In Situ and Invasive Neoplasia

Tumour staging and grading are two distinct but complementary systems used to describe the nature of a malignant neoplasm and predict its behaviour. Both have important implications for prognosis and treatment planning.

Staging describes the anatomical extent of tumour spread. It is based on three parameters: the size of the primary neoplasm, the extent of spread to regional lymph nodes, and the presence or absence of blood-borne distant metastases — together formalised in the TNM system (Tumour, Node, Metastasis). Staging is the most important prognostic indicator for most malignancies and also guides selection of treatment modality. A higher stage correlates with a worse prognosis. Stage 0 — carcinoma in situ — represents disease that has not yet breached the basement membrane. Stage 1 indicates the tumour is confined to the organ of origin. Stage 4 indicates distant metastatic disease.

Grading, by contrast, describes the degree of differentiation of the tumour — how closely the tumour cells resemble the normal cells from which they arose. Well-differentiated (low grade) tumours closely resemble normal tissue and generally behave less aggressively. Poorly differentiated (high grade) tumours have lost morphological similarity to normal tissue and tend to behave more aggressively. Staging is generally of greater clinical significance than grading, but for certain tumour types, grading is the more useful prognostic tool — particularly soft tissue sarcomas (which do not reliably spread via lymph nodes), primary brain tumours, lymphomas (which are already in the lymphatic system, making lymph node staging less informative), and prostate cancer (assessed by the Gleason grading system).

Precancerous lesions represent a critical intermediate stage between normal tissue and invasive malignancy. They are sites of cellular proliferation accompanied by genetic alterations, likely reflecting changes in precursor adult stem cells — some degree of de-differentiation may be present. Epithelial precancerous lesions range along a spectrum from mild dysplasia to carcinoma in situ, the most severe form where cells display malignant characteristics but have not yet penetrated the basement membrane. Not all precancerous lesions progress to invasive cancer — some regress spontaneously.

Precancerous lesions are clinically important because they represent the window of greatest opportunity for cure. However, they are characteristically difficult to detect: they are usually small, non-invasive in nature, and unlikely to cause symptoms. They are also unlikely to be detectable using serum markers unless the affected cell type produces hormones or secretions (for example, precancerous ovarian lesions in the context of hormone-producing tumours).

Ultraviolet (UV) radiation — particularly UVB light — is a well-characterised carcinogen relevant to skin cancers. UV light causes several cellular effects: inhibition of cell division, inactivation of enzymes, induction of DNA mutations, and at high doses, cell death. The specific carcinogenic mechanism of UVB is the formation of pyrimidine dimers in DNA — abnormal covalent bonds between adjacent pyrimidine bases on the same DNA strand, which distort the helix and cause mutations in critical genes including RAS (an oncogene) and p53 (a tumour suppressor gene).

Normally, pyrimidine dimer damage is repaired by the nucleotide excision repair (NER) pathway, which recognises and removes the damaged sequence and replaces it with correct nucleotides. When the NER pathway is overwhelmed or genetically impaired, DNA damage accumulates, leading to large transcriptional errors and, in some cases, cancer.

Xeroderma pigmentosum is a rare autosomal recessive disorder caused by mutations in one of several genes involved in the NER pathway. Affected individuals have extreme photosensitivity and an up to ten-thousand-fold increased risk of skin cancer in sun-exposed areas, providing compelling clinical evidence for the importance of NER in protecting against UV-induced carcinogenesis. At least seven genetic variants of the condition exist, each caused by a mutation in a different NER gene.