Radiation Protection Dosimetry (2008), Vol. 128, No. 4, pp. 485–490 Advance Access publication 6 October 2007
doi:10.1093/rpd/ncm435
SCIENTIFIC NOTE
PATIENT DOSIMETRY IN INTERVENTIONAL CARDIOLOGY AT THE UNIVERSITY HOSPITAL OF OSIJEK Dario Faj1, * , Robert Steiner1, Dejan Trifunovic´2, Zlatan Faj3, Mladen Kasabasˇic´1, Dragan Kubelka2 and Zoran Brnic´4 1 University Hospital of Osijek, J. Huttlera 4, 31000 Osijek, Croatia 2 State Office for Radiation Protection, Frankopanska 11, 10000 Zagreb, Croatia 3 Croatian Electro Power Industry, F. Sˇepera 81, 31000 Osijek, Croatia 4 University Hospital Merkur, 10000 Zagreb, Croatia
Received January 15 2007, revised August 8 2007, accepted August 18 2007 The interventional cardiology was recently implemented at the University Hospital of Osijek. Patients’ absorbed doses during coronary angiography (CA) and the percutaneous transluminal coronary angioplasty (PTCA) procedures were measured and compared with published data and international standards. All patients undergoing CA or PTCA procedures during a 1month period were included in the study. Patients’ doses are expressed in terms of dose area product (DAP) per procedure. The patients’ DAPs ranged from 2.6 to 210 Gy cm2 (average of 59 Gy cm2) during CAs, and from 61 to 220 Gy cm2 (average of 120 Gy cm2) during PTCAs. Patients’ doses during CAs and PTCAs at the University Hospital of Osijek are in good agreement with the published ones. In complex cases, the radiochromic dosimetry films were used to show possible dose distributions across patient’s skin. The film dosimetry showed a limitation of using only DAP values for the estimation of skin injuries risk.
INTRODUCTION The interventional cardiology (IC) is a very common interventional radiology practice and recently there has been a substantional increase in number of procedures(1 – 4). The coronary angiography (CA) and the percutaneous transluminal coronary angioplasty (PTCA) are the procedures with the highest doses to the patient (2 – 4) and to the staff. The European Union Medical Exposures Directive 97/43/ Euroatom identify the interventional radiology (including IC) as an area of special concern5. Radiation induced skin injury has been recognised for the past decade as a potential complication of IC(6). Although there is a biological variation in radiation sensitivity of the patient, the dose of 2 Gy is considered to be the threshold for the skin erythema(6). It should be noticed that the threshold is significantly reduced if the skin was previously irradiated(6). Measurements of the patient doses are very important from the radiation safety point of view and can be a useful tool to detect problems in the clinical practice. If the doses are higher than expected, they indicate possible problems in optimisation of either equipment or procedures, or they can be related to the cardiologist’s skill(1,3). To investigate the radiation safety of the patients and possible problems in clinical practice during the recently
*Corresponding author:
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implemented IC procedures in our hospital, doses to the patients were measured. Patients’ doses were measured at the collimator level. Dose distribution measurements were done at the patient’s skin level. Comparison of those measurements has been done in two different cases. Although the radiation protection law in Croatia insists on its existence, there is still no detailed framework for the estimation of patient doses and establishment of the national diagnostic reference levels. To the best of our knowledge, this paper presents the first results of patient dose measurements in IC in Croatia.
MATERIALS AND METHODS The radiological unit Digitex Premier manufactured by Shimadzu was installed at the University Hospital of Osijek in 2005. At the cardiology department, all of the CA and PTCA procedures were followed during the 1-month period and total of 14 PTCA and 28 CA procedures was included. There were no repeated procedures on the same patient during the measurements. Out of the 42 patients included in the study, 34 were males and eight females. The average weight of the patients was 78.8 + 12.2 kg (mean + SD) and the average height was 167.1 + 8 cm. The procedures are done by a senior cardiologist and a nurse, with a cardiologist in training present at some of the procedures. A technologist is also involved, but mostly in the
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D. FAJ ET AL.
control room. Lead aprons (0.5 mm lead equivalent), thyroid collars, lead equivalent glasses with side protection, a lead protective curtain attached to the patient table and a ceiling mounted protective screen are routinely used. Since the radiological unit at our department is not equipped with an in-built dose area product (DAP) meter, the external one (Diamentor M4KDK, PTW Freiburg, Germany) was installed. It measures the DAP of the radiation impinging upon the patient. This requires determination of the DAP that enters the patient after attenuation and scattering in the patient’s couch and the mattress. Since these conditions depend on the radiological unit, we calibrated the DAP meter for our unit. The calibration has to account for the differences between the DAP displayed by the transmission chamber placed on the collimator and the DAP of the radiation impinging on the patient. These differences are not only due to the attenuation and scattering of the patient’s couch and mattress but they also include the energy dependence of the transmission chamber, inhomogeneity of the beam throughout the cross section, extra focal radiation, radiation scattered in the collimator and filters and recombination effects taking place in the transmission chamber(7). The DAP meter was calibrated for two different kilovolt values, representing the range of kilovolt usually encountered in existing practice at the department. Values of 60 and 80 kV were chosen. The calibration factor is the ratio of the DAP for the radiation which actually impinges on the patient, and the value displayed by the DAP meter: k¼
Dref Anom : DAP
The calibration factor, k, should be applied to the transmission chamber to obtain the patient’s DAP. Dref is the dose value measured by the reference chamber ( plane parallel type 23342, PTW Freiburg, Germany) on the top of the patient’s couch and mattress. Anom is the real area of the beam at the reference chamber plane. Anom was determined by exposing a radiographic film placed on the table top under the reference chamber. The set-up is illustrated in Figure 1. The absorber protects the image intensifier from direct irradiation and drives the automatic exposure control to the kilovolt values required. The distance from the tube to the table top was approximate to the one used in practice for an average patient. The distance of the image intensifier to the reference chamber was sufficient to minimise the backscatter from the copper absorber to the reference chamber. The calibration factor, k, was measured for the two beam energies and the mean value was used,
Figure 1. Set-up of the equipment for the calibration of the DAP meter.
with uncertainty as +half of the range expressed in percentage. The patient data were obtained with calibration factor k ¼ 0.884 + 5%. The DAP meter measures the entire dose received by a patient at the collimator level. Consequently, the measured value does not take into account that the beam is passing through different parts of the skin. Possible distributions of the dose across the patient’s skin are examined using radiochromic dosimetry films. Reflective Gafchromic XR films manufactured by the International Specialty Products were used. Gafchromic XR is the dosimetry film media for measuring patient’s skin dose during fluoroscopically guided medical procedures. They are very suitable because visual information, similar to the film image, is immediately available since no development and no packaging is needed. The size of the films is 1400 1700 . The useful range for measurements of dose is up to 15 Gy and the film has minimal dependence on the dose rate and photon energies between 60 and 120 keV(8). All film measurements further in the text are done using Gafchromic films. The film measurements are done only when a complex interventional procedure is expected. Then a film was put under the patient, over the mattress, with its long side transversal to the patient. From the film, the maximum dose was estimated just after the procedure, comparing the darkest area of the Gafchromic films with the comparison strip provided by the manufacturer of the film. Uncertainty of the calibration strip is +25%(8). To have more precise estimations of doses and to be able to read dose distributions from the films later, own calibration of the films had been done. Each batch of the films requires new calibration(8), so the Lot number of the batch was recorded and calibration curve has been determined as follows:
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To obtain a high dose rate at the fluoroscopic unit, using 80 kVp, the films and the reference chamber were positioned as close as possible to the focal spot of the tube. While doing this, the image of the ion chamber on the monitor was observed to assure that the chamber is well centred within the field. Then, the copper absorber was placed in the beam in the same way as in Figure 1 to increase the tube output. Obtained dose rate was 0.51 Gy min21. The approximate size of the uniform area at the centre of the beam was measured to assure proper positioning of film pieces. To produce calibration curve, 20 film pieces of 4 4 cm and labelled them were cut. One sample was not irradiated and was used as unexposed reference. Then the samples were exposed with the air kerma values from around 0.1 to 7 Gy. The air kerma was measured with the reference chamber put on the top of the film pieces. The most of the doses estimated just after the procedures from the Gafcromic films using the strip provided by the manufacturer were less or near 1 Gy, so smaller steps of delivered air kerma were chosen under 1 Gy and larger steps above 1 Gy. Absorbed doses to the skin are then obtained from air kerma by multiplying the measured air kerma by 1.06. The optical absorbance of the exposed film increases in time, but the change in absorbance approaches zero 24 h after the exposure(8). The film pieces were irradiated within 2 h so that the post-exposure density growth will be uniform for all of the films when they are measured 24 h later. The same time distance was used for scanning all of the films exposed under the patients. Then the films have been digitalised with a flatbed scanner (Lexmark X2350) in the reflection mode at 24 bit RGB mode, using 75 pixels per inch. Uniformity of the scanner as proposed(9) was checked, and it was better than 2%. First set of the pieces of the film were scanned with a black sheet of paper on the back, with automatic image adjustment on. After that automatic image adjustment was turned off and scanned the rest of the films using the same parameters. The images were saved in TIFF format. Since Gafchromic dosimetry film produces blue images, the greatest response is obtained in the red colour channel(8). Scan data obtained in RGB mode were split into individual channels to extract the data for the red channel using imaging freeware ImageJ(10). Using 256 steps of red levels (R values) and the dose delivered to the film pieces, the calibration curve was constructed (Figure 2) and used for acquiring dose distributions from the films exposed under the patients. Every film piece was scanned five times to check the reproducibility of the scanner. Though standard deviations are represented by error bars in Figure 2 they are not distinguishable from the data symbols.
Figure 2. Determined calibration curve for a batch of a XR Gafchromic films. Calibration curve is well fitted with a cubic function.
Calibration curve is fitted with a function: D ¼ 0:000138R3 þ 0:0641R2 11:349R þ 783:81: Dose distributions obtained from the films are presented using graphs with doses on the x axis, and areas of the film on the y axis. Areas with the same doses were determined using pixel sizes recalculated in square centimetres. The representation in this way is similar to the dose volume histograms used in radiotherapy. RESULTS Table 1 presents ranges and averages of DAPs for patients undergoing CA and PTCA compared to the data from the reference. Although the maximum DAP per CA procedure is 210 Gy cm2, it should be noticed that it was obtained during the CA of a very big patient (102 kg) and the next largest DAP value per CA procedure is 140 Gy cm2. The correlation between Table 1. Measurements of DAP for CA and PTCA procedures in comparison to the some of the published data. Source
This study Delichas et al. (1) Delichas et al. (1) Padovani et al. (11)
487
CA range DAP (Gy cm2)
2.6–210
CA PTCA average range DAP DAP (Gy cm2) (Gy cm2) 61– 220
PTCA average DAP (Gy cm2)
59
120
84
125.5
76.6
59.8
39.3
101.9
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body weight and DAP was analysed using linear regression and a poor correlation was found (r ¼ 0.17). In all of the Gafchromic films taken, different patterns of dose distribution can be recognised. Peak skin doses (PSD) in points, extracted from the films did not exceed the value of 1.25 Gy. Figures 3 and 4 show two of the Gafchromic films taken during the PTCA procedures. Figure 3 shows three areas of large dose across the film and in Figure 4 almost the entire dose is delivered at the
same area of the film. The DAP measurement of the procedure examined by the film shown in Figure 3 was 180 Gy cm2, and the PSD estimated from the points in the film was 0.80 Gy. The DAP value of the film shown in Figure 4 was 120 Gy cm2 and the PSD was estimated to be 1.25 Gy. It is easy to observe the inconsistency of the DAP and the PSD values in cases presented in Figures 3 and 4. Figure 5 is a comparison of the dose distributions showed in Figures 3 and 4. The curve calculated from the case where the dose is concentrated on the
Figure 3. Gafchromic film taken during the PTCA procedure. Only the region with the dose is scanned and the red image is presented. It can be seen that the dose is distributed across different parts of the skin. The DAP for this procedure was measured to be 180 Gy cm2, and the PSD read from the film was 0.80 Gy.
Figure 4. Gafchromic film taken during the PTCA procedure. Only the region with the dose is scanned and the red image is presented. Almost the entire dose is delivered to the one particular area of the skin. The DAP for this procedure was measured to be 120 Gy cm2, and the PSD read from the film was 1.25 Gy.
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Figure 5. Graph representing the dose distribution of the Gafchromic films taken during the PTCA procedures showed in Figures 3 and 4. Doses in areas calculated from the film in Figure 3 are presented with squares and from the Figure 4 by triangles.
different parts of the skin (Figure 3) show large areas of doses under the 0.5 Gy and there are areas between 0.6 and 0.8 Gy (PSD 0.8 Gy). The second curve (Figure 4) implies that almost the entire dose is concentrated in the same area. This can be concluded because in the most of the areas of the film the dose is ,0.3 Gy and there are areas with doses between 0.9 Gy and 1.25 Gy (PSD 1.25 Gy). The sums of the doses over areas in both curves represent DAPs. Calculated values were 170 against 180 Gy cm2 measured by DAP meter in Figure 3 and 140 against 120 Gy cm2 measured by DAP meter in Figure 4. Differences between DAPs measured using DAP meter and DAPs measured using Gafchromic films are due to inaccuracy of film dosimetry. Also, some of the beams missed the film under the patient, while they are obtained in DAP meter measurements. DISCUSSION Patient doses expressed in terms of DAP per procedure during CAs and PTCAs at the University Hospital of Osijek compare well with the published doses, as can be seen from Table 1(1,3,10). It is easy to observe significantly lower average values in CA than in PTCA procedures as reported before(1,3,11). Since the CA procedure is frequently a part of the PTCA, this result is expected. A simple relation between patient weight and DAP measurements was not found. It was expected because DAP variations due to differences in the complexity of nominally
identical IC procedures dominate the DAP variations due to patient size(12). The PSDs in points on patient’s skin estimated from the Gafchromic films after procedures did not exceed the value of 1.25 Gy. It is significantly under 2 Gy, which is considered to be the threshold for the detectable effect of the radiation on the skin(6). Still, this is a very high dose compared to other radiology examinations and needs special concern from the radiation safety point of view(2 – 5). The film dosimetry showed different patterns of dose distributions across the patient’s skin (Figures 3 and 4). Since the DAP meter measures in the collimator plane, it measures a cumulative dose during the procedure. The fact that dose is concentrated at different parts of the skin is not considered in the DAP result. PSDs estimated in Figure 5 show larger risk for the skin injuries in the case shown in Figure 3 than in Figure 4, though DAP (measured using DAP meter or Gafchromic film) is larger in the case shown in Figure 4. Figure 5 shows that in one case, dose is spreaded through the patient’s skin and in the other mostly concentrated in one area. The cases in Figures 3 and 4 show that the measurement using the DAP meter only, can result in the overestimation of the skin injuries risk, because not the entire dose is delivered to the particular point of the skin. The comparison of DAP meter and Gafchromic film measurements showed that patient doses measured at the collimator level should be supported with dose distribution data, especially when high dose to the patient is expected.
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Though the film measurements are not made in real time as the DAP measurements, and they are not useful to the cardiologists during the procedure, they make the cardiologist aware of different dose spatial distributions in IC. If the cardiologist has to do a procedure on a patient with previously irradiated skin, or prolong procedure because of the complications, he can try to spread the dose over the skin by working from as many as possible different angles. FUNDING This work was supported by the International atomic energy agency and the Ministry of science education and sports of the Republic of Croatia. REFERENCES 1. Delichas, M., Psarrakos, K., Molyvda-Athanassopolou, E., Giannoglou, G., Sioundas, A., Hatziioannou, K. and Papanastassiou, E. Radiation exposure to cardiologist performing interventional cardiology procedures. Eur. J. Radiol. 48, 268–273 (2003). 2. Padovani, R. and Rodella, C. A. Staff dosimetry in interventional cardiology. Radiat. Prot. Dosim. 94(1– 2), 99–103 (2001). 3. Neofotistou, V. Review of patient dosimetry in cardiology. Radiat. Prot. Dosim. 94(1– 2), 177–182 (2001). 4. Filippova, I. Patient and staff doses in radiology and cardiology in Estonia. Radiat. Prot. Dosim. 117(1– 3), 59–61 (2005).
5. European Communities. Council Directive 97/43/ Euroatom on health protection of individuals against the dangers of ionizing radiation in relation to medical exposure. Official Journal of the European Communities L180/22 (1997). 6. Miller, D. L., Balter, S., Noonan, P. T. and Georgia, J. D. Minimizing radiation-induced skin injury in interventional radiology procedures. Radiology 225, 327– 328 (2002). 7. Larsson, J. P., Persliden, J., Sandborg, M. and Carlsson, G. Transmission ionization chambers for measurements of air kerma integrated over beam area. Factors limiting the accuracy of calibration. Phys. Med. Biol. 41, 2381– 2398 (1996). 8. GafChromic XR type R radiochromic dosimetry film background information and characteristic performance data. Available on http://www.ispcorp.com/products/dosimetry/index.html. 9. Thomas, G., Chu, R. Y. L. and Rabe, F. A study of Gafchromic XR type R film response with reflective-type densitometers and economical flatbed scanners. J. Appl. Clin. Med. Phys. 4, 307–314 (2003). 10. Rasband, W. S. and ImageJ, U. S. National Institutes of Health, Bethesda, Maryland, USA (1997– 2007). Available on http://rsb.info.nih.gov/ij/. 11. Padovani, R., Novario, R. and Bernardi, G. Optimization in coronary angiography and percutaneous transluminal coronary angioplasty. Radiat. Prot. Dosim. 80(1–3), 303 –306 (1998). 12. Reay, J., Chapple, C. L. and Kotre, C. J. Is patient size important in dose determination and optimization in cardiology. Phys. Med. Biol. 48, 3843–3850 (2003).
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