Lymphomas and Leukemias: What’s the Difference?

Cancer, despite being responsible for 1 in 6 deaths in 2015, is often misunderstood. Cancer is actually an umbrella term for many different diseases which share one characteristic:

“The growth of abnormal cells beyond their usual boundaries that can then invade adjoining parts of the body and/or spread to other organs”WHO 2017.

Cancers are actually so numerous that scientists have actually defined more than 200 separate diseases. Each one of these diseases has a distinct ‘recipe’ which is unique, but due to similarities in how they develop, scientists group some cancers together: 

  • Carcinomas
  • Sarcomas
  • Spinal and brain cancers
  • Lymphomas
  • Leukemias

Whilst the first three cancer categories in this list are cancers of bodily tissues, i.e. solid tumors, the latter two are cancers of the blood; these will be the focus of this article.

Blood cancers are also known as haematological cancers. Unlike those which form in bodily tissues, blood cancers don’t usually form a solid tumour. Instead, the cancer cells float around in the blood and build up to create a ‘heavy tumour burden’. Just like a bag of rice could be considered a ‘tumour’, if the rice is scattered all over the floor, it is no longer a solid mass but the ‘burden of rice’ is the same. In cancer, it is only when the tumour cells migrate (known as metastatis), that tumours begin to develop.

As with all cancers, haematological cancers are named for where they originate.

Lymphoma

Lymphatic system

Lymphomas develop within the lymph nodes; small, bean-shaped glands which form part of the lymphatic system. This is a network of vessels and nodes filled with lymph fluid, which protects the body from infection. When an organ or other tissue excretes waste, the lymph fluid is transported by the lymphatic system to a nearby lymph node, where the waste is filtered out.

Lymphomas themselves can be broken down further into Non-Hodgkins Lymphoma (NHL), and Hogkins Lymphoma (HL). NHL form the majority of all lymphomas and are usually caused by an excess of mutated B cells, an immune cell which circulates in the blood. HL, on the other hand, forms ~12% of lymphomas and has a distinctive trait of Reed-Sternberg cells. These are distinctive, malformed B cells which are excessively large. HL also has some characteristic symptoms such as fever, which distinguish it from NHL.

1200px-reed-sternberg_lymphocyte_nci-vol-7172-300

Confusingly, the world’s most proliferant lymphoma is actually called a leukemia: chronic lymphocytic leukaemia (CLL).

This nomenclature exists because, despite involvement of the lymph nodes (which is how lymphomas are defined) the cancerous cells are mostly found in the blood and bone marrow. 

Leukaemia

Leukaemias begin in stem cells within the blood. There are two stem cell systems in the body, lymphoid and myeloid. Progenitor stem cells from these systems can differentiate down a chain into different ‘child’ cells, all of which can mutate to cause cancer. This is one of the main reasons there are multiple types of leukaemia.

Diagram of development of blood cells

Leukaemia occurs at the blast cell stage of development before cells become white blood cells. These are immature cells which incorrectly mutate and eventually over-run the healthy blood cells in the body.

Leukaemia is common in both children and the elderly, though the specific disease does vary depending on age group. CLL, acute myeloid leukaemia (AML), and acute lymphoblastic leukaemia (ALL) are all most common in older patients, however ALL is also common in patients under 25 years of age. Chronic myeloid leukaemia (CML) can occur at any age but is most common in the middle-aged to elderly population.

Although childhood leukaemia is rare, it actually accounts for one third of all childhood cancers. ALL is the most common type of childhood leukemia at 75% prevalence, with most of the remaining cases being AML.

As clear from the disease names, most leukaemias are either acute (i.e. fast-growing, aggressive cancers) or chronic (i.e. slow-growing, indolent cancers) and this trait can have a significant impact on treatment. Some chronic cancers such as CLL have a ‘watch-and-wait’ strategy, whereby no treatment is administered for many years following diagnosis.

Treatment strategies for all haematological cancers will vary depending upon a number of factors; these include the disease type, comorbidities (additional conditions or illnesses), the ‘fitness’ of the patient, chromosomal abnormalities (which can have both positive and negative impacts on prognoses) and whether the patient has recieved treatment for cancer in the past.

For further information on specifics regarding leukemia or lymphoma, please speak to your local health service or visit Cancer Reseach UK or Cancer.org.

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