There is a group of diseases considered to be the cancers of the blood system. They are generally referred to as leukaemias and lymphomas, or malignant diseases of the lymphoreticular system.
White blood cells are produced in bone marrow and in lymphoid tissue, which means the spleen and the lymphatic glands located in various parts of the body. This is collectively referred to as lymphoreticular tissue.
Just as cancers can occur in any system, the regions producing blood cells are not immune. For a variety of reasons (many now clearly understood. others as yet unknown), these cell-production sites may suddenly begin to produce cells at a rate far in excess of normal. It is due to a disorder of the basic structure of the cell-producing material. As increased numbers of cells are poured into the blood system, more and more immature forms find their way into the circulation.
Under normal conditions blood cells reach maturity and, only when they are in a finished fully developed state, capable of carrying out their appointed duties.
Blood cells go through a distinct routine of development, and the very early cell is called a blast cell. As disease of these systems progresses, more immature blast cells appear in the circulating blood, and this, plus the great augmentation in gross numbers, often leads to the diagnosis of the disease.
There is an extensive classification of these diseases. Once they were considered to be individual disorders. But with greater knowledge and more work being carried out in research centers, it is now quite evident that they are all closely interrelated.
There are two main types:
(1) Malignant disease affecting the bone marrow.
This interferes with production of the cells normally produced here. It may be an acute form, giving rise to the disease known as acute leukaemia when the white cells are affected, or an erythraemia when the red-cell system is affected.
It may be a slower type of disorder, producing chronic forms of disease, such as chronic leukaemia, affecting the white cells, or polycythaemia vera that affects red-cell production, or essential thrombocythaemia (producing excessive platelets).
(2) Malignant disease affecting the lymphoid tissues.
This may produce the well-known disorder called Hodgkin’s disease, as well as many other, less frequently occurring, subclassifications.
In comparison to simple iron-deficiency anaemia, these diseases are rare, and in comparison to cancers in general, the proportion of mortalities attributed to them is also small. In Australia, no more than 1200 deaths annually are due to leukaemia, a small fraction of the 30,000 annual deaths due to cancers in general.