Introduction to Neoplasia

Neoplasia literally means "new growth." A neoplasm is a clonal expansion of cells that have undergone neoplastic transformation — an irreversible alteration in their genetic programme that allows them to proliferate in an abnormal, unregulated manner. It is important to distinguish neoplasia from the related terms tumour and cancer. A tumour simply means a swelling and is not synonymous with neoplasia. Cancer is a subgroup of neoplasia — specifically, malignant neoplasia. Not all neoplasms are cancers.

A neoplasm can be initiated by a chemical, physical, or biological agent (or event) that directly and irreversibly alters the cell genome. Examples include carcinogenic chemicals in tobacco smoke, ionising radiation, and oncogenic viruses such as human papillomavirus (HPV). The altered cells escape normal regulatory mechanisms governing proliferation, differentiation, and death.

A critical concept in the microscopic assessment of neoplastic tissue is dysplasia, which means abnormal growth. Dysplastic cells display characteristic features when examined histologically — most notably nuclear pleomorphism, meaning that nuclei within the tissue appear in multiple different sizes and shapes. Other features of cellular dysplasia include increased nuclear-to-cytoplasmic ratio, abnormal mitotic figures, and loss of normal tissue architecture.

Neoplasms are broadly classified according to two axes: the type of cell of origin and the biological behaviour (benign versus malignant). Tumours originating from mesenchymal tissues (connective tissue, bone, muscle, fat) follow a particular naming convention: benign mesenchymal tumours are named using the suffix -oma (e.g. fibroma, lipoma, osteoma), while malignant mesenchymal tumours are called sarcomas (e.g. fibrosarcoma, osteosarcoma). Tumours originating from epithelial tissues follow a different convention: benign epithelial tumours are called adenomas, while malignant epithelial tumours are called carcinomas. A malignant epithelial tumour with gland-forming morphology is an adenocarcinoma. Important exceptions to these naming rules exist: melanoma and lymphoma are both malignant despite the -oma suffix, and teratoma (derived from pluripotent stem cells) can be either benign or malignant.

Benign neoplasms expand locally without invading surrounding normal tissues and do not — as a rule — metastasise. However, this does not mean they are clinically insignificant. A benign tumour may cause serious problems by virtue of its location, size, or secretory activity. A pituitary adenoma, for example, may cause clinical disease by compressing the adjacent pituitary gland, disrupting normal hormone secretion.

Malignant neoplasms, by contrast, invade surrounding tissues and have the capacity to metastasise — to spread to distant organs via the lymphatic system or bloodstream — establishing secondary tumours at sites remote from the primary. This invasive and metastatic capacity is the defining biological feature of malignancy.

Some tumours — both benign and malignant — produce biologically active substances, including hormones. When a tumour produces systemic effects remote from the primary lesion itself, these are called paraneoplastic syndromes. One particularly important paraneoplastic manifestation is cachexia — a multi-organ syndrome of systemic inflammation and negative energy balance. Cachexia is driven by cytokines, including tumour necrosis factor (TNF) and interleukin-1 (IL-1), produced both by the tumour itself and by the host's immune response to it.

Staging is used to describe the extent of spread of a malignant tumour and has critical prognostic and therapeutic implications. Stage 0 indicates no spread beyond the site of origin; Stage 1 indicates the tumour is confined to the organ or tissue of origin; Stage 4 indicates distant metastases are already present in another organ. The staging system is tumour-specific and varies by cancer type.