Benign neoplasm: pushing margin and encapsulation
A benign neoplasm (a new growth of cells that has escaped normal control) grows by expansion, pushing neighbouring tissue aside, which typically gives a well-circumscribed (clearly outlined) lump.
Remember
A benign neoplasm pushes neighbouring tissue aside rather than growing into it, so it is typically well circumscribed with a pushing margin and does not metastasise (spread to distant sites); a capsule is common but not required.
Memory aid
benign tumours push, malignant tumours invade
Use it to sort a margin: an edge that pushes the neighbours aside points to a benign tumour, and an edge that grows into them points to a malignant one.
A neoplasm is a new growth of cells that has escaped normal control. A benign neoplasm is non-invasive, which means it does not grow into the surrounding tissue. It stays confined to its site of origin and grows by expanding, so it pushes neighbouring tissue aside.
This expansion gives the tumour a pushing margin, an edge that pushes the surrounding tissue aside instead of growing into it. The lump is often well circumscribed, which means clearly outlined. It may also be encapsulated, wrapped in a thin layer of connective tissue. A capsule may be present but is not required, so judge the whole picture rather than one feature.
Under the microscope the cells of a benign tumour are typically well differentiated: they still look like the normal tissue they came from. They usually grow slowly, with few mitoses (dividing cells).
Because the cells do not break through into the surrounding tissue, they cannot reach the lymphatics (lymph vessels) or blood vessels that would carry them away. A benign tumour therefore does not metastasise (spread to distant sites).
A malignant neoplasm is different. Its defining abilities are to invade surrounding tissue and to metastasise, and its edges are often irregular. A tumour that invades the surrounding tissue is malignant even before it metastasises.
Key facts
- Growth pattern
- a benign neoplasm expands and pushes neighbouring tissue aside instead of growing into it.
- Margin
- it is typically well circumscribed (clearly outlined), with a pushing margin.
- Capsule
- the tumour may be wrapped in a thin layer of connective tissue, but a capsule is not required.
- Cells
- they are typically well differentiated, so they resemble the tissue of origin.
- Spread
- it stays confined to its site of origin and does not metastasise.
- Caution
- pleomorphism (cells varying widely in size and shape) and frequent mitoses support malignancy, but they are not present in every cancer.
Exam traps
- Invasion of the surrounding tissue without distant spread is not a benign pattern, because invasion is itself a hallmark of malignancy, so a tumour that invades is malignant even before it metastasises.
- A clear outline does not make a tumour benign on its own, because cells that look little like the tissue of origin (poor differentiation) point towards malignancy.
- Rapid growth that outstrips the blood supply can occur in malignant tumours, which often grow faster than benign ones and often develop central necrosis (death of tissue), whereas a benign tumour typically grows more slowly.
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