Radiation Dose to the Breast by 64-slice CT

Rationale and Objectives

This work aimed to study the effects of scanner model and study protocol on radiation dose received by breast tissues from 64-slice computed tomography (CT) studies.

Materials and Methods

Four scanner models and three study protocols were used in scanning an anthropomorphic phantom with breast modules. Each protocol follows recommendations or guidelines from the American Association of Physicists in Medicine and the American College of Radiology. Twenty thermoluminescent dosimeters were placed inside the breast modules to measure breast tissue doses. Both the absolute and the normalized breast tissue doses were analyzed.

Results

The mean glandular doses of a lung cancer screening CT, a chest/abdomen/pelvis CT, and a virtual colonoscopy CT are equivalent to less than 1, 5–7, and 1–3 two-view digital mammograms, respectively, for a standard-sized patient. The normalized breast dose differs significantly ( P < 0.01) between lung cancer screening CT and chest/abdomen/pelvis CT; however, it shows less than ±10% variation among scanner models for the same protocol. In virtual colonoscopy CT, breast tissue dose decreases with the distance between local tissues to the edge of the x-ray field, although the decreasing trend varies for different scanner models and protocol settings.

Conclusions

When breasts are entirely included in the primary x-ray field, breast dose by 64-slice CT is mainly protocol dependent, with the normalized breast dose about 15% lower for protocols with modulated mA than for those with constant mA; when breasts are only partially included in the primary beam field, breast dose by 64-slice CT is dependent on both the scanner model and the protocol settings.

Introduction

In recent years, public concerns over computed tomography (CT) radiation dose and its associated risks have spurred efforts to understand, manage, and optimize patient dose from CT studies . Among these efforts, the recording and reporting of patient dose play an important role for patient management in clinical practice. Although CT dose index (CTDI), a metric directly available from scanners, has long been used in reporting scanner radiation output, its practical value is limited because of its inability to account for patient variation. Organ dose has been considered a more valuable and suitable metric to meet clinical needs; it gives physical characterization of patient-specific radiation dose and forms the basis for risk estimates.

The determination of organ dose is a challenging task. The organ is affected not only by the scanner radiation output level, usually characterized by CTDI, but also by various other factors, such as patient size, study protocol, scanner model, and x-ray energy spectrum. A number of studies have been conducted with the aim of developing a robust yet simple method with acceptable accuracy for organ dose estimation . These studies employed either experimental methods using physical phantoms or numerical methods using validated Monte Carlo programs, although the majority of them were based on the latter because of the flexibility of simulation.

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Materials and Methods

Phantom and CT Scan Protocols

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TABLE 1

Acquisition Parameters for the Lung Screening CT Protocols Used on Different Systems

GE LightSpeed VCT Siemens SOMATOM Sensation 64 Philips Brilliance 64 Toshiba Aquilion 64 kVp 120 120 120 120 mAs 20 25 18 15 Rotation time (s) 0.5 0.5 0.75 0.5 Pitch 0.969 1.0 0.673 0.828 Detector configuration 32 × 0.625 mm 64 × 0.6 mm \* 64 × 0.625 mm 64 × 0.5 mm Bow-tie filter Large body Body Body Large CTDI vol (mGy) 1.9 1.8 1.7 2.1

CT, computed tomography; CTDI vol , volume CT dose index.

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TABLE 2

Acquisition Parameters for the Chest/Abdomen/Pelvis CT Protocols Used on Different Systems

GE LightSpeed VCT Siemens SOMATOM Sensation 64 Philips Brilliance 64 Toshiba Aquilion 64 kVp 120 120 120 120 Tube current modulation technique Auto mA and Smart mA Care Dose 4D D-DOM and Z-DOM Sure Exposure Rotation time (s) 0.4 0.5 0.75 0.5 Pitch 0.984 0.9 0.891 0.828 Detector configuration 64 × 0.625 mm 64 × 0.6 mm \* 64 × 0.625 mm 64 × 0.5 mm Bow-tie filter Large body Body Body Large CTDIvol (mGy) 20.0 20.0 20.0 20.0

CT, computed tomography; CTDI vol , volume CT dose index.

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Breast Dose Measurements by Thermoluminescent Dosimeter (TLD)

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Data Analysis

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Normalizedbreastdose=AbsolutebreastdoseRegionalCTDIvol, Normalized

breast

dose

Absolute

breast

dose

Regional

CTDI

vol

which characterizes the resulting breast tissue dose per unit radiation output to the breasts, and allows comparison among protocols with different radiation output levels. The regional CTDI vol is defined as follows:

CTDIvol,regional=CTDIvol,global×meanmAforimageslicesscontainingbreasttissuesmeanmAforallimageslices, CTDI

vol

regional

CTDI

vol

global

mean

for

image

slicess

containing

breast

tissues

mean

for

all

image

slices

where CTDI vol,global is the displayed CTDI vol from the scanner, representative of the dose index averaged over the entire scanner anatomy. For protocols using tube current modulation, CTDI vol,regional is more accurate than CTDI vol,global in characterizing the radiation output used on breast tissues. For protocols using constant mA, CTDI vol,regional becomes equal to CTDI vol,global .

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Results

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TABLE 3

Acquisition Parameters for the Virtual Colonoscopy CT Protocols Used on Different Systems

GE LightSpeed VCT Siemens SOMATOM Sensation 64 Philips Brilliance 64 Toshiba Aquilion 64 kVp 120 120 120 120 mAs (supine position) 100 150 100 75 mAs (prone position) 100 25 100 75 Rotation time (s) 0.5 0.5 0.75 0.5 Pitch 1.375 1.4 0.891 1.48 Detector configuration 64 × 0.625 mm 32 × 0.6 mm \* 64 × 0.625 mm 64 × 0.5 mm Bow-tie filter Large body Body Body Large CTDI vol (mGy) 10.0 10.0 10.0 10.0

CT, computed tomography; CTDI vol , volume CT dose index.

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Discussion

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Conclusion

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Acknowledgment

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Radiation Dose to the Breast by 64-slice CT

Rationale and Objectives

Materials and Methods

Results

Conclusions

Introduction

Materials and Methods

Phantom and CT Scan Protocols

Breast Dose Measurements by Thermoluminescent Dosimeter (TLD)

Data Analysis

Results

Discussion

Conclusion

Acknowledgment

References

Further Reading

Algorithm Variability in the Estimation of Lung Nodule Volume From Phantom CT Scans Results of the QIBA 3A Public Challenge

Anomalous Coronary Arteries

Association between Breast Parenchymal Complexity and False-Positive Recall From Digital Mammography Versus Breast Tomosynthesis