Taxonomy of Cancer
Classification
Benign & Malignant Tumours
A tumour is an abnormal mass of tissue. There are two types of tumours: benign tumours and malignant tumours. Only malignant tumours are considered cancerous. The difference between benign and malignant tumours is summarised below.
| BENIGN TUMOURS | MALIGNANT TUMOURS |
|---|---|
| Small | Large |
| Well-demarcated | Poorly demarcated |
| Slow-growing | Rapidly growing with haemorrhage and necrosis |
| Non-invasive | Locally invasive |
| Non-metastatic | Metastatic |
| Well-differentiated | Poorly differentiated |
Cancer Types
Cancers may be classified into four types.
Carcinoma: cancers that start their growth in epithelial tissues.
Sarcoma: cancers that start their growth in connective tissues.
Lymphoma: cancers of the lymphatic system
Leukaemia: cancers of the blood system.
Grading
Grading describes a tumour based on how abnormal the tumour cells and tissue look under a microscope. It is an indicator of how quickly a tumour is likely to grow and spread.
See this fact sheet from the US National Cancer Institute for more information on grading.
If the tumour cells and tissue are close to normal cells and tissue, the tumour is said to be well-differentiated. If the tumour cells and tissue are substantially different from normal cells and tissue, the tumour is said to be poorly differentiated. A well-differentiated tumour tends to grow and spread at a slower rate than a poorly differentiated tumour.
Grading systems differ depending on the type of cancer. In general, tumours are graded as 1, 2, 3 or 4, depending on the amount of abnormality. If a grading system for a tumour type is not specified, the following system is generally used:
GX Grade cannot be assessed (undetermined grade)
G1 Well-differentiated (low grade)
G2 Moderately differentiated (intermediate grade)
G3 Poorly differentiated (high grade)
G4 Undifferentiated (high grade)
Note that tumour grade is not the same as cancer stage. While tumour grade is a good indicator of how poorly or well-differentiated a tumour is, tumour grade gives much less information than cancer stage (discussed further below).
Staging
Staging describes the severity of a person’s cancer based on the size and/or extent of the primary tumour and whether or not the cancer has spread in the body.
See this fact sheet from the US National Cancer Institute for more information on staging.
The TNM system is one of the most widely used cancer staging systems. The TNM system is based on the size and/or extent of the primary tumour (T), the amount of spread to regional lymph nodes (N), and the presence of distant metastasis (M).
Primary Tumour (T)
- TX Primary tumour cannot be evaluated.
- T0 No evidence of primary tumour.
- Tis Carcinoma in situ (CIS). Abnormal cells are present but have not spread to neighbouring tissue. Although not a cancer, CIS may become a cancer and is sometimes called preinvasive cancer.
- T1, T2, T3, T4 Size and/or extent of primary tumour.
Regional Lymph Nodes (N)
- NX Regional lymph nodes cannot be evaluated.
- N0 No regional lymph node involvement.
- N1, N2, N3 Degree of regional lymph node involvement.
Distant Metastasis (M)
- MX Distant metastasis cannot be evaluated.
- M0 No distant metastasis.
- M1 Distant metastasis is present.
For example, “breast cancer classified as T3 N2 M0” refers to a large tumour that has spread outside the breast to nearby lymph nodes but not to other parts of the body. Likewise, “prostate cancer classified as T2 N0 M0” refers to a medium-sized tumour that is located only in the prostate and has not spread to nearby lymph nodes or any other part of the body.
Choose the correct statement.