Benign and Malignant Tumours
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Direct answer
A tumour or neoplasm is an abnormal mass of tissue whose growth exceeds that of normal tissue and continues after the original stimulus has gone. Benign tumours are slow-growing, well differentiated, encapsulated, non-invasive and never metastasise; malignant tumours grow rapidly, show variable anaplasia, infiltrate surrounding structures and spread to distant sites — metastasis being the single definitive hallmark of cancer.
What you must remember
- Nomenclature: benign epithelial tumours end in adenoma or papilloma; malignant epithelial tumours are carcinomas; benign mesenchymal tumours use the root plus -oma (lipoma, leiomyoma, osteoma) and malignant ones are sarcomas (liposarcoma, osteosarcoma).
- Beware exceptions to the -oma rule: lymphoma, melanoma, mesothelioma and seminoma are all malignant despite their suffix; a teratoma contains tissue from more than one germ cell layer.
- Anaplasia — the histological mark of malignancy — includes pleomorphism, hyperchromatic nuclei, high nuclear-cytoplasmic ratio, prominent nucleoli, atypical mitoses and tumour giant cells.
- Benign growth is expansive with a capsule and rare, normal mitoses; malignant growth is rapid, infiltrative, poorly demarcated and outstrips its blood supply, producing necrosis and haemorrhage.
- Routes of spread: direct local invasion; lymphatic spread, typical of carcinomas to regional nodes first; haematogenous spread, typical of sarcomas and of renal, hepatic, thyroid and choriocarcinoma, seeding lung, liver, bone and brain; transcoelomic spread across peritoneal surfaces, classically ovarian carcinoma.
- Premalignant progression runs from dysplasia through carcinoma in situ (basement membrane intact) to invasive carcinoma, best illustrated by the cervix.
- Every tumour has neoplastic parenchyma and supportive stroma; abundant collagenous stroma is desmoplasia, and tumour angiogenesis driven by VEGF is essential for growth beyond a few millimetres.
- Grading scores histological aggressiveness, whereas staging (TNM) maps anatomical spread; staging is generally the stronger prognostic determinant.
Common confusion
Grading and staging are perpetually swapped. Grade describes how aggressive the tumour looks — differentiation and mitotic activity under the microscope; stage describes how far it has travelled — tumour size, nodal status and metastases. A low-grade tumour that is widely disseminated is high-stage; the two are independent assessments that combine for prognosis.
Exam-focused takeaway
Theory answers should tabulate benign versus malignant on differentiation, rate of growth, capsule, invasion, metastasis and mitoses, then cover nomenclature, routes of spread and the dysplasia-carcinoma sequence. Viva examiners ask for the definition of anaplasia and carcinoma in situ and the differences between carcinoma and sarcoma. MCQs test the malignant -oma exceptions, which carcinomas spread by blood, the meaning of desmoplasia and the TNM system.
Frequently asked questions
Which single feature best separates malignant from benign tumours?
Metastasis, together with local invasion; benign tumours may grow large but never spread to distant sites.
What is carcinoma in situ?
Severe dysplastic change with all the cytological features of malignancy but still confined by an intact basement membrane — pre-invasive disease.
How do carcinomas and sarcomas usually spread?
Carcinomas spread first through lymphatics to regional nodes; sarcomas favour haematogenous dissemination, with notable carcinoma exceptions including renal, hepatic and thyroid primaries.
Differentiate grading from staging.
Grading assesses histological differentiation and mitotic activity; staging maps anatomical spread using the TNM system and is the better prognostic guide.
What is a teratoma?
A germ cell tumour containing recognisable tissue from more than one germ layer, such as hair, teeth and thyroid tissue in an ovarian dermoid.