Since there are different types of tumours with different characteristics, it is useful to classify a tumour. Characteristics considered for classification are malignancy, location, size and spread of the tumour.
The degree of malignancy (malignancy grade) of the examined tumour tissue is determined by comparing the tumour cells with the healthy cells in their tissue of origin (e.g. liver cells, skin cells, etc.). The various characteristics and functions of a healthy cell are also referred to by the term of “differentiation”. The more characteristics and functions a tumour cell still shares with its original tissue, the higher its degree of differentiation and the lower the malignancy Conversely, malignancy increases as the degree of differentiation decreases and tumour cells lose almost all characteristics and functions except for an unnatural and useless capability to replicate.
Histological classification of tumour cells
Histological classification (grading) of tumour cells (according to UICC = Union Internationale Contre le Cancer)
- Grade 1 (G1): well-differentiated malignant tissue (“low-grade”), many shared characteristics with tissue of origin
- Grade 2 (G2): moderately differentiated malignant tissue
- Grade 3 (G3): poorly/lowly differentiated malignant tissue
- Grade 4 (G4): non-differentiated or undifferentiated or anaplastic malignant tissue (“high-grade”), so that the tumour sometimes can no longer be assigned to a specific initial tissue or can only be assigned by immunohistochemical examination
- Grade 9 (G9): the degree of differentiation cannot be assessed
As soon as possible, the physician will try to classify the tumour not only by examining a specimen of representative cells but by assessing it in its entirety. Often an accurate classification can only be made after surgery. The most common system for classifying tumour diseases is the internationally valid TNM scheme. The classification allows physicians to summarize the essential data about the tumour using a short letter/number combination. Thus, this information is usually found in case reports and patient records.
The letters are abbreviations of the words “tumour” (tumor), “node” (lymph node) and “metastases”
- T = tumour: size and spread of the original tumour, also called “primary tumour”.
- N = lymph nodes: Quantification of cancer cells in adjacent lymph nodes. These are called lymph node metastases.
- M = Metastases: Detection of cancer cells in other parts of the body or organs. One then speaks of “distant metastases”.
In order to be able to specify the extent of the disease more precisely, each letter is accompanied by a number. Possible are T1 to T4, N0 to N3 as well as M0 and M1. The use of zero (N0, M0) indicates that lymph node or distant metastases are not detectable.
T1 to T4 for the primary tumour
For category T, the numbers 1 to 4 describe the size and spread of the primary tumour. For some tumours, experts subdivide even more precisely. The T-digits are then supplemented with the lowercase letters a to d. In prostate cancer, for example, there are not only stages T1 to T4, but also stages T2a to T2c and T3a and T3b in between. The criteria for category T classification depend on the tumor type.
N0 to N3 for the lymph nodes
N-category numbers are based on the number and location of “regional” lymph nodes affected by cancer cells. Lymph nodes are like filter stations within the lymphatic system. Regional is the name given to lymph nodes associated with the lymphatic drainage system of an organ or region of the body. Thus, when cancer cells detach from a tumour or travel through the lymphatic system, they can often be first detected inside the lymph nodes in the tumor’s immediate vicinity. For example, breast cancer surgery usually includes removal of the axillary lymph nodes, as these are the closest lymph nodes in breast cancers to be examined for tumor metastasis.
N0 means that no cancer cells could be detected in the regional lymph nodes of the organ affected by the tumour. N1, N2 and N3 describe the involvement of more and more regional lymph nodes or of those that are located further away from the tumour. The criteria for category N classification also depend on the tumour type.
M0 and M1 for distant metastases
Category M only distinguishes whether distant metastases have been found (M1) or not (M0). For instance, the classification “breast carcinoma, T1N1M0” means that in this case of breast cancer, the tumour is up to 2 cm in size (which is the definition of T1 for breast cancer), in at least one lymph node, cancer cells have been detected, but no metastases have been found. Based on this classification, a prognosis can be made. It is a prediction about the course of the disease.
The prognosis is based on worldwide experience and statistics on cancer. Perhaps phrases such as “The prognosis is good” or “It is unfavourable” will be dropped; perhaps more specific clues will be given, such as an expected survival time. Thus, there are statistical cure probabilities, survival rates and survival times for all types of cancer. For the “T1N1M0 breast carcinoma” example above, this means that the 5-year statistical survival rate is between 83 to 87% and the 10-year survival rate is about 82%.. Thus, after 10 years, more than 80% of patients will still be alive after this type of breast cancer diagnosis, indicating that the chances of a cure are high.
However, it must be remembered that statistics only make a statement about probabilities. After all, no one can predict whether a patient with a tumour that has a 99% unfavorable outcome will not count among the 1% that will survive.
In any case, the following applies: The statistical prognosis of a cancer disease is always only a statement about the totality of all patients and has only limited significance for the individual situation.