Benign Versus Malignant Tumours

Expansion

Malignant tumours invade, metastasise, grow rapidly and show cellular atypia

Mnemonic

“Benign stays, malignant spreads”, with the supporting features:

	Benign	Malignant
Growth	Slow	Rapid
Border	Encapsulated, well circumscribed	Infiltrative
Differentiation	Well differentiated	Variable to anaplastic
Nuclei	Uniform	Pleomorphic, hyperchromatic, high nuclear to cytoplasmic ratio
Mitoses	Few, normal	Many, abnormal
Necrosis	Rare	Common
Metastasis	Never	Defining feature

Metastasis is the only absolute criterion; everything else is a matter of degree, which is why some histologically bland tumours behave badly.

Nomenclature follows a rule: -oma for benign (lipoma, adenoma), carcinoma for malignant epithelial, sarcoma for malignant mesenchymal.

The exceptions catch people out: melanoma, lymphoma, mesothelioma, seminoma, glioma and myeloma are all malignant despite the ending.

Expansion

Feature	Benign	Malignant
Growth	Slow	Rapid, with many mitoses
Border	Encapsulated, well circumscribed	Invasive, irregular
Metastasis	Never	Yes
Differentiation	Well differentiated, resembles tissue of origin	Variable, may be anaplastic
Nuclei	Normal	Pleomorphic, hyperchromatic, high nuclear to cytoplasmic ratio
Necrosis	Uncommon	Common, as growth outstrips supply
Recurrence after excision	Uncommon	Common

Nomenclature

Benign: tissue plus -oma (lipoma, adenoma, leiomyoma)
Malignant epithelial: carcinoma
Malignant mesenchymal: sarcoma
Misleading exceptions that are malignant despite the -oma suffix: melanoma, lymphoma, mesothelioma, seminoma, glioma, myeloma

Benign does not mean harmless: a benign meningioma can be fatal by pressure, a phaeochromocytoma by hormone secretion, and a colonic adenoma by progressing to carcinoma.

Anaplasia, meaning loss of differentiation, is the histological hallmark of high grade malignancy, and includes giant cells, abnormal mitotic figures and loss of polarity.