How DOSE assists medical physicists in identifying and analysing significant events - part 2
August 02, 2021
By Dr. rer. nat. Hans Dieter Nagel
How DOSE supports the Medical Physics Expert in identifying and analysing significant events - Introduction
Part 1: Identification of significant events according to StrlSchV Annex 14 I and II (1)
Part 2: Identification of significant events according to StrlSchV Annex 14 I (2) a and 14 II (2+3) a
Part 3: Determining the causes of dose limit exceedances in the CT application area
Part 2: Identification of significant events according to StrlSchV Annex 14 I (2) a and 14 II (2+3) a
The possibility to identify and process significant events is one of the most important func-tions of a dose management system (DMS). The criteria in the German legislation are men-tioned in the new Radiation Protection Ordinance (StrlSchV) Annex 14 . With regard to an individual person, a significant event is suspected as soon as:
- in computed tomography, the CTDIvol exceeds 120 mGy for an application to the brain and 80 mGy for an application to the body (Annex 14 I (2) a);
- for X-ray fluoroscopy, the total DAP exceeds the limit of 20,000 cGyxcm2 (also Annex 14 I (2) a);
- in the case of interventions for the purpose of examination, the total DAP exceeds the limit value of 20,000 cGyxcm2 (Annex 14 II (2) a);
- in the case of interventions for the purpose of treatment, the total DAP exceeds the limit of 50,000 cGyxcm2 associated with the occurrence of deterministic skin damage of a second or higher degree, acutely or within 21 days (Annex 14 II (3) a).
In the CBCT application area, the limits for CT or X-ray fluoroscopy apply, depending on which value was exceeded first.
DOSE provides an algorithm that applies the aforementioned criteria for identifying such significant events. In the RF and XA application areas (criteria 2-4), the total DAP is used for this purpose. However, in the CT application area (criterion 1), controversial opinions have existed since the publication of the Ordinance as to whether the criterion 1 is meant for CTDIvol per examination (i.e. study) or per individual scan series. Likewise, if it is meant per examination, what does 'CTDIvol' refer to? Until today, there is no binding clarification of this fundamental question by the legislator (Federal Ministry for the Environment, Nature Conservation and Nuclear Safety or the representative Federal Office for Radiation Protection).
Both formal and conceptual considerations support interpreting the two CT thresholds of criterion 1 as examination-related cumulative CTDI values:
- The term 'application' is defined in the German radiation law, StrSchG §5 (3), as 'technical performance of an examination with ionizing radiation or radioactive sub-stances and the reporting of the examination'; therefore, there is no indication that the limits are series-related.
- The corresponding limit values for X-ray fluoroscopy and intervention (criteria 2-4) are clearly examination-related.
- For dynamic CT scans (perfusion, bolus monitoring) in which the same area is ex-posed multiple times, the CTDIvol is reported as the product of CTDIvol of the single rotation times the number of rotations, i.e., cumulative CTDI.
- For intermittent CT interventions with multiple scans separated in time and CT exam-inations consisting of multiple scan series, multiple exposure of the same area with corresponding dose accumulation usually occurs too.
- The CTDIvol correlates with the dose of the organs in the scan area; the cumulative CTDIvol represents the organ dose, which increases accordingly with multiple expo-sure.
Originally (i.e. in the first draft of the legal document), the effective dose was intended to limit the dose of individuals. However, its determination is not standardized and therefore unsuitable for setting limits. Instead, the CTDIvol was chosen. The obvious intention of the legislator in the field of CT application is the prevention of excessive organ doses. Therefore, the interpretation as an examination-related limit value is logical. However, this interpreta-tion is at least conservative, which means on the safe side, which is usual practice in radia-tion protection. As long as a binding clarification by the legislator is pending, it is left to the medical physicist or the local supervisory authority to determine how the rule in criterion 1 should be used.
Qaelum's dose management system, DOSE, can be configured for both interpretation vari-ants by selecting the 'group type' (Fig. 1). With 'study-based notifications', monitoring is per-formed on an exam-by-exam basis using the cumulative CTDI, whereas with 'series-based notifications', monitoring is performed using the CTDIvol of each individual scan series.
However, similar to the criteria in relation to a group of persons, according to Annex 14 I+II (1) , a review is required anyway before a significant event notification is send to the su-pervisory authority. If one of the scan series already exceeds the limit value, the situation is clear. In the case of examination-related interpretation, it must be checked whether the scan areas overlap or are spatially separated from each other.
DOSE assists in the following way:
- by listing the dose contributions of the individual scan series.
- by displaying the overlap area.
Fig. 2 illustrates this using the example of a combined CCT + carotid CTA examination in which the carotid CTA incl. bolus monitoring had to be repeated. The cumulative CTDI in the body region is 98 mGy, thus above the limit, so DOSE generated a warning message. The function 'series overlap' shows that scan series and bolus monitoring of both CTA trials over-lap. Therefore, if interpreted as 'examination-related', this is a reportable significant event. If interpreted as 'series-related', on the other hand, it would not be a significant event, since even the CTDIvol of the first monitoring sequence ('tracker', series 5) with 58 mGy remains below the limit value.
For both interpretation variants, DOSE provides the information required for decision-making. However, a prerequisite for the availability of the 'series overlap' function is that the device provides an RDSR, the images of all scan series are available in the PACS or on the modality, and the image data contain the required DICOM tag ('IrradiationEventUID') for unambiguous identification of the scan series. If not, the question must be clarified by 'man-ual work' or answered conservatively, in case of missing image data in the PACS.
From my experience, reportable significant events in relation to a group of persons, accord-ing to Annex 14 I+II (1) , occur very rarely, at least in the field of CT application. At the facilities supervised by me as Medical Physics Expert (MPE), all reportable significant events in the area of CT were exceedances of the rule for individual persons (i.e. criterion 1: 120 mGy or 80 mGy limits), which were avoidable, with the exception of brain perfusion. The most common causes of excessive cumulative CTDIvol values are:
- multiphase CT examinations with non-optimized scan protocols, especially in combi-nation with bolus monitoring and ECG triggering;
- unauthorized protocol modifications by device operators, especially in bolus monitor-ing settings;
repeated bolus monitoring at the same location;
- technical limitations in bolus monitoring mode of devices from a particular manufac-turer that require dose settings more than 10times higher than usual;
- non-optimized CT intervention protocols;
- use of unfavourable operating modes for CT intervention (e.g., CT fluoroscopy).
Brain perfusion is a special case, as extensive compromises are necessary to comply with the 120 mGy limit, which would significantly affect the reliability of the method. The last na-tionwide CT survey in 2012 showed a mean value of 293 mGy for brain perfusion examina-tions , which is realistic based on my own experience. This seems to have been 'forgotten' when drafting Annex 14, especially since the German Federal Office for Radiation Protection also failed to prepare a separate Diagnostic Reference Level (DRL) for this. It is to be hoped that in the course of the clarification, brain perfusion will either be excluded or that a sepa-rate limit value will be set up for it.
The following examples show cases where excessive cumulative dose values have occurred in practice:
Example 1 in Fig. 3 was provided to me by colleagues MPEs and is from a native abdominal examination on a heavily obese patient where the device had reported a 'scan conflict' due to insufficient tube power. As a result, the device operator modified the settings for both mAs and voltage thereby increasing the original dose setting by a factor of 2.5. As a result, a CTDIvol of 119 mGy was already applied with a single scan series.
Example 2 in Fig. 4 was also provided to me by colleagues MPEs and originates from a com-bined CCT+head angio study in which the CTA incl. bolus monitoring had to be repeated. The protocol settings for the premonitoring and monitoring series, which were already higher by a factor of 3 than usual mAs (factory setting: 120 kV/20 mAs), were additionally increased by a factor of 2 to 3.3 by the operator. The dose level for bolus monitoring is thus 6 to 10 times higher than usual. The sum of all CTDI values is 1009 mGy (Head), of which 286 mGy is from the first and 453 mGy is from the second monitoring sequence.
Example 3 concerns CT devices of a certain manufacturer with technical limitations in bolus monitoring mode. With the factory protocol settings, CTDIvol values of up to 530 mGy are applied in a monitoring sequence over the full 60 s monitoring time. The protocol settings I found at the time I started my MPE activity on a device from this manufacturer were only marginally reduced, with up to 400 mGy. As a consequence, exceeding the limit was the rule. In a pulmonary embolism study, 293 mGy were applied with the monitoring sequence alone. Even after optimization in the meantime within the framework of the technical possi-bilities, the limit values continue to be exceeded on the device as soon as unfavourable cir-cumstances occur (obese patients, repeated bolus monitoring).
The examples illustrate, that dynamic and multiphase examinations, as well as changes in dose settings by the operator, are the most serious problems in the field of CT application and that dose monitoring based on the cumulative CTDIvol per examination is the method of choice for this. Regardless of how the open question of the interpretation of the criterion 1 (Annex 14 I (2) a) will be finally clarified, DOSE identifies such suspected significant event cases and also provides the information needed for decision-making as to whether this is a reportable significant event.
Identifying such cases, however, is only the first step in the processing of significant events. How DOSE helps to identify the root causes will be explained in another blog post.
Stay tuned for some educational weeks!
- Radiation Protection Ordinance, Federal Ministry for the Environment, Nature Con-servation and Nuclear Safety, Germany. https://www.bmu.de/en/law/radiation-protection-ordinance
- Part 1: Identification of significant events according to StrlSchV Annex 14 I and II (1). https://qaelum.com/news/news/part-1-identification-of-significant-events-according-to-strlschv-annex-14-i-and-ii-1-1
- A A Schegerer, H-D Nagel, G Stamm, G Adam, G Brix. Current CT practice in Germa-ny: Results and implications of a nationwide survey, Eur J Radiol, 2017 May; 90: 114-128. doi: 10.1016/j.ejrad.2017.02.021.
|Dr. Nagel worked for many years as a clinical scientist for one of the leading medical imaging manufacturers. In 2009, he founded his own company focusing on MPE services in the application areas of CT and XA.|