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The major difference between these two groups of malignancies is that leukaemia is a defect in the growth and maturation of any of the blood forming cell lines of the bone marrow. Different leukaemias have different faults at different places in the growth and maturation cycle of blood forming cells. The result is infiltration of the bone marrow with lots of individual abnormal cells that are then released into the blood stream. This can not only involve both major types of white cells (myeloid or lymphoid) but also red blood cells and megakaryocytes (from which platelets derive). Leukaemia is often referred to as a liquid tumour.

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Lymphoma, however, is an abnormality of the lymphoid cells only and originates in lymphatic tissue, especially lymph glands, although it can also be found in gut, skin or indeed bone marrow tissue. These cells form structured lumps where the lymphoma cells\' relationships with each other and supporting tissues are important. Lymphoma is therefore an example of a solid tumour.

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To confuse matters, lymphoma can ‘spill’ over into blood later in the disease in a leukaemic phase and leukaemia can infiltrate

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organs, typically the spleen, enlarging them and forming a lump. Leukaemia cells can occasionally form tumours as well, for example chloromas, in acute myeloid leukaemia.

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Cancer Cytogenetics