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Can tumour cells get passed to another person through blood contact, for example from blood donations or used needles?

When cancer in one part of the body spreads to another part of the body, the outlook for a patient is rarely positive. Given how frequently this happens, it may come as a surprise to know that the spread of cancer from one person to another is actually incredibly rare.

"Generally, in well people who are not immune-suppressed, getting cancer from one individual to another via blood is very unlikely," says Dr Ashley Ng from the cancer and haematology division of the Walter and Eliza Hall Research Institute.

Several large studies have been conducted to specifically examine this question.

"One study looked at about one-third of a million blood recipients, of which about 12,000 were at risk of being transfused blood from a donor with sub-clinical cancer and they found no increase in risk," says Ng.

This evidence fits with what we know about how the immune system responds to foreign matter. In the case of blood transfusion, blood type (such as A, B, AB and O) is carefully matched between the donor and recipient so the recipient's immune system doesn't see the red blood cells as foreign and destroy the red blood cells.

If there are cancer cells in that blood, there are other unique proteins on the surface of those cells that in the majority of cases, mark them out as foreign. The recipient's immune system therefore identifies them and destroys them before they can settle in.

Blood banks also carefully screen donors to rule out anyone who's had cancer, just in case.

But if the recipient's immune system isn't working well — for example, if they are immune-suppressed by illness or because they have had an organ transplant which requires immunosuppression of the recipient to prevent rejection of the donor organ — then they are less likely to be protected by this mechanism.

"When we do blood transfusion into immunocompromised people, we can irradiate the actual red cell units," says Ng. This is done already to reduce the risk of the transfused white blood cells attacking the recipient's body — something called graft versus host disease. This irradiation can also kill any sub-clinical cancer cells which may be circulating in the donor's blood.

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Organ donation and pregnancy

In the case of solid organ transplants, such as liver or kidneys, there have been reports of cancer being unknowingly transmitted from the donor to the recipient. While donors and their organs are screened for cancer it can, on very rare occasions, slip through undetected. However the risk is incredibly low — around 0.015 per cent, according to Australian guidelines.

Finally, there is also evidence that cancer can be transmitted from a mother to her unborn child but again, this is very rare.

"In review back in 2003, there were only 14 reported cases in the literature where the mum had a type of cancer and the child also developed the same cancer," says Ng. The cancers documented generally included aggressive types cancer and unfortunately likely to be during advanced stages of disease in the mother. Such cancers included leukaemia, melanoma, solid organ cancers such as lung, and sarcomas."

Transmission between mother and foetus can occur because of the unusual immunological relationship that exists between the two during pregnancy — one in which the foetal immune system is still relatively immature and may tolerate foreign cells. However, transmission of a maternal cancer to the foetus is very unlikely as this requires the cancer cells to be travelling in the mother's circulation, and in addition, cross the placental barrier to the foetus. In most pregnancies, unless this placental barrier is breached such as with accidental trauma, foetal circulation remains completely separate from the mother's blood supply.

Then there are the very rare, very unusual cases of person-to-person transmission, such as the surgeon who contracted cancer from a patient after accidentally cutting himself during surgery, and transmission of colon cancer through a needlestick injury.

In the case of the surgeon, it turned out that the cancer itself had performed a genetic miracle and incorporated some of the surgeon's genes into itself. Dr Ng suggests that the explanation for these rare cases could also be related to immunologically similarities between the donor and recipient, or that the cancer cells somehow were able to evade immune detection.

Dr Ashley Ng is a haematologist at Royal Melbourne Hospital and post-doctoral fellow at the Walter and Eliza Hall Research Institute. He spoke to Bianca Nogrady.