CT Scans Increase Cancer Risk 11/10/19

CT Scans increase the risk of cancer by 24% for one scan and 15% for each subsequent scan. This is based on a 2013 Melbourne study published in the British Medical Journal, which looked at people who had had CT scans at least a year before they developed any type of cancer.

The method used was to compare the people who had scans under Medicare in the period 1985-2005 with the Cancer registries.   It was a very large study using the whole Medicare database for Australia, hence has a lot of statistical power and credibility.  The mean time of follow up was 9.5 years, so it is possible that the numbers will go up with time.

It was also found that the effect was worse the younger the patient.

CT,( also called CAT scans) standing for Computerised Tomography, or Computerised Axial Tomography involves a narrow X ray beam shooting through the patient at many different angles as the source revolves around the patient.  A computer then adds the beams from many different directions for each little area of the body to assemble a 3D picture.  The problem is that the beam goes for longer than a normal X-ray and so the radiation dose is higher. 

Newer CTs have slightly lower radiation doses, but the best solution is to have MRIs rather than CTs. Australia has far more CT machines per head of population than other countries, and consequently has been slow to change to the newer MRIs (Magnetic Resonance Imaging) which involves no radiation at all!  In 2014 the number of CT machine per million population were Australia 56, US 44, Germany 19, NZ 18, UK 8 and Australia also had the highest rate of total cancers!

The reason for the inertia may be the fact that many doctors and radiologists are unaware of this literature, and that governments and many radiology practices have invested heavily in CT technology.

The Australian result is consistent with US Food and Drug Administration which states that ‘A CT examination with an effective dose of 10millisieverts may be associated with an increase in the possibility of fatal cancer of approximately 1 chance in 2000’.

The current limit of radiation exposure for radiographers is 20 millisieverts (mSv) per year averaged over 5 years, and not more than 50 mSv received in any one year for effective (whole body) dose. This level is set by ARPANSA, the Australian Radiation Protection and Nuclear Safety Agency.

Median doses in milliseiverts for some procedures estimated by the US National Cancer Institute for CTc were as follows with the minimum and maximum in brackets after: Routine head 2.1 (0.27-5.8), suspected stroke 14 (4.1-56), Routine chest 8.2 (1.7-24), coronary angiogram 22 (6.3-39), Routine abdomen-pelvis 15 (2-9-43), multiphase abdomen-pelvis 24 (6.4-90).  

Put another way, a Routine Head CT is the same dose as 30 chest X-rays, a stroke CT 199, a chest CT 117, a coronary angiogram 309, a routine abdo-pelvis 220 and a multiphase abdo-pelvies 442. (ncbi.nlm.nih.gov/pmc/articles/PMC4635397/)

This is a big and credible story and it is a worry that so little has changed six years after this seminal paper.

The reference is:

Mathews JD, Forsythe AV, Brady Z, et al. British Medical Journal 2013 May 21; 346:f2360

A new Masters Course in Advanced Imaging was launched at University of Western Sydney Clinical School on Saturday 26 October 9am-1pm with a lecture by the US inventor of the MRI machine, Dr Raymond Damadian, with supporting lectures on the use of MRIs and the dangers of CTs.