- Email: [email protected]
Dose Index Analytics: More Than a Low Number
MAHADEVAPPA MAHESH, MS, PhD RICHARD L. MORIN, PhD
THE MEDICAL PHYSICS CONSULT
Dose Index Analytics: More Than a Low Number Ehsan Samei, PhD, Olav Christianson, MS
What do we know about CT dose, what is its variability across medical practices, and how can we ensure that our patients are safe, considering the complexities and heterogeneity of clinical care within and across medical facilities? The foundational reason we care about CT dose is radiation risk. The risk to an individual patient from a CT scan is likely to be very small. The exact magnitude has been very difficult to ascertain, as the estimates have been derived from data with large uncertainties. Although this uncertainty has given rise to debates, experts agree that individual patients should be exposed only to the minimum amount of radiation necessary for a clinical diagnosis. This position is consistent with the principal precept of bioethics, “primum non nocere” (first, do no harm). In the face of uncertainty, we are morally obligated to do the safest thing. One may approach minimizing patient dose by prospectively prescribing imaging protocols that deliver low dose. Although this step is necessary, it relies on the assumption that the prescribed dose matches the actual delivered dose. There are numerous documented cases in which this assumption has failed, leading to radiation overdoses causing deterministic effects. Even in the absence of deterministic effects, however, many more instances of overdosing (ie, using doses more than necessary) can go unnoticed. Without a mechanism in place to retrospectively assess actual delivered radiation doses, it is impossible to ensure that dose problems do not occur. Therefore, dose monitoring is an essential element of safe and consistent imaging practice. 832
The ACR initiated the CT Dose Index Registry in 2011 as an effort to monitor CT radiation dose. Since then, hundreds of facilities have registered with the Dose Index Registry, leading to millions of collected dose data. Although this monumental feat represents the most substantial effort to understand and manage CT radiation dose across the nation, one may ask, what is the precise use of dose data? We can direct the effort to lower patient dose. But precisely how, and is lowering dose as a goal by itself sufficient to ensure good imaging practice? To construct a meaningful dosemonitoring system, 3 major requirements must be in place. First, we need to have access to doserelevant information. The Dose Index Registry provides a mechanism to do so. Second, dose-relevant data are not the same thing as dose. “Monitoring” implies tracking individual patient dose. Even if one is to use dose aggregates, the aggregates are dependable only to the extent that they are based on individual data points that are meaningful. Finally, and most importantly, access to data—even good data—by itself is meaningless unless the information is analyzed, understood, and used as a way to improve imaging performance. Collecting data without a process to use them toward improved care or operation is inconsistent with the mandates of stewardship and evidence-based practice. To be most effective, dose monitoring should be paired with an equally necessary analytic tool to extract generalizable knowledge from the data and to put that knowledge into practice toward the betterment of clinical operation. Here we outline
a few analytic methods that can be applied to the dose data to extract meaningful knowledge toward quality improvement. Dose-monitoring data can be used to establish most meaningful reference and alert levels. Contrary to the most intuitive strategy that might come into mind, universal dose alerts are of little clinical utility (Fig. 1). Radiation doses differ among protocols and patient sizes because of differences in anatomic regions and imaging tasks. Therefore, for dose reference or alert levels to be meaningful, they should be both protocol and size specific, and that applies to both volume CT dose index and size-specific dose estimate reference levels. Reference or alert levels that ignore patient size can lead to excessive false alarms for large patients while leaving possible overdose cases for small patients unnoticed. The establishment of protocoland size-specific dose reference levels enables the identification of misexposures. Individual cases in which the doses have exceeded or have not met the threshold levels can be tagged as over- and underexposure cases, which can be processed to investigate root causes and to devise solutions to improve the consistency and quality of the imaging operation. In this process, identifying underexposure cases is as important as identifying the overexposure ones. A dose that is too low to provide sufficient diagnostic information is actually excessive, as it unnecessarily exposes a patient for the sake of questionable medical benefit. One of the most challenging aspects of protocol optimization involves standardizing dose and image quality across systems. Each manufacturer has a unique dosing strategy
ª 2014 American College of Radiology 1546-1440/14/$36.00 http://dx.doi.org/10.1016/j.jacr.2014.05.004
The Medical Physics Consult 833
Fig 1. Volume CT dose index (CTDIvol) versus patient size (A) and patient number (B) for abdominal-pelvic CT examinations on a typical commercial CT system (GE Discovery CT 750 HD; GE Medical Systems, Milwaukee, Wisconsin). The circles indicate cases in which the CTDIvol was justified given the patient size. The squares indicate cases in which an inappropriate CTDIvol was used given the patient size. The lines represent dose reference levels determined from the 95th percentiles of the current practice. Cases exceeding these reference levels correspond to large patients, for whom the higher radiation dose was warranted to generate a diagnostic image. Conversely, when patient size is considered, several cases considered meeting the dose threshold actually received radiation doses inappropriate for their size. FN ¼ false-negative; FP ¼ false-positive.
and automatic exposure control methodology. Protocols on two scanners that indicate roughly the same volume CT dose index or sizespecific dose estimate across patients can deliver dramatically different dose levels for large and small patients because of the differences in the automatic exposure control algorithms. The system dependency of dose metrics, characterized through the dose-monitoring program, can be used as a basis from which to adjust system-based protocols to achieve a higher level of consistency across systems and models. Valuable information can be gained by analyzing trends in the dose data over both short and long periods of time. Short-term trends, like differences in radiation dose as a function of time of day, reveal differences in how imaging examinations are administered between shifts. In contrast, long-term dose trends can aid in evaluating the overall efforts of the facility to work toward minimizing radiation dose. For instance, changes in imaging protocols and purchases of new equipment can be evaluated to see how much they minimize the actual radiation dose. An important use of a retrospective dose-monitoring system is to investigate whether a protocol applied to a patient matches the prescribed written protocol definition. There are many instances when such alterations are warranted to accommodate a particular examination condition (eg, increasing the rotation time to accommodate an obese patient). However, errors can also occur, such as when a scan parameter is justifiably modified for an individual patient and accidentally saved to the system as the default protocol. By comparing the doses delivered to patients against an expected value on the basis of the documented protocol definition, these mistakes can be identified and corrected. The methodologies outlined above provide a first-order analytic
834 The Medical Physics Consult
strategy that should be applied to dose-monitoring data. However, this is only the first step in the process. Additional techniques need to be developed as we further engage with the data and understand their utility. Although speculative sporadic interrogations of the data can provide beneficial value, they are neither “safe” nor efficient. We simply do not have the resources to sufficiently interrogate the vast amount of dose data to ensure
that nothing is being missed. As such, there is a need to develop streamlined processes that standardize the dose analytics in the most efficient manner. This type of meaningful summarization of the data should be a major focus of dose monitoring in the coming months and years. In any effort to manage the radiation doses from medical imaging, it is important to keep in mind that the purpose of imaging
is not dosing the patient but rather using sufficient dose for effective medical care. As such, dose monitoring should be used as a tool to manage and to right-size the patient dose, not just lower it. Meaningful dose-monitoring analytics are essential to take advantage of the wealth of information included in the dose database and use that knowledge toward that goal and toward improved clinical operation.
Ehsan Samei, PhD, and Olav Christianson, MS, are from Carl E. Ravin Advanced Imaging Laboratories, Duke Clinical Imaging Physics Group, Department of Radiology, Duke University Medical Center, Durham, North Carolina. Ehsan Samei, PhD, Carl E. Ravin Advanced Imaging Laboratories, Department of Radiology, Duke University Medical Center, Durham, NC 27710; e-mail: [email protected].
THE MEDICAL PHYSICS CONSULT
Dose Index Analytics: More Than a Low Number Ehsan Samei, PhD, Olav Christianson, MS
What do we know about CT dose, what is its variability across medical practices, and how can we ensure that our patients are safe, considering the complexities and heterogeneity of clinical care within and across medical facilities? The foundational reason we care about CT dose is radiation risk. The risk to an individual patient from a CT scan is likely to be very small. The exact magnitude has been very difficult to ascertain, as the estimates have been derived from data with large uncertainties. Although this uncertainty has given rise to debates, experts agree that individual patients should be exposed only to the minimum amount of radiation necessary for a clinical diagnosis. This position is consistent with the principal precept of bioethics, “primum non nocere” (first, do no harm). In the face of uncertainty, we are morally obligated to do the safest thing. One may approach minimizing patient dose by prospectively prescribing imaging protocols that deliver low dose. Although this step is necessary, it relies on the assumption that the prescribed dose matches the actual delivered dose. There are numerous documented cases in which this assumption has failed, leading to radiation overdoses causing deterministic effects. Even in the absence of deterministic effects, however, many more instances of overdosing (ie, using doses more than necessary) can go unnoticed. Without a mechanism in place to retrospectively assess actual delivered radiation doses, it is impossible to ensure that dose problems do not occur. Therefore, dose monitoring is an essential element of safe and consistent imaging practice. 832
The ACR initiated the CT Dose Index Registry in 2011 as an effort to monitor CT radiation dose. Since then, hundreds of facilities have registered with the Dose Index Registry, leading to millions of collected dose data. Although this monumental feat represents the most substantial effort to understand and manage CT radiation dose across the nation, one may ask, what is the precise use of dose data? We can direct the effort to lower patient dose. But precisely how, and is lowering dose as a goal by itself sufficient to ensure good imaging practice? To construct a meaningful dosemonitoring system, 3 major requirements must be in place. First, we need to have access to doserelevant information. The Dose Index Registry provides a mechanism to do so. Second, dose-relevant data are not the same thing as dose. “Monitoring” implies tracking individual patient dose. Even if one is to use dose aggregates, the aggregates are dependable only to the extent that they are based on individual data points that are meaningful. Finally, and most importantly, access to data—even good data—by itself is meaningless unless the information is analyzed, understood, and used as a way to improve imaging performance. Collecting data without a process to use them toward improved care or operation is inconsistent with the mandates of stewardship and evidence-based practice. To be most effective, dose monitoring should be paired with an equally necessary analytic tool to extract generalizable knowledge from the data and to put that knowledge into practice toward the betterment of clinical operation. Here we outline
a few analytic methods that can be applied to the dose data to extract meaningful knowledge toward quality improvement. Dose-monitoring data can be used to establish most meaningful reference and alert levels. Contrary to the most intuitive strategy that might come into mind, universal dose alerts are of little clinical utility (Fig. 1). Radiation doses differ among protocols and patient sizes because of differences in anatomic regions and imaging tasks. Therefore, for dose reference or alert levels to be meaningful, they should be both protocol and size specific, and that applies to both volume CT dose index and size-specific dose estimate reference levels. Reference or alert levels that ignore patient size can lead to excessive false alarms for large patients while leaving possible overdose cases for small patients unnoticed. The establishment of protocoland size-specific dose reference levels enables the identification of misexposures. Individual cases in which the doses have exceeded or have not met the threshold levels can be tagged as over- and underexposure cases, which can be processed to investigate root causes and to devise solutions to improve the consistency and quality of the imaging operation. In this process, identifying underexposure cases is as important as identifying the overexposure ones. A dose that is too low to provide sufficient diagnostic information is actually excessive, as it unnecessarily exposes a patient for the sake of questionable medical benefit. One of the most challenging aspects of protocol optimization involves standardizing dose and image quality across systems. Each manufacturer has a unique dosing strategy
ª 2014 American College of Radiology 1546-1440/14/$36.00 http://dx.doi.org/10.1016/j.jacr.2014.05.004
The Medical Physics Consult 833
Fig 1. Volume CT dose index (CTDIvol) versus patient size (A) and patient number (B) for abdominal-pelvic CT examinations on a typical commercial CT system (GE Discovery CT 750 HD; GE Medical Systems, Milwaukee, Wisconsin). The circles indicate cases in which the CTDIvol was justified given the patient size. The squares indicate cases in which an inappropriate CTDIvol was used given the patient size. The lines represent dose reference levels determined from the 95th percentiles of the current practice. Cases exceeding these reference levels correspond to large patients, for whom the higher radiation dose was warranted to generate a diagnostic image. Conversely, when patient size is considered, several cases considered meeting the dose threshold actually received radiation doses inappropriate for their size. FN ¼ false-negative; FP ¼ false-positive.
and automatic exposure control methodology. Protocols on two scanners that indicate roughly the same volume CT dose index or sizespecific dose estimate across patients can deliver dramatically different dose levels for large and small patients because of the differences in the automatic exposure control algorithms. The system dependency of dose metrics, characterized through the dose-monitoring program, can be used as a basis from which to adjust system-based protocols to achieve a higher level of consistency across systems and models. Valuable information can be gained by analyzing trends in the dose data over both short and long periods of time. Short-term trends, like differences in radiation dose as a function of time of day, reveal differences in how imaging examinations are administered between shifts. In contrast, long-term dose trends can aid in evaluating the overall efforts of the facility to work toward minimizing radiation dose. For instance, changes in imaging protocols and purchases of new equipment can be evaluated to see how much they minimize the actual radiation dose. An important use of a retrospective dose-monitoring system is to investigate whether a protocol applied to a patient matches the prescribed written protocol definition. There are many instances when such alterations are warranted to accommodate a particular examination condition (eg, increasing the rotation time to accommodate an obese patient). However, errors can also occur, such as when a scan parameter is justifiably modified for an individual patient and accidentally saved to the system as the default protocol. By comparing the doses delivered to patients against an expected value on the basis of the documented protocol definition, these mistakes can be identified and corrected. The methodologies outlined above provide a first-order analytic
834 The Medical Physics Consult
strategy that should be applied to dose-monitoring data. However, this is only the first step in the process. Additional techniques need to be developed as we further engage with the data and understand their utility. Although speculative sporadic interrogations of the data can provide beneficial value, they are neither “safe” nor efficient. We simply do not have the resources to sufficiently interrogate the vast amount of dose data to ensure
that nothing is being missed. As such, there is a need to develop streamlined processes that standardize the dose analytics in the most efficient manner. This type of meaningful summarization of the data should be a major focus of dose monitoring in the coming months and years. In any effort to manage the radiation doses from medical imaging, it is important to keep in mind that the purpose of imaging
is not dosing the patient but rather using sufficient dose for effective medical care. As such, dose monitoring should be used as a tool to manage and to right-size the patient dose, not just lower it. Meaningful dose-monitoring analytics are essential to take advantage of the wealth of information included in the dose database and use that knowledge toward that goal and toward improved clinical operation.
Ehsan Samei, PhD, and Olav Christianson, MS, are from Carl E. Ravin Advanced Imaging Laboratories, Duke Clinical Imaging Physics Group, Department of Radiology, Duke University Medical Center, Durham, North Carolina. Ehsan Samei, PhD, Carl E. Ravin Advanced Imaging Laboratories, Department of Radiology, Duke University Medical Center, Durham, NC 27710; e-mail: [email protected].