About… Transplants

I said in an ealier post that I would write a little about the transplant process, so here it is. I must add that this is entirley my understanding from a lay point of view, so the medics in the audience (yes, you at the back – you know who you are!) can stand by to correct me!

There are two basic types of transplant – self donated (autologous) and donor transplants. In both cases the aim is to repair the damage caused by the treatment for the cancer. Let me explain.

The aim of the chemotherapy is to destroy all the cancer cells in the body. Unfortunately the majority of drugs are not selective, and will attack any cells. However cells are only vulnerable when they are in an active or multiplying phase, and it is a characteristic of cancer cells that they have a longer active phases and shorter dormant phases than normal cells. In fact the more aggressive the cancer, the more responsive it can be to treatment. (Although that is not always the case)

The effectiveness of the chemo also depends on the dose – the higher the dose, the more effective, but the limiting factor is the damage it causes to other cells in the organs of the body. Susceptible organs are the bone marrow, liver, and kidneys, and if the dose of chemo is too high, this can result in death – which from a patient perspective is undesirable! The sore mouth and other side effects from the chemo are because the cells lining the mouth and gut are renewed frequently, and therefore susceptible to the effects of the chemotherapy.

However, stem cells transplantation is a relatively easy procedure, so by harvesting stem cells from the patient first, a higher dose of chemo (or of total body irradiation) which will kill off the bone marrow, which is then regenerated from the stem cells.

With donor cells, there is an added, and probably more important effect. It is thought that we produce cancer cells all the time, but they are destroyed by the immune system before they cause problems. In some cases, for reasons not always understood, the immune system may become blind to a specific cancer, which then goes on to grow. The reasons are not fully understood, but they may be environmental factors (smoking is the obvious one) and some viruses have been implicated, such as the Epstein-Barr virus in lyphomas. The E-B virus causes glandular fever.

So by using a donor, the whole immune system of the recipient is regenerated, which will not be blind to the cancer cells, and will therefore destroy them.

There are added complications with a donor transplant though. The most obvious one is tissue compatibility. The most desireable option would be a donation from an identical twin. Sadly I don’t have one! The next preferred option is from a matching sibling, where there is a 1 in 4 chance of a match. Sadly y sister was only a 50% match, and my suggestion that my parents might like to try for another fell on stony ground! That only left the option of an unrelated donor transplant. 80 potential matching donors were identified from the Anthony Nolan Bone Marrow Trust and the NHS registers. The 80 was down selected to 5, all of whom were excellent matches, and finally one donated his cells on Wednesday, for the transplant yesterday.

The actual mechanism of the transplant can vary too. For my self donated transplants, the bone marrow was completely destroyed (ablated is the technical term, I think) and that can be done for donor transplants too, but the risks increase significantly after the age of 40. This is because of the thalamus gland, which plays a part in immune system development. It is the size of a walnut at birth, grows to the size of a fist (I’m told) by puberty, then atrophies to the size of a walnut by the age of 40. Full donor transplants are very successful in treating some childhood leukaemias.

As I am over 40, the risks of a full donor transplant are too high, so an alternative , the mini or reduced intensity transplant is used. This is where the marrow is suppressed to ‘make room’ for the incoming graft, which then slowly takes over in a controlled fashion, the control being via the application of immuno-suppressing drugs.

Think of it as a field of weeds. In a full donor transplant, the field is sprayed with weedkiller (the chemo) and then new seed sown. In the reduced intensity transplant, a corner of the field is cleared and re-sown, the rest of the weeds are just cut down. Gradually the plants in the cleared corner will take over from the weakened weeds.

For details of the Anthony Nolan Trust, follow the link on the right under the blogroll.