 points. ALT typically has very few mitotic figures.
• Necrosis — Necrosis is a form of cell death that appears as areas of dead tissue within the tumor. No necrosis scores 0 points; necrosis in less than half the tumor scores 1 point; necrosis in half or more of the tumor scores 2 points. Necrosis is not typically seen in ALT.

If areas of the tumor begin to look very different from normal fat and show features of a higher-grade sarcoma, this is called dedifferentiation (see below). Finding dedifferentiation changes the diagnosis entirely.

Tumor size

The tumor is measured in three dimensions, but only the largest single measurement is typically recorded in your pathology report as the tumor size. Tumor size is used to determine the pathologic tumor stage (pT). Tumors 5 cm or smaller are associated with a better prognosis than larger tumors. The final tumor size is measured from the surgically removed specimen rather than from a biopsy, which captures only a small portion of the tumor. ALTs in the retroperitoneum often grow to very large sizes — sometimes 20 cm or more — before they are discovered, because this location provides little external resistance to growth.

Tumor extension

ALT typically starts within a fat-containing compartment of the body but can grow into adjacent structures as it enlarges. This spread beyond the original site is called tumor extension. The pathologist carefully examines all tissue submitted with the resection specimen to determine whether tumor cells have grown into surrounding muscles, organs, nerves, or blood vessels. In retroperitoneal tumors, the kidney, adrenal gland, colon, and major vessels may become involved. Extension into surrounding structures increases the pathologic tumor stage (pT) and may affect whether complete surgical removal is possible.

Dedifferentiation in atypical lipomatous tumor

Over time, a small proportion of ALTs undergo a process called dedifferentiation. This means that a portion of the tumor’s cells stop resembling fat and transform into a completely different type of high-grade sarcoma cell. When this happens, the diagnosis changes from ALT to dedifferentiated liposarcoma. This distinction is critically important: dedifferentiated liposarcoma is a much more aggressive cancer with a significantly higher risk of spreading to distant organs and a substantially worse prognosis than ALT alone. Dedifferentiation may be present in the original resection specimen, or it may develop in a tumor that recurs after surgery. Your pathology report will specify whether any dedifferentiated component was identified and, if so, how much of the tumor it represents.

Surgical margins

In pathology, a margin is the edge of tissue removed during surgery. Margin status indicates whether the entire tumor was removed or whether tumor cells remain at the cut edge of the tissue.

• Negative margin — No tumor cells are present at the cut edge. This suggests the tumor was completely removed. The distance from the nearest tumor cells to the margin may also be recorded, as a wider clear margin is associated with a lower risk of local recurrence.
• Close margin — Tumor cells are very close to the cut edge but do not reach it. This may still be associated with a higher risk of local recurrence and may prompt further treatment planning.
• Positive margin — Tumor cells are present at the cut edge, meaning some cancer may remain in the body. Additional surgery or radiation therapy may be recommended.

Achieving clear margins is one of the most important factors in reducing the risk of local recurrence for ALT. In the retroperitoneum, where tumors are frequently very large and closely surrounded by vital structures, achieving wide clear margins is often not possible, and this is a major reason why retroperitoneal ALT/well-differentiated liposarcoma has a higher recurrence rate than tumors in the extremities.

Lymph nodes

Spread of ALT to lymph nodes is extremely uncommon. ALT is a locally aggressive tumor that grows and recurs at the primary site rather than spreading through lymphatic vessels. For this reason, lymph nodes are not routinely removed during surgery for ALT unless they are visibly enlarged or there is another specific clinical reason to examine them. If lymph nodes were removed and examined, the pathologist will report the number of nodes examined and whether any tumor cells were found. Finding tumor cells in a lymph node — called metastasis — would be an unusual finding in pure ALT and might raise concern for a dedifferentiated component.

Biomarker and molecular testing

For most patients with ALT, there are currently no established biomarker tests to guide the selection of a targeted drug. The molecular testing most commonly performed on this tumor — MDM2 and CDK4 FISH — is primarily used to confirm the diagnosis rather than to guide treatment decisions. That said, research is active in this area:

MDM2 and CDK4 amplification

As described above, amplification of the MDM2 and CDK4 genes is the hallmark genetic finding in ALT and well-differentiated liposarcoma. Both genes are confirmed by FISH testing on the tumor tissue. While these findings currently serve a diagnostic rather than therapeutic purpose for localized ALT, they are the subject of ongoing research into targeted therapies. MDM2 inhibitors — which aim to restore the normal p53 braking system that MDM2 disrupts — are being investigated in clinical trials for well-differentiated and dedifferentiated liposarcoma. CDK4 inhibitors are also under study. If your tumor recurs or progresses, your oncologist may discuss whether trial participation is appropriate.

Next-generation sequencing (NGS)

In patients with recurrent, locally advanced, or metastatic disease, comprehensive molecular profiling using next-generation sequencing (NGS) may be performed to look for additional genetic changes that could identify eligibility for a clinical trial or targeted therapy. This is more relevant if dedifferentiation has occurred, since dedifferentiated liposarcoma can harbor a wider range of additional mutations. Your oncologist will advise whether molecular profiling is appropriate in your situation.

For more information about biomarkers and molecular testing in cancer, visit the Biomarkers and Molecular Testing section of this website.

Pathologic stage (pTNM)

The pathologic stage for ALT is based on the TNM staging system developed by the American Joint Committee on Cancer (AJCC), 8th edition. This system uses information about the primary tumor (T), lymph nodes (N), and distant metastatic disease (M) to describe the full extent of cancer. The M stage — whether the cancer has spread to distant organs — is determined by imaging rather than by the pathology report. In general, a higher stage reflects more advanced disease. Because ALT can arise in many body sites, the tumor (pT) staging criteria vary based on where the tumor started.

Tumor stage (pT)

Head and neck:

• pT1 — Tumor is 2 cm or smaller.
• pT2 — Tumor is more than 2 cm but 4 cm or smaller.
• pT3 — Tumor is more than 4 cm.
• pT4 — Tumor has grown into surrounding structures such as the bones of the face or skull, the eye, major blood vessels of the neck, or the brain.

Trunk and extremities (chest wall, back, abdomen, arms, and legs):

• pT1 — Tumor is 5 cm or smaller.
• pT2 — Tumor is more than 5 cm but 10 cm or smaller.
• pT3 — Tumor is more than 10 cm but 15 cm or smaller.
• pT4 — Tumor is more than 15 cm.

Thoracic visceral organs (organs inside the chest):

• pT1 — Tumor is confined to one organ.
• pT2 — Tumor has grown into the connective tissue immediately surrounding the organ where it started.
• pT3 — Tumor has grown into at least one adjacent organ.
• pT4 — Multiple separate tumors are present.

Retroperitoneum (the space at the back of the abdominal cavity):

• pT1 — Tumor is 5 cm or smaller.
• pT2 — Tumor is more than 5 cm but 10 cm or smaller.
• pT3 — Tumor is more than 10 cm but 15 cm or smaller.
• pT4 — Tumor is more than 15 cm.

Orbit (the bony socket surrounding the eye):

• pT1 — Tumor is 2 cm or smaller.
• pT2 — Tumor is more than 2 cm but has not grown into the bones surrounding the eye.
• pT3 — Tumor has grown into the bones surrounding the eye or other bones of the skull.
• pT4 — Tumor has grown into the eye itself or surrounding structures including the eyelids, sinuses, or brain.

• pT0 — No viable tumor found in the resection specimen. This may occur after a complete response to neoadjuvant therapy.
• pTX — The tumor cannot be reliably assessed, for example because the specimen was received as multiple small fragments.

Nodal stage (pN)

• pN0 — No cancer found in any lymph nodes examined.
• pN1 — Cancer found in one or more regional lymph nodes.
• pNX — Lymph nodes were not assessed.

What is the prognosis?

ALT has a generally favorable prognosis compared to most other sarcomas. Because it is low-grade by definition and virtually never spreads to distant organs in its pure (non-dedifferentiated) form, the primary concern with ALT is local recurrence rather than metastatic disease.

The prognosis differs substantially by location:

• Extremity ALT — When the tumor arises in an arm or leg and can be completely removed with negative margins, long-term local control is excellent. Five-year survival rates for completely resected extremity ALT approach 90% or higher. The risk of local recurrence is low when margins are clear.
• Retroperitoneal ALT (well-differentiated liposarcoma) — Prognosis is less favorable because these tumors are typically very large at diagnosis, negative margins are difficult to achieve, and local recurrence is common. Despite this, because the tumor rarely spreads to distant organs in its pure form, many patients survive for many years even with recurrent disease requiring repeat surgery. Five-year overall survival rates for retroperitoneal well-differentiated liposarcoma are approximately 70–80%, though local recurrence remains a persistent challenge.

The following features are associated with a worse outcome:

• Dedifferentiation — The single most important adverse prognostic event in ALT. Once dedifferentiation occurs, the tumor behaves as a high-grade sarcoma with meaningful risk of distant spread and a substantially worse prognosis.
• Positive surgical margins — Tumor cells at the cut edge of the resection are the strongest predictor of local recurrence.
• Retroperitoneal location — Tumors in this location are more likely to recur locally and less likely to be completely resected than tumors in the extremities.
• Large tumor size — Tumors greater than 5 cm are associated with higher recurrence rates, particularly in retroperitoneal disease.

What happens after the diagnosis?

After the diagnosis is confirmed, care is ideally managed by a multidisciplinary team including a surgical oncologist, medical oncologist with experience in sarcoma, and radiation oncologist. Referral to a sarcoma center with expertise in retroperitoneal and soft tissue tumors is strongly recommended, particularly for large or retroperitoneal tumors.

Surgery is the primary treatment for ALT. The goal is complete removal of the tumor with negative margins. For extremity tumors, limb-sparing surgery is almost always possible. For retroperitoneal tumors, surgery is more complex and may involve removal of adjacent organs (such as a kidney) to achieve the widest possible margins. This approach is increasingly used at specialized sarcoma centers and is associated with lower recurrence rates.

Radiation therapy is used selectively. For extremity ALT with close or positive margins, postoperative radiation can reduce the risk of local recurrence. For retroperitoneal tumors, the role of radiation therapy is more limited due to proximity to sensitive abdominal structures, though it may be considered in selected cases. Some centers use preoperative radiation for retroperitoneal tumors to improve local control.

Chemotherapy is not routinely used for pure ALT because these tumors grow slowly and are generally less responsive to chemotherapy. If a dedifferentiated component is identified — particularly one that is large or represents a significant portion of the tumor — chemotherapy may be considered, as dedifferentiated liposarcoma is treated more similarly to other high-grade sarcomas. The most commonly used agents in this setting include doxorubicin and ifosfamide.

Clinical trial enrollment should be discussed for any patient with recurrent or unresectable disease and for those whose tumor has dedifferentiated. MDM2 inhibitors, CDK4 inhibitors, and other targeted agents are being actively investigated. Your oncologist can advise on currently available trials.

Surveillance after treatment includes regular physical examination and cross-sectional imaging (CT or MRI) of the primary tumor site and the chest (to monitor for lung metastases). Because ALT rarely spreads to distant organs in its pure form, surveillance is primarily focused on early detection of local recurrence, which can often be treated with repeat surgery. The frequency and duration of surveillance will depend on the tumor location, margin status, and the presence of a dedifferentiated component.

Questions to ask your doctor

• Is my tumor called an atypical lipomatous tumor or well-differentiated liposarcoma, and what is the practical difference for my treatment?
• Was MDM2 amplification testing (FISH) performed to confirm the diagnosis?
• Were the surgical margins clear, and how close was the tumor to the edge of the removed tissue?
• Was any dedifferentiation found in the tumor, and if so, how much of the tumor was involved?
• What is the pathologic stage of my tumor, and does the location affect my risk of recurrence?
• Will I need radiation therapy or any other additional treatment after surgery?
• How will my tumor be monitored for local recurrence, and how often will imaging be performed?
• If the tumor comes back, what are my treatment options?
• Should molecular profiling (next-generation sequencing) be performed on my tumor?
• Are there clinical trials I should consider, particularly for MDM2 or CDK4 inhibitors?
• Should I be seen at a sarcoma specialty center?
• Is there any risk that this tumor will transform into a more aggressive cancer over time, and how can I reduce that risk?