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PA6.1 | Neoplasia — Definitions, Nomenclature & Characteristics — SDL Guide (Part 2)

Critical Exceptions — Malignant Tumours with '-oma' Suffix

The most examination-critical exceptions are malignant neoplasms that carry the '-oma' suffix — a historical accident that now demands rote memorisation:

Tumour name	Why it looks benign	Actual behaviour
Melanoma (malignant melanoma)	-oma suffix; once called 'naevocarcinoma'	Highly aggressive; metastasises early
Lymphoma	-oma suffix	Malignant haematopoietic neoplasm
Seminoma	-oma suffix	Malignant germ cell tumour of testis
Mesothelioma	-oma suffix	Malignant tumour of pleura/peritoneum
Hepatoma	-oma suffix; also called hepatocellular carcinoma	Malignant
Glioma (glioblastoma multiforme)	-oma suffix	Malignant CNS tumour

Memory hook: 'MLSMH-G' — Melanoma, Lymphoma, Seminoma, Mesothelioma, Hepatoma, Glioma.

Special Tumour Categories

A five-panel medical diagram compares mixed tumours, teratomas, hamartomas, and choristomas with organ examples and labeled tissue components. — **Panel A:** Comparison map showing definitions of mixed tumour, teratoma, hamartoma, and choristoma with the key right tissue/right place distinction.. **Panel B:** Pleomorphic adenoma of parotid gland showing epithelial nests, myoepithelial cells, and chondroid stroma from one germ-layer lineage mixture.. **Panel C:** Mature cystic teratoma of ovary showing skin lining, hair, tooth, sebaceous material, ectoderm, mesoderm, endoderm, and malignant immature neuroepithelium inset.. **Panel D:** Pulmonary hamartoma showing disorganized native lung-region tissue elements: cartilage, smooth muscle, fat, and bronchial-type epithelium.. **Panel E:** Choristoma showing ectopic pancreatic tissue in stomach wall and gastric mucosa in Meckel diverticulum as normal tissue in abnormal locations..

Mixed tumours contain two or more cell lineages arising from one germ layer (e.g., pleomorphic adenoma of the parotid = epithelial + myoepithelial + chondroid stroma; benign but locally recurrent).

Teratoma arises from totipotent germ cells and contains tissues derived from all three germ layers (ectoderm, mesoderm, endoderm). The classic example is the mature cystic teratoma (dermoid cyst) of the ovary — benign, contains hair/teeth/skin. Immature teratomas (containing fetal-type neuroepithelium) are malignant. Teratomas of the testis tend to be malignant regardless of maturity.

Hamartoma — a disorganised but non-neoplastic overgrowth of normal tissue elements native to a given organ (e.g., pulmonary hamartoma with cartilage + muscle + fat in the lung). It is NOT a true neoplasm.

Choristoma — a congenital rest of normal tissue in an abnormal location (e.g., pancreatic tissue in the stomach wall; gastric mucosa in Meckel's diverticulum). Also not a neoplasm.

Key distinction: Hamartoma = right tissue, right place, wrong organisation. Choristoma = right tissue, wrong place.

SELF-CHECK

A 35-year-old woman has a painless testicular mass. Biopsy shows a tumour with sheets of large uniform cells with clear cytoplasm and prominent nucleoli, resembling spermatocytes. The tumour is called a 'seminoma'. Which statement is MOST accurate?

A. It is benign because it carries the '-oma' suffix

B. It is a malignant germ cell tumour despite the '-oma' suffix

C. It is a mixed germ cell tumour arising from all three germ layers

D. It is a hamartoma of testicular tissue

Reveal Answer

Answer: B. It is a malignant germ cell tumour despite the '-oma' suffix

Seminoma is a classic exception — a malignant germ cell tumour of the testis that carries the misleading '-oma' suffix. It is exquisitely radiosensitive and carries a good prognosis if detected early, but it is unequivocally malignant. Options A and D are wrong by definition; option C describes a teratoma, not a seminoma.

Characteristics of Benign vs Malignant Tumours — Overview

The distinction between benign and malignant neoplasms rests on four fundamental characteristics. None is absolute on its own — the diagnosis is always a combination of histological and clinical features.

Characteristic	Benign	Malignant
Differentiation / Anaplasia	Well-differentiated; resembles parent tissue	Variable; often poorly differentiated or anaplastic
Rate of growth	Slow; few mitoses	Rapid; many mitoses (often abnormal)
Local invasion	Non-invasive; often encapsulated	Invasive; infiltrates surrounding tissue
Metastasis	Absent	Present (defining feature of malignancy)

IMPORTANT: Metastasis is the single most reliable criterion for malignancy. A tumour that has metastasised is by definition malignant — there are no exceptions.

A four-panel comparison diagram summarizes differentiation, growth rate, local invasion, and metastasis as key features distinguishing benign from malignant tumours. — **Panel A:** Overview checklist showing differentiation/anaplasia, rate of growth, local invasion, and metastasis as combined diagnostic criteria.. **Panel B:** Side-by-side histology comparison showing well-differentiated benign cells with few mitoses versus anaplastic malignant cells with pleomorphism and abnormal mitoses.. **Panel C:** Tissue cross-section comparing an encapsulated non-invasive benign tumour with an infiltrative malignant tumour invading adjacent tissue.. **Panel D:** Metastatic spread pathway showing malignant cells entering a vessel or lymphatic channel and forming a distant metastatic deposit..