Estimating cancer risks from pediatric CT: going from the qualitative to the quantitative

these discussions, and this is really our rationale – to get some sort of feel for the absolute risks that may be involved. Let me start with a bit of background. It is clear, and I think everybody in the audience knows this, that pediatric CT is different from adult CT and also from any other sort of radiological exam as our article [1] points out. The organ doses are clearly higher for children than for adults [2]. Pediatric CT is of course increasing in frequency quite rapidly and probably more so than adult CT [3], and as Eric mentioned children are much more sensitive to radiation-induced cancer than adults [4]. I will go through these differences in some detail. First, let’s talk about the organ doses. For a given set of machine parameters (including the mAss), organ doses are larger in a child compared to a (larger) adult. Consider, for example, an organ located on the proximal side of the body relative to the x-ray source. This organ will get roughly the same dose in both adult and child (Recall that dose is energy deposited divided by mass.). As the x-ray source rotates, that same organ will be on the distal side of the body relative to the x-ray source; now that organ is partly shielded by the body tissue proximal to it, reducing the organ dose. But this dose-reducing, partial shielding will be much less for a thin individual, such as a child, compared to a thicker adult. Thus organ doses for children are larger than for adults. Pediatric CT usage is rapidly increasing. The following are some very rough numbers, and it must be said there is still a need for more surveys on pediatric CT usage in the USA. In 1989, around 4% of all CTs were pediatric [5], and this rose to around 6% in 1993. Today that number is about 1 in 10, making an estimated 2.7 million pediatric CT examsper year in this country [3]. This is clearly a large increase in the use of pediatric CT. CTs contribute disproportionately to the overall radiation dose from radiological sources. Perhaps 10% of all diagnostic radiological procedures are CT procedures, but their contribution to the overall collective dose is probably 67% simply because the doses are higher [3, 6]. These numbers are from a fine paper by Mettler and colleagues published recently [3]. Although CTs are not the most common radiological exam, they are the most important to the population in terms of the dose. We also need to think about the issue of multiple CTs. Again quoting data from Mettler [3], 30% of patients who undergo CT have at least 3 scans, 7% have at least 5, and 4% have at least 9. Thus, we need to get a better feel for the average number of CTs that any given individual has, and multiply the dose by that number. Finally, we should again discuss the issue that children are clearly more sensitive to radiation than adults [4]. Figure 1 presents more recent data from the A-bomb survivors which show an even bigger age effect. The graph indicates lifetime cancer mortality risk versus age at time of exposure. There is an order of magnitude increase in risk in children versus adults and a significant sex differential, a factor of 2 difference in sensitivity between girls and boys. In general, the reason for the shape of this curve is twofold. One is that children have more time to express a cancer than do adults. Hopefully, they have their whole lives in front of them [7]. Second, it appears that children are inherently more sensitive to radiation simply because they have more dividing cells and radiation SESSION I: HELICAL CT AND CANCER RISK