RSS feedIonising radiation is extensively used in medicine. It is used to sterilize medical equipment, food and, most of all, is used in radiology for imaging the human body. While ionising radiation benefits mankind, it can cause diseases too. Epidemiological studies in nuclear industry workers, residents living in proximity to nuclear installations or in high radon content environments have shown that low-dose ionising radiation is a definite risk factor for causing cancer.
In 1978 researchers removed blood samples from 20 infants and children before and immediately after cardiac catheterization for investigating their underlying heart problems. All 20 samples showed chromosomal damage following the X-ray procedure. While some believed that the damage was mainly due to the contrast agent used, others were not so sure. In 2000, Israeli researchers studied children with congenital heart anomalies who underwent cardiac catheterization in 3 major Israeli medical centres (Int J Epidemiol 2000; 3: 424 – 428). In the period 1950 to 1970, 674 children were imaged. The expected number of malignancies in all sites for these children was 4.75 but the actual observed number was 11.0 cases! Of the 11 cancer cases, the expected number of lymphomas occurring was 0.63 cases but the actual observed number of lymphomas was 4. The expected number of melanoma cases was 0.62 but the actual number was 3.0. While current technology uses lower radiation doses, cardiac catheterizations are getting more common and lasting longer because of increasing options available for performing therapeutic interventions under X-ray guidance. This will translate into more radiation exposure.
Advances in CT scanning technology have led to the development of CT colonography (CTC) to look at the colon and as a potential screening tool for the detection of colon polyp. The ultimate aim is to use CTC imaging to replace colonoscopy for colorectal cancer screening. In most western developed countries, CTC’s role and cost effectiveness is still being evaluated in a clinical trial setting. What is not studied and known is will CTC lead to an increase in cancer incidence in the screened population? I would guess that it will not be known for years to come.
Since not everyone who received ionising radiation during X-ray imaging procedures such as cardiac catheterization developed cancer, perhaps there are people who are genetically predisposed to developing cancer when exposed to ionising radiation. An interesting Israeli study tried to assess whether genetic factors can modify the risk for meningioma following ionising radiation exposure (Lancet Oncol 2007; 8: 403 – 410). They hoped to get an answer by comparing the frequency of meningiomas in families that included irradiated and non-irradiated siblings. (Ionising radiation is an established risk factor for meningioma, a form of brain tumour. Ionising radiation exposure, especially during childhood, increases the risk for meningioma 6-fold.)
Where did they get willing families with irradiated and non-irradiated siblings? Tinea capitis, a scalp fungal infection, was a particular problem among immigrant children pouring into the newly established state of Israel. In order to control this contagious infection, a treatment programme involving the delivery of a dose of X-rays to the head to induce temporary hair loss was used as standard treatment in the 1950s. Often several siblings from a family were irradiated at the same time. This treatment fell out of favour by the 1960s when the oral antifungal agent griseofulvin became available.
The researchers found 160 families in which there was an index case who had irradiation and developed meningioma (RAM), 145 families with an index case who had irradiation but did not develop meningioma, 85 families with an index case who was not irradiated but developed meningioma and 135 families with an index control case who did not have radiation or meningioma. In the RAM group, 17 families (11%) had additional 1st degree relatives who developed meningioma. In contrast, in the other 3 groups, only 1 or 2 families (1%) had additional 1st degree relatives developing meningioma. Furthermore, 22 families (10%) in the RAM group had family members with cancers in the irradiated sites (including head, neck and chest) as compared with only 9 families (5%) in the irradiated but no meningioma group.
This study has shown that for a small group of individuals, ionising radiation may well have triggered their genetic predisposition to developing cancer. For most people, ionising radiation is relatively safe. However, this is not a carte blanche for indiscriminate use of X-ray studies to screen for cancers or illnesses. Not infrequently I hear patients telling me that they want this CT scan and that X-ray to reassure them that they have no health problems. They fail to appreciate that ionising radiation has potential health risks. Moreover, all imaging modalities have limitations; the cancer may well be below the detection limit of the imaging methods used. The health problem is actually there, its just that we cannot see it yet!
