Adipose tissue, commonly known as fat, is a type of body tissue made up of cells called adipocytes. These cells store energy, help cushion and protect internal organs, and provide insulation to maintain body temperature. Adipose tissue surrounds many organs throughout the body and is a normal part of the tissue layers examined in pathology specimens. When adipose tissue is mentioned in a pathology report, it may simply describe the normal fat present in the sample — or it may appear in the context of a tumor growing into or through the surrounding fat, which is an important finding that affects staging and treatment.
Where is adipose tissue found in the body?
Adipose tissue is found throughout the body. It accumulates under the skin (subcutaneous fat), around internal organs (visceral fat), and in the spaces between organs. In pathology, several specific fat compartments are frequently named when a tumor grows beyond its organ of origin:
- Pericolorectal fat — the fat surrounding the colon and rectum. Invasion into this fat is a significant finding in colorectal cancer staging.
- Perinephric (perirenal) fat — the fat surrounding the kidney. Invasion into this layer affects the stage of kidney cancer.
- Peripancreatic fat — the fat surrounding the pancreas. Extension into this tissue is commonly reported in pancreatic cancer.
- Perirectal fat — the fat surrounding the rectum, distinct from the pericolorectal fat of the broader colon.
- Subcutaneous fat — the layer of fat beneath the skin, which may be involved in skin tumors or soft tissue tumors growing near the surface.
What does adipose tissue look like under the microscope?
Under the microscope, adipose tissue appears as large, round, clear cells grouped together in a honeycomb or bubble-like pattern. Each adipocyte is filled almost entirely with a single large droplet of fat, which pushes the nucleus — the cell’s control center — to one side of the cell. Because fat does not retain standard laboratory stains well, the fat droplet appears empty or clear, giving adipocytes their characteristic transparent appearance.
What does it mean when a tumor invades adipose tissue?
One of the most common reasons adipose tissue is mentioned in a pathology report is to describe a tumor that has grown beyond its organ of origin and into the surrounding fat. Phrases such as “extends into pericolorectal adipose tissue,” “invades perinephric fat,” or “tumor present in peripancreatic adipose tissue” all mean that cancer cells have been found in the fat layer surrounding the organ.
This finding is important for two main reasons:
- Staging. Invasion of the surrounding fat is often a threshold that moves a tumor from one pathologic stage to the next. For example, in colon cancer, extension through the bowel wall into pericolorectal fat represents at least pT3 disease. In kidney cancer, invasion into perinephric fat indicates at least pT3a staging. A higher stage generally reflects a more advanced tumor and may influence decisions about additional treatment after surgery.
- Margins. When a tumor has grown into the surrounding fat, the surgical margin — the edge of the tissue removed — becomes particularly important. If the tumor extends close to or reaches the margin, there is a higher risk that some cancer cells may remain in the body, which can affect recommendations for further treatment.
What tumors arise from adipose tissue?
In addition to being invaded by cancers from nearby organs, adipose tissue can give rise to its own tumors:
- Lipoma — a common, noncancerous (benign) growth made up of mature fat cells. Lipomas are soft, slow-growing lumps usually found just beneath the skin. They rarely require treatment unless they cause discomfort.
- Liposarcoma — a malignant (cancerous) tumor that develops from fat cells. Liposarcomas are rare but can be aggressive, particularly when they arise deep within the body such as in the abdomen or thigh. Several subtypes exist, ranging from slow-growing to highly aggressive.
Questions to ask your doctor
- My report says the tumor has grown into the surrounding fat — what does this mean for my stage?
- Does invasion into adipose tissue affect my surgical margins or the need for additional treatment?
- Is there any concern about a primary tumor of adipose tissue, such as a liposarcoma?
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