Introduction
Tumours are classified morphologically by their degree of differentiation (grading) and clinically by their extent (staging), for which the international TNM system is used. The grade predicts the aggressiveness of the cancer; the stage determines treatment and prognosis.
Morphological classification: grading
The classification is based on the differentiation of tumour cells, that is, the extent to which they resemble the normal cells they originate from. All cancers can be grouped into 4 grades, G1-G4, which help predict the aggressiveness of the cancer.
- G1 - well differentiated: closest resemblance to normal cells; they look like normal cells. Examples: squamous cell carcinoma (with or without keratinization); trabecular, alveolar, papillary or mucous adenocarcinoma.
- G2 - moderately differentiated: tumour cells moderately resemble normal cells.
- G3 - poorly differentiated: minimal resemblance to normal cells; glandular, nested, scirrhous or solid patterns of growth.
- G4 - undifferentiated: the cells do not resemble normal cells and their histogenesis cannot be determined morphologically; they are immature, "primitive" and atypical (anaplastic). Examples: small-cell or large-cell carcinoma.
Mesenchymal tumours (sarcomas) are graded low, intermediate or high based on subcellular characteristics which imply the aggressive nature of the cancer; higher-grade tumours are more likely to metastasize.