Spine computed tomography doses and cancer induction.

STUDY DESIGN

Computer modeling using patient computed tomography (CT) exposure data.

OBJECTIVE

To adequately consent patients, radiation dose needs to be converted into a relative risk of inducing a cancer. This article estimates different radiation doses and their relative risk of inducing a cancer from spine CT.

SUMMARY OF BACKGROUND DATA

There has been a marked increase in imaging, particularly CT, and medical exposures make up the majority of background radiation. There is little in the literature about radiation does form spine radiograph and CT imaging.

METHOD

Based on Monte Carlo simulations and the use of software designed for CT dosimetry, the anatomic region of the spine was mapped onto a mathematical phantom. The routine CT protocol was applied with corrections made to reflect the variation in radiation exposure along the length of the spine, resulting from automatic exposure control. The effective dose was calculated for each protocol and the relative risk of cancer induction calculated.

RESULTS

Risk ratio for inducing a cancer when CT scanning the whole lumbar spine was about 1 in 3200, which was much less than the risk of CTing the whole dorsal spine (about 1 in 1800) due to the longer coverage required and the anatomic implications of scanning in the region of the cervical dorsal junction. Quantitative CT of the lumbar spine is a low dose technique with estimated effective dose about 0.1 mSv with an estimated cancer risk of 1 in 200,000 compared to a typical chest radiograph estimated effective dose of 0.02 mSv, which gives a relative risk of causing cancer of about 1 in 1,000,000. Undertaking evaluation of the dorsal and lumbar markedly reduces the amount of radiation and therefore reduces the risk, for instance the estimated effective dose of CT from L3 to L5 is about 3.5 mSv, with an estimated cancer risk of 1 in 5200.

CONCLUSION

Precise CT technique of the spine, covering the smallest area necessary to answer the clinical question, has a dramatic effect on the estimated cancer risk for individual patient. Cancer risks are summative, so spine CT imaging needs to be considered in the light of the total radiation risk to the patient over their lifetime.