BEIR VII Research Articles

CT Radiation Dose: Trending in the Right Direction

Better definition of the cancer risk on the basis of patient age and clinical condition at exposure are just two small parts of the overall radiation risk equation.

Estimating cancer risks to adults undergoing body CT examinations

The purpose of the study is to estimate cancer risks from the amount of radiation used to perform body computed tomography (CT) examination. The ImPACT CT Patient Dosimetry Calculator was used to compute values of organ doses for adult body CT examinations. The radiation used to perform each examination was quantified by the dose-length product (DLP). Patient organ doses were converted into corresponding age and sex dependent cancer risks using data from BEIR VII. Results are presented for cancer risks per unit DLP and unit effective dose for 11 sensitive organs, as well as estimates of the contribution from 'other organs'. For patients who differ from a standard sized adult, correction factors based on the patient weight and antero-posterior dimension are provided to adjust organ doses and the corresponding risks. At constant incident radiation intensity, for CT examinations that include the chest, risks for females are markedly higher than those for males, whereas for examinations that include the pelvis, risks in males were slightly higher than those in females. In abdominal CT scans, risks for males and female patients are very similar. For abdominal CT scans, increasing the patient age from 20 to 80 resulted in a reduction in patient risks of nearly a factor of 5. The average cancer risk for chest/abdomen/pelvis CT examinations was ∼26 % higher than the cancer risk caused by 'sensitive organs'. Doses and radiation risks in 80 kg adults were ∼10 % lower than those in 70 kg patients. Cancer risks in body CT can be estimated from the examination DLP by accounting for sex, age, as well as patient physical characteristics.

WE‐A‐211‐03: BEIR VII: What It Does and Doesnˈt Say

Background: Over the last 2 years, there has been considerable attention focused on the potential cancer risks from medical imaging procedures, particularly with the widespread use of multislice CT. Almost all studies rely upon the BEIR VII report (Biological Effects of Ionizing Radiation panel formed under the auspices of the National Academy of Sciences) to generate an estimate of the risk of cancer from a given procedure. In doing so, most studies treat the BEIR VII report as a solid basis of scientific evidence for estimation of cancer risk. In reality, a close reading of this report shows that there are a large number of assumptions inherent in all aspects of the estimates of cancer risk — a fact that is often and clearly stated in the report.This presentation will review some of the assumptions used by the BEIR panel to calculate cancer risk, including the impact of different models (excess absolute risk vs. excess relative risk), the dose and dose rate effectiveness factor (DDREF) and relative biological effectiveness (RBE), on cancer estimates.Often lost in the discussion on medical exposure is a reference level by which exposure levels can be compared to something that unavoidable and impacts everyone on earth — background radiation. This presentation will review the consequences of applying the BEIR model to background radiation and known variations in levels of background radiation throughout the U.S., and how the cancer rate from background compares with that from medical imaging.Learning Objectives1. Understand the assumptions underlying the BEIR VII report2. Be able to compare the relative risks of medical imaging procedures to that from background radiation

SU‐E‐T‐46: Evaluation of Different Cancer Risk Models on Second Cancer Risk Calculation for Conventional Breast Cancer Radiation Therapy

Purpose: To determine the influence of different cancer risk models on second cancer risk when these models are applied to full‐body dose distribution of two conventional breast treatments computed by Monte Carlo. Methods: Two conventional breast treatments (2‐fields and 4‐fields) were simulated in a full‐body voxelised anthropomorphic phantom with a Monte Carlo model of the Siemens Primus 6MV linac. Dose distribution for each organ at risk was extracted. Linear risk models from the ICRP 103 and BEIR VII were applied to the average dose of specific organs. Both cancer incidence risk and cancer mortality risk were determined for the BEIR calculations. Risks were determined when considering all organs together and out‐of‐field organs exclusively. In addition to these linear models, cancer risks were determined with non‐linear dose risk models applied to the full body dose distribution yielding “Organ Equivalent Doses” (OED). for all models used, both the absolute risks were determined for the two treatments and the relative risk between the two treatment plans. Results: The radiation‐induced second cancer risk ranged from 0.12% to 145% for the 2‐field treatment and from 0.84% to 458% for the 4‐field treatment depending on the choice of models. The calculated risks can be much higher than the values reported by epidemiological studies (2–4%). The ratio of risks between the two treatments varied from 1.2 to 9.1. Conclusions: Risk models should be applied with great caution. Great uncertainties underlie absolute risk calculations yielding unrealistic risks; therefore many investigators use the ratio of risk to compare different treatments. However, the ratio of risk between two treatments may also greatly vary depending on the risk models used. This may explain inconsistent findings on the potential increase in risk of IMRT techniques compared to conventional treatments.This work was supported by the Swiss National Science Foundation FNRS 320000‐114101

MO‐D‐211‐01: Low Dose Molecular Breast Imaging

Background: Several new nuclear medicine‐based breast imaging techniques have emerged over the last few years. These include Positron Emission Mammography (PEM), Breast Specific Gamma Imaging (BSGI) and Molecular Breast Imaging (MBI). These techniques usually employ doses of ∼ 10 mCi F‐18 FDG in the case of PEM, or 20–30 mCi Tc‐99m sestamibi in the cases of BSGI or MBI. In addition to their potential value as adjunct diagnostic tools, these techniques hold promise as potential adjuncts to screening mammography particularly for women with dense breast tissue where the sensitivity of mammography is known to be reduced. A previous study has shown MBI to have 2–3 times the sensitivity and equivalent specificity to screening mammography in women with dense breasts. However, the current administered doses employed in these techniques deliver effective radiation doses that are ∼10 times higher than mammography.From the BEIR VII tables, the lifetime associated risks from PEM, BSGI and MBI can be calculated and compared to that of screening mammography. These estimates indicate that in order to achieve a comparable radiation burden to mammography, the administered doses of F‐ 18 FDG and Tc‐99m sestamibi need to be reduced to less than 2 mCi F‐18 FDG and 4 mCi Tc‐99m sestamibi.This presentation will briefly review the clinical literature demonstrating the value of these new techniques in both diagnostic and screening applications, and will review the dosimetry and estimated lifetime associated risks from these technologies. It will then focus on one of these technologies — MBI, and will illustrate how we can optimize the physics of the collimation and detector to achieve a 5‐fold reduction in the required dose of Tc‐99m sestamibi for clinical studies. We will also review possible post‐acquisition processing techniques that may enable further reduction in dose, along with alternative radiopharmaceuticals that deliver low doses due to their biodistribution.A total of 4 dose reduction schemes will be discussed — a) development of optimized collimation for single‐photon imaging, b) improved utilization of the energy spectral characteristics of the gamma cameras, c) implementation of noise reduction and resolution recovery algorithms, and d) use of adaptive geometric mean algorithms for combination of images from opposing detectors. In addition we will consider alternative radiopharmaceuticals, such as Tc‐99m MDP that deliver a lower radiation burden than Tc‐99m sestamibi.We will demonstrate that image quality obtained with low‐dose MBI performed with 4 mCi Tc‐99m sestamibi matches that of standard MBI performed with 20 mCi dose. Low‐dose MBI presents radiation risks to the patient comparable to that of digital screening mammography, allowing its safe implementation as a screening technique.

Risk of Low-Level Ionizing Radiation from Medical Imaging Procedures

Medical imaging procedures with x-ray, computed tomography (CT) and nuclear isotopes use ionizing radiation.1–4 On the other hand, ultrasound and magnetic resonance imaging (MRI) do not. When biological tissues are exposed to radiation, the effects are related to the type of radiation and the amount absorbed. Units of absorbed dose are grays (Gy) (joules/kilogram). One rad (radiation absorbed dose) is 0.01 Gy.5 As not all types of radiation affect tissue the same, the equivalent dose (measured in Sieverts) takes this into account. X-ray, CT, and gamma ray radiation have a “weighting factor” of 1.0, therefore with these medical procedures 1 Gy = 1 Sv. Medical imaging doses are usually measured in units of milliSieverts (mSv). In addition, one REM is 0.01 Sv (10 mSv). An average exposure from natural background radiation in the United States for an individual is ∼3–5 mSv/year.5–8 Also, a small amount of radiation exposure occurs with tobacco, the domestic water supply, building materials, and to a lesser extent televisions, and computer screens.9 The usual radiation dose of a chest radiograph is 0.02–0.1 mSv.10–12 Representative radiation doses for other radiological studies are shown in Figure 1.13–17 These medical studies contribute up to 20% of the total annual radiation exposure to the population of the United States.18 The National Research Council’s Committee to Assess Health Risks from Exposure to Low Levels of Ionizing Radiation has published their most recent report, the BEIR VII Report.19 The purpose of this report is to advise the U.S. Government on the relationship between exposure to ionizing radiation and human health. The BEIR VII Report describes a cancer risk model (linear no-threshold model) in which cancer risk diminishes proportionately to diminishing radiation exposure.

Evaluation of the dose and of the risk of cancer induction associated with the use of transmission X-ray body scanners using the Monte Carlo MCNPX code

In recent years, X–ray body scanners have been introduced at airports, penitentiaries and other places with considerable movement of people in order to combat drug trafficking, the entry of illegal materials and terrorism. However, although the application of this equipment in the national security area is indeed relevant, its use has caused a great deal of controversy, especially with regard to the doses absorbed and to the cancer induction risk associated with these exposures. The aim of this study is to use the Monte Carlo MCNPX code and the male adult voxel (MAX) and female adult voxel (FAX) phantoms, to evaluate the absorbed dose, effective dose and risk of cancer incidence attributable to exposures of individuals submitted to transmission X–ray body scanners in several projections. The effective dose values were calculated as recommended by the ICRP Publication 103 and the risk of cancer incidence were estimated through the BEIR VII Report.

Comparison of radiation exposure and associated radiation‐induced cancer risks from mammography and molecular imaging of the breasta)

Recent studies have raised concerns about exposure to low-dose ionizing radiation from medical imaging procedures. Little has been published regarding the relative exposure and risks associated with breast imaging techniques such as breast specific gamma imaging (BSGI), molecular breast imaging (MBI), or positron emission mammography (PEM). The purpose of this article was to estimate and compare the risks of radiation-induced cancer from mammography and techniques such as PEM, BSGI, and MBI in a screening environment. The authors used a common scheme for all estimates of cancer incidence and mortality based on the excess absolute risk model from the BEIR VII report. The lifetime attributable risk model was used to estimate the lifetime risk of radiation-induced breast cancer incidence and mortality. All estimates of cancer incidence and mortality were based on a population of 100 000 females followed from birth to age 80 and adjusted for the fraction that survives to various ages between 0 and 80. Assuming annual screening from ages 40 to 80 and from ages 50 to 80, the cumulative cancer incidence and mortality attributed to digital mammography, screen-film mammography, MBI, BSGI, and PEM was calculated. The corresponding cancer incidence and mortality from natural background radiation was calculated as a useful reference. Assuming a 15%-32% reduction in mortality from screening, the benefit/risk ratio for the different imaging modalities was evaluated. Using conventional doses of 925 MBq Tc-99m sestamibi for MBI and BSGI and 370 MBq F-18 FDG for PEM, the cumulative cancer incidence and mortality were found to be 15-30 times higher than digital mammography. The benefit/risk ratio for annual digital mammography was >50:1 for both the 40-80 and 50-80 screening groups, but dropped to 3:1 for the 40-49 age group. If the primary use of MBI, BSGI, and PEM is in women with dense breast tissue, then the administered doses need to be in the range 75-150 MBq for Tc-99m sestamibi and 35 MBq-70 MBq for F-18 FDG in order to obtain benefit/risk ratios comparable to those of mammography in these age groups. These dose ranges should be achievable with enhancements to current technology while maintaining a reasonable examination time. The results of the dose estimates in this study clearly indicate that if molecular imaging techniques are to be of value in screening for breast cancer, then the administered doses need to be substantially reduced to better match the effective doses of mammography.

MO‐E‐204B‐08: How Well Do Effective Doses Predict Carcinogenic Risks in Body CT?

Purpose: To compare the radiation‐induced cancer risks per unit effective dose for five types of body CT scans. Method and Materials: CT examinations investigated were chest, abdominal, pelvic, abdomen/pelvis and chest/abdomen/pelvis. Effective doses and organ doses were obtained using ImPACT CT Patient Dosimetry Calculator (version 1.0). Organ doses were converted into gender and age‐dependent patient carcinogenic risks using data in BEIR VII. Values of cancer incidence risk per unit effective dose (mSv) were obtained for males and females ranging from 20 to 80 year old. Results: For males, the highest cancer risk was 14 per 100,000 people per mSv for 20 year olds undergoing pelvic CT, and the lowest risk was 1.2 per 100,000 people per mSv for 80 year olds undergoing chest CT. For females, the highest cancer risk was 17 per 100,000 people per mSv for 20 year olds undergoing chest CT, and the lowest risk was 1.4 per 100,000 people per mSv for 80 year olds undergoing abdominal CT. The age‐averaged value (± Standard Deviation) of the ratio of the highest to lowest risk per mSv for males and females were 2.8 ± 0.2 and 2.2 ± 0.2, respectively. The average carcinogenic risk for females per mSv was about 23% higher than that for males. Increasing the patient age from 20 to 80 reduced the radiation risk by nearly a factor of five for males, and nearly a factor of six for females. Conclusion: For the same effective doses in body CT, the risk to any individual patient can vary by up to 300%. Patient age is much more important for determining patient risk than patient sex.

TH-C-201B-09: Calculating Cumulative CT Dose in the Trauma Patient Population

Purpose: Recent concerns about lifetime radiation dose from computed tomography(CT) exams has prompted a review of the CT dose delivered at our hospital a Level Itrauma center. A particular patient population of concern is trauma patients who routinely receive multiple CT exams during their stay. We sampled the trauma patient population and tracked their CT dose during the lengths of their stays to calculate their lifetime attributable risk of cancer incidence (LARi) and mortality (LARm) based on total CT dose sex and age. Methods and Materials: We selected as our population sample 100 patients chosen alphabetically by last name who were admitted under a trauma code in the month of November 2008. We collected information on the CTDI value the scan length and the anatomy scanned from the DICOM headers of all CT scans made during each patient's stay to calculate each patient's individual effective dose and cancer risk estimates (LARi and LARm using the BEIR VII data). Results: The mean effective dose (in mSv) for male and female trauma patients was 41.9 and 34.3 respectively and the maximum dose for any patient was 113.8 mSv. The mean LARm for males and females was 0.17% and 0.16% respectively. the LARm for males and females was 0.17% and 0.16% respectively. Conclusion: Based on our dose tracking information we have a better understanding of the risks of cancer incidence and mortality for our trauma patients due to our diagnostic tools which must be weighed against the risks of morbidity and mortality from the trauma incident itself. Our institution has adopted dose recording and dose tracking as an intrinisic part of each CT exam and as part of each patient's history which will allow a more comprehensive examination of the radiation dose to or patient population.

SU-GG-I-60: Radiation Risks in Nuclear Cardiac Imaging

Purpose To investigate how patient age, sex and weight affect risks of radiation induced cancer in cardiac imaging. Method and Materials Patients between 40–80 years old and weight > 40 kg undergoing cardiac imaging with 99mTc sestamibi, 99mTc tetrofosmin and 201Tl during a 5 year period (2002–2007) were reviewed. Effective doses (E) were obtained assuming a constant 40 mCi administered activity (10 mCi rest/30 mCi stress) for sestamibi/tetrofosmin and 3 mCi for 201Tl. An empirically derived correction factor (i.e., E proportional Wa) was used to calculate an effective dose corrected for patient weight. Patient effective doses were converted into age and sex dependent risks of cancer induction from radiation exposure using BEIR VII data. Results A total of 7023 procedures were selected (3801 females/3223 males). Median age for males was 61 years (46–75) and 59 years (46–74) for females (10th to 90th percentile ranges are in brackets). Median weight for males was 88.6 kg (68.6–113.6) and 78.2 kg (59.1–106.8) for females. The overall median cancer induction risk was 0.053% (0.031%–0.085%), 0.044% (0.026%–0.069%) and 0.11% (.066%–0.22%) for sestamibi, tetrofosmin and 201Tl respectively. The median female to male cancer induction risk ratio was 1.25 for 99mTc and 1.4 for 201Tl. On average for males and females, increasing weight from 10th percentile to 90th percentile reduced the cancer induction risk by factors of 1.6, 1.4 and 2.1 for sestamibi, tetrofosmin and 201Tl respectively, and the risk reduction factors for increasing age from 10th to 90th percentiles 2.2, 2.1 and 2.4 for sestamibi, tetrofosmin and 201Tl respectively. Conclusion Patient age was found to be the most significant factor affecting radiation risks, (2.2x) similar to choice of pharmaceutical (2x). 201Tl had a higher risk than 99mTc (2x). Weight had a slightly smaller influence on risks while sex related variations in risk were lower.

Cumulative patient effective dose and acute radiation-induced chromosomal DNA damage in children with congenital heart disease

BackgroundThe seventh Committee on “Biological Effects of Ionizing Radiation” (BEIR VII, 2006) underlines “the need of studies of infants who are exposed to diagnostic radiation because catheters have been placed...

SU‐FF‐I‐62: Radiation Risks to Patients with Suspected PE Undergoing Chest CT and V/Q Scans

Purpose. To compare the risks of radiation induced cancer to patients with a suspected pulmonary embolism (PE) undergoing chest CT scans with those of ventilation/perfusion (V/Q) nuclear medicine scans. Method. We determined absorbed doses to eight radiosensitive organs in adult male and female patients with ages that ranged from 20 to 80. Organ doses for a chest CT scan were determined for a Sensation 64 scanner operated at 120 kV (DLP 190 mGy‐cm) using the ImPACT spreadsheet. Corresponding doses were obtained for ventilation study (110 MBq 99mTc labeled DTPA aerosol) and a perfusion study (370 MBq 99mTc labeled MAA) using ICRP organ dose per unit administered conversion coefficients. Organ doses were converted into lifetime attributable risk factors provided by BEIR VII. Results. In 40 year old patients, cancer incidence rates in CT were 26 and 53 per 100,000 for males and females respectively. Corresponding cancer incidence rates in ventilation studies were 7 per 100,000 (males) and 9 per 100,000 (females), and in perfusion studies were 33 per 100,000 (males) and 68 per 100,000 (females). When averaged over all ages and sexes, the average cancer mortality rate was 67%. Reducing the patient age to 20 years increased the patient risk by an average of 51%, whereas increasing patient age to 80 reduced the cancer risk by an average 69%. For CT and perfusion studies, the highest risks were for the lung whereas for ventilation studies, the highest risk from irradiation of the bladder. Conclusion. Cancer radiation risks depend much more on patient demographics than on the choice of imaging modality (i.e., CT vs V/Q).

Radiation risks to lymphoma patients undergoing 18F-FDG studies

e22024 Background: To investigate radiation risks to patients undergoing 18F-FDG studies that take into account patient age, sex, and weight. Methods: We obtained patient demographics and weights of 54 consecutive patients with lymphoma who each underwent a18F-FDG PET study. The administered activity was converted into an effective dose using the patient size dependent dosimetry data provided in ICRP Publication 80. Patient effective doses were converted into estimates of the lifetime attributable risk of cancer incidence and cancer mortality using age and sex dependent risk factors provided in BEIR VII. Results: The average patient age/weight in this study was 43 years/68 kg for males and 33 years/61 kg for females. Patient effective dose are proportional to patient weight(-0.784) (r2 >0.99). The average administered activity was 410 MBq, which resulted in an average effective dose of 8.5 mSv for males and 8.9 mSv for females. The average cancer incidence was 0.71/1,000 cases for males, and 1.27/1,000 cases for females, with cancer mortality being about half the incidence. Cancer rates for the youngest patients were an order of magnitude higher than those for the oldest patients. Conclusions: Average lifetime attributable cancer risks to lymphoma patients from a single FDG-PET study is approximately 1 per 1,000, but are strongly influenced by patient age, gender and weight. Cancer risk in younger patients is a magnitude of order greater. No significant financial relationships to disclose.

Cumulative Radiation Exposure and Cancer Risk Estimates in Emergency Department Patients Undergoing Repeat or Multiple CT

The purpose of our study was to define a conservative estimate of the number of patients undergoing repeat or multiple emergency department CT studies and to quantify their cumulative CT radiation doses and lifetime attributable risk of developing cancer. We identified all patients at a tertiary care adult academic medical center with at least three emergency department visits within a 1-year period that included CT of the neck, chest, abdomen, or pelvis. For this cohort, we identified all diagnostic CT studies over the previous 7.7 years. We calculated cumulative radiation doses by summing typical effective doses of the anatomic regions scanned, and we calculated lifetime attributable risk using the population-averaged dose-to-risk conversion factor of one cancer per 1,000 patients receiving a 10-mSv dose, in accordance with the seventh Biologic Effects of Ionizing Radiation (BEIR VII) report. One hundred thirty emergency department patients met the inclusion criteria. Over the 7.7-year period, median, mean, and maximum values for the study count were 10, 13, and 70 with cumulative CT doses of 91, 122, and 579 mSv and lifetime attributable risk of one in 110, one in 82, and one in 17, respectively. Emergency department studies comprised 55% of those captured. Repeat imaging of the same study type represented at least half of the imaging for 72% of the cohort and all of the imaging for 12%. A small proportion (1.9%) of emergency department patients undergoing CT of the neck, chest, abdomen, or pelvis have high cumulative rates of multiple or repeat imaging. Collectively, this patient subgroup may have a heightened risk of developing cancer from cumulative CT radiation exposure.

Nuclear Medicine Radiation Risks

TO THE EDITOR: In recent times, there has been concern about the harmful effects of radiation from diagnostic procedures such as CT. The BEIR VII report ( [1][1] ) invoked data, largely but not entirely from atomic bomb survivors, to support the linear-no-threshold model of radiation-induced cancer

Analysis of Current Practice of CT examinations

Background. The number of CT examinations performed in Denmark increased from 14 500 examinations in 1979 to 301 617 in 2005. This implies increased radiation dose to the population. On this background, an analysis of the practice for CT examinations including potential limitations of radiation exposure and the associated risk is needed. Purposes. To analyse 1) the current use of CT in a university department compared to 1996, 2) the radiation dose and risk associated with the examinations and 3) the use of CT in Denmark since 1979. Material and methods. The administrative data of CT examinations performed in the Department of Radiology, Aarhus Sygehus, during 2005 and 1996, respectively, were obtained. Additionally national CT data were obtained from the database at the National Board of Health. Results. In 1996 1 840 patients obtained 5 538 CT examinations at Aarhus Sygehus. Their mean age was 46.7 years (0–88). The most frequent referring speciality was oncology followed by abdominal surgery and orthopaedic surgery. In 2005 3 769 patients obtained 11 216 CT examinations. They were generally older with a mean age of 56.9 years (0–97). The most frequent referring speciality was oncology followed by chest medicine and abdominal surgery. In 2005 the total effective dose was 71 043 mSv (mean 18.9 mSv/per patient). According to the BEIR VII model this radiation level corresponded to a risk for inducing a cancer in 7 patients, being fatal in half of them. The national data showed a gradual increase of the number of CT examinations from 1979 to 2005, most pronounced after year 2000 coinciding with the introduction of multi-slice CT (MSCT). Conclusion. The number of CT examinations at Aarhus Sygehus doubled during a 9 year period. The increase occured especially in middle and high age groups.

UPDATE ON LINEAR NON-THRESHOLD DOSE-RESPONSE MODEL AND IMPLICATIONS FOR DIAGNOSTIC RADIOLOGY PROCEDURES

Cancer risk estimates are used in the setting of radiation protection standards by international and national organizations, and for this purpose need to be developed for low doses of radiation. The approach has involved extrapolation from cancer mortality and incidence values at higher doses to predict the low-dose estimates. Such an extrapolation has generally involved the use of the linear non-threshold (LNT) theory. Recent reports from the National Research Council (BEIR VII) and the International Commission on Radiological Protection (ICRP) have considered the appropriateness of the use of LNT for the purposes of radiation protection standard setting. The overall conclusion from both committees was that current scientific evidence remains consistent with the LNT hypothesis, while appreciating that this might not rule out the possibility that other extrapolation models might well be valid but require further evaluation and additional research to establish their validity. The dose and dose-rate effectiveness factor (DDREF) is used for adjustment in the extrapolation from high to low doses and from high to low dose rates. The BEIR VII committee proposed a new Bayesian approach for estimating DDREF and concluded that a value of 1.5 best fit the data. This is a departure from the previously used value of 2, which is still proposed by ICRP in its most recent recommendations. The current cancer risk estimation process as utilized by ICRP and BEIR VII is used here to assess the potential risks from annual whole-body computed tomography (CT) screens using information and an approach published by Brenner and Ellington. The major conclusion is that potential radiation risks need to be considered along with the pros and cons of the detection limits of the procedure and the impact of false positives.

Abstract 688: Risk of Radiation Induced Cancer from Cardiac CT in a Real-Life Clinical Patient Population

Previous studies on lifetime cancer risks from radiation at cardiac CT are based on extrapolation of data unrelated to medical imaging and on presumptive scan protocols but not on actual patient cohorts. We aimed to determine the realistic risk of radiation induced cancer based on a real-life clinical patient population undergoing cardiac CT. We studied 104 consecutive patients undergoing 64-slice cardiac CT (Siemens). Data were obtained on patient demographics (sex, age, weight) as well as CT technique (kV, mAs, scan length, DLP) which were used to determine median values and the 10th and 90th percentiles. Organ dose estimates were obtained using the ImPACT dosimetry spreadsheet. Organ doses were converted into patient lifetime risk of developing radiation induced fatal and non-fatal cancer using the BEIR VII approach. Patient cancer risks were adjusted taking into account patient sex, age and weight, the latter being an often neglected factor influencing radiation risk. The majority of patients (62%) were males, with a median patient age of 59 years (41–72) and a median weight of 92 kg (69–116). All scans were performed at 120 kV, and the median DLP was 1124 mGy-cm (940–1500). In normal sized patients, the organs receiving the highest radiation doses were the lung (76 mGy) and the female breasts (92 mGy). The average patient lifetime risk for developing a radiation induced cancer was 0.12%, of which 90% would be fatal. Approximately 85% of the radiation risk arises from the irradiation of the lung. Patients with an age and weight at the 10th percentile, who receive a DLP at the 90th percentile would have a cancer risk double the average value. In a typical clinical population undergoing cardiac CT patients are predominantly male, heavier set, and in their 5th–7th decade of life. In such a patient population the realistic risk of radiation induced cancer from cardiac CT is 0.12%, with the risk to the most sensitive patients being about twice this value. Thus, in a real-life clinical patient cohort the realistic risk of radiation induced cancer from cardiac CT is substantially lower than previously reported for general populations. Appropriate patient selection and indication remain the most important means for radiation protection.

Quantification of risk-benefit in interventional radiology

The increased use of interventional radiological procedures brings with it an increased risk of cancer induction due to the possible high radiation levels used. This risk must be balanced against any viable alternatives and should take into account the individual risks and benefits. The purpose of this study is to quantify this risk benefit in terms of quality-adjusted life years (QALYs) of a procedure and its alternatives, including the risk due to the radiation. Uterine fibroid embolisation was chosen as a model study with data collected both on the outcomes of procedures in terms of complications and quality of life in the long-term and on radiation dose delivered to the patient. Dosimetry data were obtained from University Hospital Galway. The risk was then calculated using the BEIR VII model of dose-response for low linear energy transfer (LET) radiation. In this case, it was found that the radiological technique has distinct advantages over hysterectomy and myomectomy in terms of QALYs. The increased risk due to radiation from the procedure is small when compared with the overall benefit to the patient. Probability of stochastic effects based on the average dose per procedure carries an attributable risk (fatal and non-fatal cancer and hereditary effects) of 0.4% or a relative risk of 1.09%. The method described here can readily be adapted to other interventional procedures.