Diagnostic CT radiation and cancer induction

Abstract

Background radiation makes up most of an individual’s exposure; however, medical sources are the largest manmade component and have been rising rapidly [1–3]. Whilst in the UK general radiology doses have decreased dramatically [4], it is only interventional and CT doses that have increased. CT use in particular has increased, and in 7 years one US Level 1 Trauma Centre increased the use of chest CT from 2.7 to 28.7% of blunt trauma cases [5]. The deterministic effects of radiation occur after a predictable threshold dose, e.g. opacification in the lens of the eye is followed by cataracts, or skin irradiation progressively produces transient erythema, temporary epilation, skin erythema, and telangiectasia to desquamation at increasing threshold doses. Random stochastic effects, which are not dependent on the threshold, include genetic mutations and carcinogenesis. The effect of radiation from CT scans depends on the field of view scanned and the sensitivity of the organs within the area. Highly radiosensitive organs include bone marrow, lung, stomach mucosa, thyroid and breast glandular tissue. Thyroid irradiation may cause hypothyroidism and thyroid cancers, particularly at a younger age [6]. Atomic bomb survivors, exposed to up to 4 Gy, have shown a linear and statistically highly significant radiation dose response, causing breast cancers [7]. The Computed Tomography Dose Index (CTDI) works out concentration units, expressed in mGy, for a procedure, depending on the total energy deposited in the patient divided by the mass of that section. Using Monte Carlo dosimetry [8], the CTDI value can be used to derive mean organ doses in a CT section in millisieverts (mSv). The effective dose (E) is the sum of the dose to individual organs, each organ dose being weighted according to the radiosensitivity, and can be converted numerically to estimate excess radiation risk. It has been recommended [9] that effective dose values be treated with caution as additional uncertainty is introduced by applying organ weighting factors, but nevertheless the effective dose quantity is a useful measure when received as an indication of relative total stochastic risk, that is, the random risk. The International Commission on Radiation Protection (ICRP) has set the nominal risk coefficient for cancer induction at 5.5% per sievert for adults [9]. Some feel that this relationship between dose and cancer risk is controversial [4]. There is a large variation in the calculated effective dose between X-ray units and CT scanners [10–12]. For example, in a recent UK national survey, the chest, abdomen and pelvis examination mean effective dose was 12 millisieverts (mSv) with a coefficient of variation of 34% for multislice scanners [10]. Variations in effective dose have also been reported for the same model of scanner because of protocol variations [13]. Modern multislice scanners are so fast that there is an inevitable tendency to scan a larger field of view (the length of the patient) than is absolutely necessary [3] and increased scanning of the whole cervical spine rather than level-specific examinations [14]. Estimation of life time cancer risks also varies widely P. J. Richards (*) X-ray Department, University Hospital of North Staffordshire NHS Trust, Princes Road, Hartshill, Stoke-on-Trent ST4 7LN Staffordshire, UK e-mail: paula.richards@uhns.nhs.uk