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Radiation Dose to the Female Breast from 16-MDCT Body Protocols

¹ Department of Radiology, Duke University Medical Center, DUMC 3808, Durham, NC 27710.
² Radiation Safety Division, Duke University Health System, Durham, NC 27710.

Received December 17, 2004; accepted after revision July 12, 2005.

 
Address correspondence to L. M. Hurwitz.

Abstract

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OBJECTIVE. The objective of our study was to determine the radiation dose to the female breast from current 16-MDCT body examinations.

MATERIALS AND METHODS. Metal oxide semiconductor field effect transistor (MOSFET) detectors were placed in four quadrants of the breast of a female-configured anthropomorphic phantom to determine radiation dose to the breast. Imaging was performed on a 16-MDCT scanner (LightSpeed, GE Healthcare) using current clinical protocols designed to assess pulmonary embolus (PE) (140 kVp, 380 mA, 0.8-sec rotation, 16 x 1.25 mm collimation), appendicitis (140 kVp, 340 mA, 0.5-sec rotation, 16 x 0.625 mm collimation), and renal calculus (140 kVp, 160 mA, 0.5-sec rotation, 16 x 0.625 mm collimation).

RESULTS. Radiation dose to the breast ranged from 4 to 6 cGy for the PE protocol and up to 1-2 cGy in the inferior aspect of the right breast and lateral aspect of the left breast for the appendicitis protocol. The renal calculus protocol yielded less than 150 µGy absorbed breast dose.

CONCLUSION. Current clinical chest and abdomen protocols result in vairable radiation doses to the breast. The magnitude of exposure may have implications for imaging strategies.

Keywords: breast • breast cancer • CT • radiation dose • women's imaging

Introduction

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Breast cancer is newly diagnosed in more than 200,000 women per year in the United States, where the lifetime probability of developing this cancer is 13.3% [1]. These statistics underscore the intense public awareness of this disease through widely publicized personal stories and fundraising events. Risk factors for developing breast cancer include attained age, age greater than 35 at first pregnancy, genetic predisposition, and radiation exposure. In general, the risks of radiation-induced breast cancer are related to age at exposure and total radiation dose. Analysis of atomic bomb survivors and patients exposed to therapeutic levels of radiation show that substantial exposure ( 1 Gy or Sv) increases the risk of developing breast cancer for a girl or young woman [2]. The effects of low-level radiation ( 10 cGy) are less well delineated; however, some retrospective studies report an increased risk of developing breast cancer after radiation doses as low as 1.6-75 cGy during childhood or early adulthood [3-5].

There are limitations to the existing data regarding breast dose from diagnostic radiology studies (mammography, chest radiography, single-detector CT, and early MDCT). With respect to CT, some radiation doses are estimates rather than the more accurate, directly measured, absorbed organ doses. In some studies, the information has been determined in part by using results of single-detector scanners. Given fundamental differences in the design of single- and multidetector machines, such assumptions may be inaccurate for current clinical MDCT imaging [6]. More precise data can be obtained using thermoluminescent dosimeters (TLDs) or metal oxide semiconductor field effect transistor (MOSFET) technology (or both) with an anthropomorphic phantom. MOSFET technology was first used to measure patient skin entrance doses after diagnostic radiology examinations in 1989 [7]. Its application for CT dosimetry was first reported by Yoshizumi et al. [8].

The increased utilization of diagnostic radiology, especially the increased number of CT examinations, is an important source of low-level radiation. Concern regarding the radiation dose attributable to CT is warranted [9]. In particular, radiation doses to the breast, although reported for single-detector CT and electron beam CT units [10-13], have not, to the best of our knowledge, been published for 16-MDCT scanners.

View larger version (75K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A —Anthropomorphic female phantom. Photograph shows adult female phantom (model 702, CIRS).

View larger version (14K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B —Anthropomorphic female phantom. Diagram shows detector location for absorbed radiation dose to breast from 16-MDCT body protocols. Numerals indicate clock positions.

Materials and Methods

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Phantom
An adult female anthropomorphic phantom (model 702, CIRS) was used. The phantom's specifications are as follows: height, 160 cm; weight, 55 kg; and thorax dimensions, 20 x 25 cm (Figs. 1A, 1B). The breasts for this phantom have an erect and symmetric shape and are 4.3 cm tall and 10.8 cm in diameter at the base. This phantom is made of human tissue-equivalent material and has been validated to have the same X-ray interaction as living human beings [14].

Detector Placement and Calibration Methods
In this study, a high-sensitivity MOSFET apparatus (model 1002RD, Thomson-Nielson) was used to record absorbed radiation doses to the breast [15]. Individual detectors were positioned at the inferior, lateral, superior, and medial aspects of each breast at a depth of 1 cm from the skin surface. The detectors were calibrated in air with an ion chamber (model 10x5-6, Radcal) and a monitor (model 9015, Radcal). Detector calibrations were performed with a conventional radiographic X-ray tube. An additional filtration of 0.2-mm copper was added to the X-ray tube, resulting in a simulated CT scanner beam quality of half-value layer 7.27-mm aluminum equivalent at 120 kVp [16]. The chamber correction factor was obtained from the University of Wisconsin calibration laboratory and is traceable to the National Institute of Standards and Technology (NIST). The f-factor (Roentgen-to-rad conversion factor) was determined using the following equation:

where f_tissue is the f-factor for the tissue of interest, [(µ_en)_m]_tissue and [(µ_en)_m]_air are the mass energy absorption coefficients for tissue and air, and X is the exposure. The mass energy absorption coefficients are available from the NIST Web site [17]. The millivolts-to-centigray conversion factors for individual MOSFET detectors were obtained by fitting four exposed points with a least-squares fitting routine (Prism [version 2.0, 1995], GraphPad Software, Inc.). These conversion factors were stored in the MOSFET software (AutoSense PC Software version 2-1, Thomson-Nielsen) for immediate readout after the scans.

The lower limit of detection of MOSFET is approximately 0.15 cGy with 25% uncertainty at 68.3% confidence level (1 SD).

Scanning Protocols
Each CT scan was obtained three times on a 16-MDCT scanner (LightSpeed, GE Healthcare) using our institutional MDCT protocols for PE, appendicitis, and renal calculus imaging. The protocols are described in Table 1. These specific imaging examinations and protocols were chosen because they are commonly used in young adults in whom exposure is of greater concern and for whom alternative imaging techniques are available.

Determination of Organ Dose
The effective doses for these protocols were computed according to publication 60 of the International Commission on Radiological Protection [18]. To assess the surface path of the X-ray beam, a sheet of X-ray film (X-Omat V, Eastman Kodak) was placed across the breast and was exposed during scanning with the MDCT protocols that were previously established to produce breast doses greater than 150 µGy (Figs. 2A, 2B).

View larger version (132K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A —X-ray beam pattern over breast for appendicitis protocol. Unexposed film in yellow folder overlies lower thorax and upper abdomen of phantom.

View larger version (83K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B —X-ray beam pattern over breast for appendicitis protocol. Exposed film after CT shows black lines (more exposure) when X-ray beam course is anterior to patient alternating with white lines that are produced when X-ray beam is posterior to patient. Note that MOSFET (metal oxide semiconductor field effect transistor) sensor size and location in breast do not permit direct exposure unless helical course brings X-ray beam precisely over detector. Red dots indicate location of MOSFET detectors at 9-, 6-, and 3-o'clock positions.

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View larger version (12K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3 —Bar graph shows radiation dose to female breast from pulmonary embolus protocol on 16-MDCT of chest. Numbers above each column represent absorbed dose; error bars are for one standard deviation. Rt = right, Lt = left, B = breast. Numerals in x-axis indicate clock positions.

View larger version (10K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4 —Radiation dose to female breast from appendicitis protocol on 16-MDCT of abdomen and pelvis. Numbers above each column represent absorbed dose; error bars are for one standard deviation. Rt = right, Lt = left, B = breast. Numerals in x-axis indicate clock positions.

Discussion

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Previous reports of absorbed breast dose from single-detector and electron beam chest CT examinations have shown an exposure of approximately 20 mGy [10-13]. Breast doses caused specifically by a PE protocol for chest CT, an increasingly popular examination, have not been published. Extrapolation of the doses from other chest CT protocols and different scanners is problematic. Our data show that current 16-MDCT chest protocols for detection of PE induce greater absorbed breast dose than that previously reported for general chest CT. This difference is likely due to several factors including the CT equipment itself. CT scanners produced by different manufacturers may deliver unequal radiation doses at identical or similar scan settings because of varying locations of the X-ray beam relative to the patient and different filtration devices and X-ray beam quality. Advances in technology also account for differences in radiation doses. MDCT may cause greater exposure as the number of detectors (4-8) increases [6, 19]. Differences in protocol parameters—such as increasing tube current, decreasing gantry cycle time, increasing peak kilovoltage, and decreasing pitch—with helical scanning [20, 21] produce different doses. Finally, different methods of radiation dose determination contribute to different results of dose assessment. For example, using the CT dose index (CTDI) and the dose-length product (DLP) for dose estimation can be problematic. CTDI measurements used in many MDCT scanners underestimate radiation doses as much as 20-30% as compared with MOSFET technology [22]. As with studies using older CT scanners, we found that when using 16-MDCT apparatus there was no significant dose variability within the breast with chest protocols, whereas with electron beam CT the medial breast is less exposed.

To our knowledge, this is the first report of radiation doses to the breast generated by abdominal CT. For the appendicitis protocol, the dose varied in different quadrants of the breast. This variability can be explained by our analysis of the X-ray beam motion (Fig. 4). This, not surprisingly, shows that the radiation dose to breast or any other superficial structure such as the thyroid gland will be higher when the X-ray beam is directly anterior to the structure. Obviously, the specific regions of the breast that are exposed may vary depending on breast size and the patient's position in the scanner. In any case, the uppermost slices of abdominal CT protocols include the dome of the liver and often the inferior thorax; thus, most women will have at least a portion of their breasts exposed to the radiation beam. Importantly, efforts to exclude the breasts from the scan beam may be rewarded by limiting dose to that of scatter radiation (< 150 µGy), as seen with the renal calculus protocol.

The implication of more examinations being performed with advanced scanner technology is greater radiation exposure to the breast, with a greater potential for radiation-induced breast cancer. A correlation between radiation exposure at the levels received from diagnostic imaging and increasing numbers of breast cancer in children and young women has been postulated. Using a linear no-threshold model to extrapolate atomic bomb and childhood therapeutic radiation data to a dose of less than 50 cGy, there is an increase in breast cancer in this population [23, 24]. Specifically, Land et al. [25] determined that the relative risk (RR) for developing breast cancer before age 35 is 14.6 at 1 Sv when irradiation occurs before age 20 [25]. An RR of 14.6 means that breast cancer is 14.6 times more likely than in a nonirradiated population. The portion of risk associated with radiation exposure, or excess relative risk (ERR), is 13.6 at 1 Sv. Data from the atomic bomb survivors do not permit determination of the shape of the dose-response curve at low levels of exposure.

Assuming a conservative risk model where ERR is directly proportional to dose with no threshold, the risk-versus-dose ratio is 13.6 per Sievert, or 0.136 per centigray; this assumes a quality factor of 1.0 for X-ray photons. In theory, each additional centigray of dose to the breast in women irradiated before age 20 would increase the risk of early onset breast cancer (i.e., diagnosis before the age of 35) by 13.6%. A breast dose of about 5 cGy, as measured in the PE protocol, would result in an RR of 1.68. The RR for women irradiated before age 20 decreases with increasing attained age and is about 2 at 1 Sv after age 45 [24]. The linear nonthreshold model indicates a theoretic increase in RR of about 1% per centigray after age 45, implying an RR of about 1.05 for the PE protocol. In either case, it is important to limit the radiation dose to the breast to the lowest values practicable.

Reducing radiation to the breast from MDCT can be achieved in several ways. Using a lower tube current, higher pitch, and lower peak kilovoltage will decrease exposure. Bismuth breast shields have been shown to reduce breast irradiation in adults and children [26, 27]. These shields have not been shown to have deleterious effects on the diagnostic quality of general CT examinations of the lungs and mediastinum [27]. However, they have not been directly evaluated for CT examinations of the chest for the detection of PE. Radiation to the breast during appendix imaging can be decreased by limiting the amount of coverage over the diaphragm, moving the breasts out of the field of view, or using bismuth shields.

In conclusion, radiation doses to the breast from current MDCT body protocols may yield 1-6 cGy of absorbed radiation to the breast, which is more than has previously been reported. This finding may have implications regarding imaging strategies.

References

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