Radiation Protection Dosimetry (2005), Vol. 117, No. 1–3, pp. 256–259 doi:10.1093/rpd/nci759 Advance Access published on February 3, 2006
ENTRANCES SKIN DOSE DISTRIBUTION MAPS FOR INTERVENTIONAL NEURORADIOLOGICAL PROCEDURES: A PRELIMINARY STUDY O. Rampado and R. Ropolo Struttura Complessa Fisica Sanitaria, Azienda Ospedaliera San Giovanni Battista, Corso Bramante 88, 10126 Torino, Italy Dose estimation in interventional neuroradiology can be useful to limit skin radiation injuries. The purpose of this study was to evaluate the role of entrance skin dose (ESD) maps in planning exposure condition optimisation. Thirteen cerebral angiography and five embolisation procedures were monitored, measuring ESD, dose–area product (DAP) and other operational parameters. A transmission ionisation chamber, simultaneously measuring air kerma and DAP, measured dose-related quantities. Data acquisition software collected dosimetric and geometrical data during the interventional procedure and provided a distribution map of ESD on a standard phantom digital image, with maximum value estimation. Values of 88– 1710 mGy for maximum skin dose and 16.7–343 Gy cm2 for DAP were found. These data confirm the possibility of deterministic effects during therapeutic interventional neuroradiological procedures like cerebral embolisation. ESD maps are useful to retrospectively study the exposure characteristics of a procedure and plan patient exposure optimisation.
INTRODUCTION (1)
In 2000 , the International Commission on Radiological Protection (ICRP) recommended that in interventional procedures where the maximum cumulative skin dose is estimated to be at, or above, 3 Gy (1 Gy or above for procedures likely to be repeated), the interventionist should annotate suitable body map with the estimated doses. Such annotated body map should be placed in the patient’s record. Also, the European Union and Food and Drug Administration(2,3) highlighted the need of patient dosimetry in interventional radiology, in order to optimise patient dose and to decrease the risk of skin injuries. In interventional neuroradiology, entrance skin dose (ESD) to patient can frequently reach values high enough to result in injuries like erythema or temporary epilation. ESD maps are a useful tool to study procedure characteristics. This information may be synthesised by the field concentration factor, as the ratio of the maximum ESD to the average dose on the full irradiated area, quantity defined by Vano et al.(4) for cardiology procedures and adopted for neuroradiology procedures(5). In this study, ESD distribution maps obtained during interventional neuroradiological procedures are analysed to discuss the exposure condition and to plan action for optimisation. Measurements were made by a transmission ionisation chamber, simultaneously measuring air kerma, reported at a defined distance from the focus, and dose–area product (DAP). The irradiated areas and the field concentration factors have been calculated by a software that
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process geometrical and dosimetric data during the procedure. The correlation between calculated dose values and exposure parameters is also discussed. MATERIALS AND METHODS This study was undertaken in the ‘San Giovanni Battista’ Hospital, Turin, Italy. The examinations were performed on an angiographic X-ray system (Philips Integris V 5000) with a total filtration of 3.7 mm Al. This equipment used a region of interest (ROI) fluoroscopic technique, based on the effect of a uniform filter in aluminium with a central circular hole (radius 18 mm) and an increasing thickness (maximum 12 mm) from the centre to the periphery. Dose measurements were made by a transmission ionisation chamber (Diamentor M4KDK, PTW, Freiburg) with small centre measuring field (13 mm 13 mm) and large measuring field, which provided two signal outputs: one for air kerma (Kair)(1) measure and the other for DAP. The chamber was placed next to the collimator exit port of the X-ray tube and connected to the Diamentor M4 system (PTW, Freiburg). We used the interface provided by this device to send measured data to a notebook. Geometrical parameters were manually input by a trained observer present during interventional procedures. Specially developed software processed input data, providing a real-time map of ESD and storing all the information in a database. This acquisition software made a distinction between fluoroscopic and radiographic modes, according to a threshold value of Kair rate. In this way, the dose contributions of the two exposure modes were recorded separately. An image of the Alderson Rando anthropomorphic phantom was used to show the
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SKIN DOSE MAPS FOR NEURORADIOLOGY
ESD distribution map and the position of maximum value (MESD). For the head, the skin surface of the head is presented as the projection of a cylindrical surface on a plane, in order to see all dose values in a single image. More details on the method are given in a previous paper(6). Specific processing tools were developed to analyse the dose distribution map. The dose distribution image may be segmented in order to highlight the location and the size of the regions which received doses above a defined threshold. The average pixel value and the maximum value allow calculation of the field concentration factor. The total cumulative ESD was also calculated, as the total ESD that could be obtained during the procedure, considering the radiation beam fixed over a single location, at the average focus skin distance. RESULTS AND DISCUSSION Measurements were performed during 13 cerebral angiography and 5 embolisations. The average age of patients was 54 y old (range 28–74). In Tables 1 and 2 the average, the minimum and the maximum values of DAP, fluoroscopy time, number of frames, maximum entrance skin dose (MESD), total cumulative ESD and concentration factor are shown. In one case, MESD was close to the threshold for temporary erythema. The reference Table 1. Quantities measured for diagnostic procedures (cerebral angiography). Quantity DAP Fluoroscopy time Number of frames MESD Total cumulative ESD Concentration factor
Units
Mean Minimum Maximum
Gy cm2 49.3 min 5.1 — 137 mGy 267 mGy 400 —
3.6
16.7 0.7 66 80 147
76.0 11.9 274 440 686
2.0
5.2
level for MESD of 1 Gy, indicated in ICRP Publication 85 for repeated procedures, is exceeded in four embolisation procedures. DAP and MESD for diagnostic procedures are about one-fourth of those obtained during therapeutic procedures. The main difference between the two kinds of procedures is the fluoroscopy time. Time rarely exceeds 10 min for cerebral angiography, but for embolisation, the fluoroscopy time may be in the order of 1 h. Also, the number of frames is greater for embolisation. The correlation between MESD values and these exposure parameters is explored in the Discussion section. Segmentation of dose distribution allows identifying the location and the size of regions that received doses above a defined threshold. For procedures likely to be repeated, the segmented map is useful to limit the re-exposure of areas previously irradiated with doses above the specified threshold. Figure 1 shows an example of segmentation of dose distribution, where regions with doses above 0.5 and 1 Gy are indicated. Re-exposure of these areas should be limited in case of repetition of the interventional procedure. In order to provide interventionists a rule of thumb to assess the potential of skin injuries, the correlation between exposure parameters and ESD was studied. The effect of different beam projections used was investigated by analysing the correlation between the MESD and the total cumulative ESD (Figure 2). The slope value is very similar for the two types of procedure (0.67 for angiography and 0.68 for embolisations), but the correlation is greater for angiography. For embolisation, the ratio between MESD and total cumulative ESD has a minimum value of 0.54 and a maximum value of 0.9. The error
Table 2. Quantities measured for therapeutic procedures (embolisation). Quantity DAP Fluoroscopy time Number of frames MESD Total cumulative ESD Concentration factor
Units
Mean Minimum Maximum
Gy cm2 191.7 min 35.8 — 294 mGy 1330 mGy 1910 —
4.8
124.5 21.7 214 690 1285 3.4
343 51 336 1710 2810 5.8 Figure 1. Examples of ESD distribution map.
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Figure 2. Correlation between MESD and total cumulative ESD for each interventional procedure.
Figure 4. Correlation between total cumulative ESD and number of frames.
embolisations and a value of 2.3 mGy per frame is found for angiography. For average exposure conditions, 1 min of fluoroscopy is approximately equivalent to 10 radiography frames, and 36 min of fluoroscopy results in a skin dose of up to 1 Gy. Neuroradiologists working with this equipment should consider these values in order to decrease the maximum skin dose, changing geometrical or exposure parameters during the procedure. As assessed in other studies(7), a poor correlation was observed between DAP and maximum ESD (r ¼ 0.32 for embolisations and r ¼ 0.6 for angiography).
CONCLUSIONS Figure 3. Correlation between total cumulative ESD and fluoroscopy time.
bars in the figures indicate the uncertainty of estimation (20% at the 1s level) of ESD(6). The total cumulative ESD was split in the two components, fluoroscopy and radiography, for each procedure. Correlations between these contributions and exposure parameters (fluoroscopy time and number of frames) was studied and are shown in Figures 3 and 4. The ESD increment per minute of fluoroscopy is 27.3 mGy for embolisations and 15.9 mGy for angiography. The reason for this difference is, that for embolisations, the ‘HIGH’ exposure modality is more commonly used, because a better image quality is needed to correctly position the catheter and embolisation coil. The average image intensifier diameter used for the two types of procedure is also different and this explains why a value of 3.0 mGy per frame is found for
ESD distribution maps were obtained during 18 interventional neuroradiological procedures: 13 diagnostic and 5 therapeutic. Maximum ESD observed during cerebral angiography are far lower than deterministic thresholds for skin injuries and the first reference level, of 1 Gy, indicated by the ICRP. This level has been exceeded in four embolisation procedures and, in one case, MESD was close to the threshold for temporary erythema. We have demonstrated that ESD distribution maps may be used to implement optimisation actions in order to decrease patient exposure and hence decrease the probability of skin injuries. For example, segmented images give information useful for procedures likely to be repeated. Concentration factors and ratios between maximum and total cumulative ESD have been studied, together with dose contribution from fluoroscopy and radiography. We define precautionary reference
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values in terms of time of fluoroscopy and number of frames, for the equipment used in this study. 3.
ACKNOWLEDGEMENTS The study was supported by grants from the projects ‘Ricerca Sanitaria Finalizzata anno 2003, Regione Piemonte’ and ‘Riduzione del rischio associato all’esposizione a radiazioni ionizzanti per fini medici, Compagnia di San Paolo’.
5.
REFERENCES
6.
1. International Commission on Radiological Protection. Avoidance of radiation injuries from medical interventional procedures. ICRP publication 85. Ann. ICRP 30(2) Elsevier Science, Oxford (2000). 2. European Commission. Council Directive 97/43/ Euratom on Health protection of individuals against the
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dangers of ionizing radiation in relation to medical exposure. (1997). US Food and Drug Administration: FDA Advisory. Avoidance of serious X ray induced skin injuries during fluoroscopically-guided procedures. Center for Devices and Radiological Health (1994) Vano, E., Gonzalez, L., Ten, J. I., Fernandez, J. M., Guibelalde, E. and Macaya, C. Skin dose and dose-area product values for interventional cardiology procedures. Br. J. Radiol. 74, 48–55 (2001). Theodorakou, C. and Horrocks, J. A. A study on radiation doses and irradiated areas in cerebral embolisation. Br. J. Radiol. 76, 546–552 (2003). Rampado, O. and Ropolo, R. A method for a real time estimation of entrance skin dose distribution in interventional neuroradiology. Med. Phys. 31, 2356–2361 (2004). Van De Putte, S., Verhaegen, F., Taeymans, Y. and Thierens, H. Correlation of patient skin doses in cardiac interventional radiology with dose-area product. Br. J. Radiol. 73, 504–513 (2001).
