Cancerogenesis Risks between 64 and 320 Row Detector CT for Coronary CTA Screening

INTRODUCTION

The assessment of a new medical procedure requires comprehensive evaluation of all conceivable benefits and risks. In the context of coronary computed tomography angiography (CCTA), measurements of the absorbed radiation dose using 64 row detector CT have been assessed and results previously published.[1234] Despite publication of these earlier studies, radiation risk assessment including secondary cancer risks for CCTA screening guidelines is lacking.[5] Deterministic radiation risk assessment is accomplished by comparing radiation dose measurements associated with CCTA with known threshold numbers, whereas stochastic risk assessment requires more sophisticated calculations that are generally derived from past epidemiological low dose radiation exposures published in the biological effects of ionizing radiation (BEIR) VII report.[6] Stochastic risk assessments are therefore scarcer and are usually based on data of individual studies, which do not properly account for all available variables in equipment and techniques used in performing CCTA examinations.

We measured and recorded absorbed radiation dose in radiosensitive organs using standard CCTA protocols for 320 row detector CT. We employed MOSFET detectors to measure the radiation dose by placing these on and within an anthropomorphic phantom. The measured absorbed dose was then used to estimate cancerogenesis risks for several radiosensitive organs using stochastic risk assessment models published in the BEIR VII report. Those cancer risks were then compared with the natural cancerogenesis risk associated with more common risk factors. Finally, a scenario of US wide decennial coronary artery screening was assumed for the age group 50-70 years and cancerogenesis risk to radiosensitive organs was estimated based on the cumulative absorbed radiation dose associated with such screening. The estimated cumulative risk was then compared with the potential benefits of detecting clinically occult coronary artery disease within this hypothetically screened population to determine if risks of decennial screening would outweigh the benefits.

MATERIALS AND METHODS

Cancerogenesis risk estimation

The lifetime attributable risk (LAR) based on absorbed radiation dose measurements of radiosensitive organs (lung, breast, and thyroid exposed to radiation) was adopted from BEIR VII report tabulation (Table 1 2D-1, page 311). To provide the most conservative cancerogenesis risk assessment as possible, absorbed radiation doses for calcium scoring and contrast enhanced CCTA were added for each study and included in our analysis. Data acquired with 120 kVp tube voltage were selected as being both the average and most frequently used maximum tube voltage used in clinical practice.

Table 1 Protocol and absorbed organs dose measurements for 320 row detector scanner calculating (2) cancerogenesis risk estimates

RESULTS

We used the data from our previously published manuscript to calculate cancer risks from radiation.[2]

LAR calculations for radiosensitive organs for ages 50, 60, and 70 years in males and females are tabulated in [Table 2] and Figure 1a and b. Thyroid LAR is negligible for all ages and both genders regardless of which scanner is used. Among radiosensitive organs, all evaluated ages and both genders, LAR is otherwise lowest for female breast at age 70 (0.002%) and highest for female lung at age 50 (0.068%).

Table 2 Lifetime attributable risk for cancer induction based on prior organ dose measurements

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Figure 1

(a) Reduction in lifetime attributable risk (LAR) for breast cancer with 320 row CT vs. 64 row CT. (b) Reduction in lifetime attributable risk (LAR) for lung cancer with 320 row CT vs. 64 Slice CT.

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Cumulative cancerogenesis risk calculation and cancer-specific mortality based on the BEIR VII report for men and women assuming decennial screening between ages 50 and 70 are shown in [Table 3]. These risks are likewise negligible for the thyroid gland for both genders. Cumulative cancerogenesis risk for the recommended CCTA screening assumption using the 320 row detector CT is 0.17% for the female lung and 0.026% for the female breast. Similarly, the cumulative cancerogenesis risk for CCTA screening assumption is 0.079% for male lung using the 320 row detector CT. By comparison, common risk factors, which increase cancer risks, are as follows: 16-fold for lung cancer in persistent smokers,[8] 2-fold for breast cancer with a first degree family member with history of breast cancer,⁽⁹⁾ and 10-fold for thyroid cancer with family member with history of thyroid cancer.[10] By way of comparison, CCTA screening in a population assumed to be 18.8 million in size would prevent occurrence of sudden cardiac death in at least some of estimated 355,000 patients, 94% of which have at least one stenosis >75%.

Table 3 LAR (%) for cancer induction/mortality from decennial screening in ages 50–70 for 64/320 row detector scanners

DISCUSSION

In our study, we estimated cancerogenesis risks for a specific organ when using 320 row detector CT by measuring the amount of radiation absorbed by the organ using known methodologies. This is an improvement to earlier organ cancerogenesis estimates that solely relied on respective “in house” absorbed radiation dose measurements in association with CCTA and are therefore more anecdotal in nature. In several previously published studies, radiation absorbed dose was calculated based on the volume CT dose index (CTDI_vol) and dose length product (DLP) as proposed by the European Working Group for Guidelines on Quality Criteria for Computed Tomography.[111213141516171819] It should be noted that the CTDI is not meant to be used for dose calculations of individual patients, but rather as a quality assurance and an improvement tool that allows for dose comparisons for different types of CT scanners when the same protocol is applied, or for the same scanner if different protocols are evaluated. As such, it has no role in the calculation of absolute absorbed dose related to deterministic and stochastic risk assessment models.

In this study, we focused on absorbed radiation measurements of radiosensitive organs that are either directly within the CT radiation beam (lung, breast, and skin tissue) or exposed to nearby scattered radiation (thyroid gland). Only one study that had been previously performed with a 320 row detector CT could be included in this analysis. Yet, the stark differences of cancerogenesis risk reduction of >30% for the lung and >50% for the female breast for the 320 row detector CT compared with 64 row detector CT suggest that an epidemiologically meaningful impact is likely due to the padding effects and internal filtration improvements associated with the 320 row detector CT. These improvements can reduce the stochastic risk with future coronary artery screening, and may cause a paradigm shift when weighing risks and benefits of such a screening program. Specifically, the cancerogenesis estimates data that were calculated based on the 320 row detector CCTA would result in a combined cumulative cancer incidence of less than 1 in 500 for breast and lung cancer in women and a cumulative cancer incidence of less than 1 in 1000 for lung in men. As such, these cancerogenesis risk estimates are in magnitudes lower than common risk factors such as persistent smoking for the development of lung cancer (16-fold) or history of breast cancer in a first degree relative for the development of breast cancer (2-fold).[89] It should be noted, that cancerogenesis incidence estimates do not equate to cancer-induced mortality data. Our data may underestimate the role of other causes of death and also do not account for the capability of CCTA to detect other clinically relevant findings such as clinically occult pulmonary embolism, malignant pulmonary nodules, or other undetected cardiopulmonary abnormalities, which may also translate into improved patient outcome. In terms of expected benefits of a screening program as the one suggested, the population size that would be submitted to such screening was previously assumed to be 18.8 million people and the annual incidence of occurrences of sudden cardiac death as 355,000, 94% of which have at least one stenosis >75%.⁽⁷⁾ As such, we believe that CCTA screening along the lines suggested with use of 320 row detector scanning and associated improvements in padding and filtration techniques can be justifiably advocated. This is contradictory to prior statements that have identified absorbed radiation associated with CCTA as a primary concern to reject CCTA as a screening tool for coronary artery disease.[5] Recent availability of the iterative reconstruction technique could reduce the radiation dose while maintaining the image quality of CCTA.[2223]

Unlike for lung and female breast, higher absorbed thyroid doses and resulting stochastic risks were found to the same for the 320 row detector CT when compared with a 64 row detector CT. This discordance can be explained by differences in measurement technique. Specifically, measurements with the 320 row detector CT were performed with the MOSFET detectors on top of the phantom surface, whereas Hurwitz et al., obtained measurements within the phantom and more closely simulated the anatomic position of the thyroid gland, thus resulting in less scatter and more dose absorption of interposed tissue and hence lower absorbed dose readings of the detectors.[20] However, thyroid dose absorptions are negligibly low in both studies, and the dose that is absorbed by the thyroid gland in association with CCTA can be safely considered as clinically inconsequential as the likelihood of cancer induction is close to zero based on BEIR VII data for patients between the ages of 50-70 years.

CONCLUSION

During CCTA, among radiosensitive organs at risk, LAR is highest for the lung but this risk remains relatively small compared with other more common cancerogenic risk factors such as smoking. Compared with the 64 row detector CT, technical improvements associated with the 320 row detector CT reduce radiation dose absorbed by the organs in the field of view (lung and female breast).