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1 Clinical Physics Group, St Bartholomew's Hospital, Queen Mary University, University of London, London EC1A 7BE and 2 Clinical Physics Group, St Bartholomew's Hospital, Barts and the London NHS Trust, London EC1A 7BE, UK
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Abstract |
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Introduction |
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One IR procedure that involves high radiation doses to patients is cerebral embolisation (CE). It is used for the occlusion of aneurysms and/or arteriovenous malformations (AVMs) from the blood supply. Alternative methods are surgery and radiosurgery. The efficacy of the procedure is monitored by injecting contrast media through a catheter introduced in the femoral artery and directed into the area of interest in conjunction with fluoroscopy and DSA. Materials used for the occlusion of aneurysms are metal coils and for the occlusion of AVMs are chemical agents such as superglue. The embolisation of AVMs is likely to be repeated in a short time period and it is likely to be followed by radiosurgery.
None of the studies that has investigated the radiation doses from CE [2–9] gave any information about the irradiated area and radiation field and the number of patients was small except in the study of O'Dea et al [5].
In order to assess a radiation-induced injury of the skin, the absorbed dose and the irradiated area should be known. Entrance skin dose (ESD), which is used to assess the likely severity of deterministic effects, may be measured by means of thermoluminescent dosimeters (TLDs) attached to the patient's skin, but TLDs make point measurements and they do not give any information about the irradiated area, unless they are incorporated in a grid. Also, if the most heavily irradiated area is not known prior to the procedure there is a risk of placing the TLD outside that area. In conjunction with a TLD grid, slow films such as radiotherapy verification films can be used in order to measure the ESD and visualize the radiation field. Another method for estimating the patient's dose is the dose–area product (DAP), which is a useful parameter, but it does not give information about the ESD unless the radiation field coverage on the patient's skin and the focus to skin distance (FSD) is known throughout the procedure.
Vano et al [10] defined the field concentration factor for cardiology procedures, as the ratio of the maximum skin dose (MSD) to the average dose. The average dose is the ratio of the DAP and the total irradiated area. The field concentration factor is higher for procedures for which the radiation field is located in specific skin regions throughout the procedure than for procedures for which the radiation field is spread over different skin areas. Procedures with high concentration factors would have a higher risk of deterministic effects than those with low concentration factor. Therefore procedures of similar complexity and of the same clinical protocol can be compared and assessed by using the field concentration factor.
This study measures the ESD by means of a TLD grid, the DAP and the eye and thyroid doses for 30 patients having undergone CE and the dose to the physician's left eye. From the TLD grid and the film results, the irradiated area and the field concentration factor have been calculated. The relationship between DAP and ESD is investigated.
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Methods and materials |
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TopAbstractIntroductionMethods and materialsResultsDiscussionConclusionsReferences |
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In Table 1
the field sizes, the dose levels and pulses s-1 available are summarized.
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the parameters for the three protocols are shown.
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The TLDs were read out using a Toledo reader model 654 (D.A Pitman Limited, Surrey, UK). The TLD-100 were annealed for 1 h at 400°C and for 2 h at 100°C and then cooled at room temperature. The TLD-100H were annealed for 10 min at 240°C followed by rapid cooling. The sensitivity of both TLD-100 and TLD-100H was routinely checked by calibration using a 6 MV linear accelerator. The TLD-100 were also calibrated at diagnostic energies and a correction factor to account for the supralinear effect was also applied. TLD-100H did not require supralinear and energy correction because the supralinear region starts at 10 Gy.
The TLDs were arranged in a grid form in order to measure the dose distribution. The TLDs were mounted on two exposed films acting as holders. One film was placed on the back of the patient's head to measure the ESD from the PA plane and the other was placed on the right side of the patient's head to measure the ESD from the LAT plane.
The TLD grid was square with dimensions either (15 x 15) cm2 with TLDs placed every 3 cm or (10 x 10) cm2 with TLDs placed every 2 cm, giving a total of 36 TLDs for each grid.
TLD-100H were placed on the patient's left and right eyes and over the patient's left and right lobe of the thyroid gland.
TLD-100H was also used to measure the doctor's left eye dose for 17 cases. The left eye dose was chosen because of the position of the neuroradiologist with respect to the position of the LAT X-ray tube, which is always on the left side of the neuroradiologist. The TLD-100H was placed in a headband and attached to the neuroradiologist's forehead. The angiographic unit is equipped with ceiling suspended shielding devices, but they are rarely used by the neuroradiologists.
Visualization of the radiation field
Kodak X-Omat V, 10 in x 12 in (Eastman Kodak Co., Rochester, NY, USA) radiotherapy verification film was used to visualize the radiation field for both planes for 20 patients. Films were processed in a Dupont Cronex T-6 processor (Willmington Delaware, USA). The films were changed during each procedure and placed in the same position as the previous one. The films were changed every 200 mGy (ESDDAP) as displayed by the system so that the saturation region of the film was not reached. The films were scanned and then superimposed by software (Adobe Photoshop 5.0). After all the films were scanned and superimposed, the isodose curves, which were obtained from the TLDs, were added on the final image.
Data recorded during each procedure
During each procedure the ESDDAP, the DAP for both modes, fluoroscopy and DSA, and for both planes the technical parameters such as the tube voltage, the tube current, exposure time, magnification pulses s-1, frames s-1, DSA time and number of images were recorded.
Patient data
Measurements were made on 30 patients. In Figure 1 the number of patients that underwent coil, glue and stent embolisation are shown. Furthermore, the number of male and female patients is shown.
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Results |
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the frequency distribution of the maximum ESDTLD for both planes is shown.
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the frequency distribution of the total DAP for both planes is shown.
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the average, the median, the minimum and the maximum values of the ESDTLD, ESDDAP, DAP, fluoroscopy time, number of images and the technical parameters are shown.
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, for the PA plane the ESDTLD is within the range of 0.12 Gy to 2.81 Gy and for the LAT plane the ESDTLD is within the range of 0.03 Gy to 3.4 Gy. The mean value for the PA plane is 0.77 Gy and for the LAT plane is 0.78 Gy. From the frequency distribution for both planes (Figure 2) it may be seen that the most frequent ESDTLD values lie at the range of 0–0.5 Gy and 0.5–1 Gy. For both planes most of the doses are below 1 Gy. Comparing the two planes the LAT ESDTLD tends to be higher than the PA ESDTLD.
DAP results
For the PA plane (Table 3) the DAP is within the range of 6–166 Gy cm2 and for the LAT plane the DAP is within the range of 1–321 Gy cm2. The mean value for the PA plane is 48 Gy cm2 and for the LAT plane is 58 Gy cm2. For the PA plane, for one patient the DAP is higher than 100 Gy cm2 and for the LAT plane five patients have a DAP higher than 100 Gy cm2 and two out of five have a DAP higher than 200 Gy cm2.
From the DAP frequency distributions it may be seen for the PA plane that the most frequent values are 20–40 Gy cm2 and 40–60 Gy cm2 while for the LAT plane the most frequent DAP values are 20–40 Gy cm2. Comparing the DAP values for the two planes, the DAP for the LAT plane seems to have a wider distribution.
Relation between DAP and ESD
There is a good linear correlation between the total PA DAP and the ESDTLD with a correlation factor of r2=0.86 while for the LAT plane the correlation factor, r2=0.79, was lower. The patients for whom the PA TLD grid received a dose from the LAT plane were excluded from the calculations and four patients for whom the LAT plane received a dose from the PA plane were also excluded. The relationship between ESDTLD and DAP for the PA plane is ESDTLD=0.014 x DAP and for the LAT plane ESDTLD=0.011 x DAP.
Fluoroscopy time and number of images
The mean fluoroscopy time and the number of images are higher for the LAT plane (16 min and 222 images) than those for the PA plane (12 min and 172 images) although for the PA plane the fluoroscopy time has reached values as high as 50 min.
For the PA plane the relationship between the number of images and the total DAP is linear with a correlation factor of r2=0.73 and for the LAT plane the correlation factor is r2=0.67.
Technical parameters
The magnification for the LAT plane is 17 cm field of view (FOV) and for the PA plane is 20 cm FOV. The tube voltage is higher for the PA plane. The tube voltage for both planes is higher than that given from the DSA protocol as may be seen from Table 2.
Dose distribution
In Figures 4 and 5 typical isodose curves obtained with the TLD grid for the PA and LAT plane, respectively, are superimposed on the images taken from the films. The units in both images are in mGy. Figures 4 and 5 correspond to different patients and they have been chosen because they show in the best possible way the localization of the radiation field, the advantage of using the TLD grid with slow films for measuring the maximum ESD and for visualizing the radiation field.
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) and an area of 39 cm2 (isodose curve b) received a dose above 400 mGy for the LAT plane (Figure 5).
Table 4 shows irradiated areas in cm2, derived from the isodose distributions, for seven patients identified as A, B, C, etc. who received doses above 1 Gy for both planes. For the PA plane and for patient D, the area that exceeded the threshold for transient erythema (2 Gy) is 35 cm2 with a surrounding area of 32 cm2 receiving a dose of 1 Gy to 2 Gy. For patient B (PA plane) the area receiving a dose between 2 Gy and 3 Gy is 5 cm2 surrounded by an area of 37 cm2, which received a dose between 1 Gy and 2 Gy. For the same patient (B) and for the LAT plane, the irradiated area that received a dose above 3 Gy is 9 cm2, which lies on the threshold for temporary epilation (3 Gy) and it was surrounded by an area of 44 cm2 which received a dose within the range of 2 Gy to 3 Gy. For both planes and for the same patient (patient B), the area that received a dose above 2 Gy is 58 cm2.
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the eye doses and the thyroid gland doses for the 30 patients are presented as box plots. The box covers 50% of the values, between the 25% and 75% quartile, with the line in the box representing the median value. The lines on both sides of the box extend out to 1.5 times the interquartile range. Values that lie outside that range are plotted as points. The circles represent doses with values between 1.5 and 3 box-lengths from the 75th percentile or 25th percentile while the asterisks represent doses with values more than 3 box-lengths from the 75th percentile or 25th percentile. The maximum right eye dose was found to be 0.5 Gy.
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Discussion |
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TopAbstractIntroductionMethods and materialsResultsDiscussionConclusionsReferences |
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it may be seen that the ESDDAP is higher than the ESDTLD for both planes. The FSD that is used during the procedure is always greater than 55 cm, at which the ESDDAP is calculated, ranging from 65 cm to 75 cm. Therefore, the ESDDAP tends to overestimate the actual ESD and it gives a rough estimation of the actual ESD. There are only four patients, for the PA plane, for which the ESDTLD is higher than the ESDDAP. In those cases the PA TLD grid received a dose from the LAT plane because the right side of the PA grid overlapped the lower side of the LAT grid. For the LAT plane and for four patients the ESDTLD is higher than the ESDDAP and again this is because the LAT TLD grid received a dose from the PA plane. The ESDTLD and the DAP for the LAT plane were found to be slightly higher than for the PA plane and that was expected since the number of images and the fluoroscopy time for the LAT plane were higher than those for the PA plane. The average ESDTLD for the PA plane is 0.77 Gy and for the LAT plane 0.78 Gy, which means that CE is a high dose IR procedure.
Seven patients received an ESD above 1 Gy. For the PA plane two of these patients received an ESD above 2 Gy, which is the threshold for transient erythema [1] and three received an ESD above 1 Gy. For the LAT plane, five patients received an ESD above 1 Gy and one patient received an ESD above 3 Gy, which is the threshold for temporary epilation [1].
For both planes there is a good correlation between the ESDTLD and the total DAP, which means that if the DAP is known for each plane separately the ESDTLD may be calculated.
In this study it was also found that the major contribution to the ESD comes from the DSA mode and not from fluoroscopy. This implies that by reducing the number of images the ESD will be reduced and fluoroscopy should be used instead of DSA when DSA is not essential. In this study it was found that there is good correlation between the number of images and the total DAP, which means that by knowing total DAP the number of images can be calculated approximately. After each procedure only the total DAP and the fluoroscopy time is recorded as standard protocol at this hospital.
Dose distribution and field concentration factor
In many IR procedures the area of the maximum dose is not known prior to the procedure making necessary the use of several TLDs. When several TLDs, or a TLD grid as in this study, are used, the risk of placing the TLD outside the region of highest dose is eliminated. By using films the radiation field can be visualized giving useful information about the procedure.
From Figures 4 and 5 it may be seen that the X-ray tube was moved throughout the procedure and different field sizes were used. Moreover, the major contribution to the dose comes from radiation field A for both images and so it can be assumed that the dose is localized, which increases the risk of causing a radiation injury.
In this study two patients exceeded the threshold for transient erythema. The irradiated area for doses above 2 Gy, for the LAT plane and for the first patient (patient B) is 53 cm2 while for the second patient (patient D) and for the PA plane it is 35 cm2. If the areas for both planes and for doses above 1 Gy are added together then the total irradiated area for patient B is 116 cm2 and for patient D the total area is 201 cm2. Examining the films of these two patients and comparing the DAP values with similar DAP values and technical parameters (FSD, magnification) of other patients for whom the ESD was much lower, it can be concluded that the radiation field was localized throughout the procedure in both cases and that explains the high ESDs that these two patients received.
By using films throughout the procedure the localization of the radiation field over specific skin regions can be assessed during the procedure and if there is a risk of causing a radiation injury the clinical protocol might be altered during the procedure so that the radiation injury will be avoided.
Vano et al [10] have calculated the field concentration factors for four different cardiac facilities and identified the facilities as HC-I, HC-O, RI and RJB. In Table 5 the average field concentration factors from [10] and from this study and the number of cases are shown. For this study the concentration factor for the PA plane was 2.0 and for the LAT plane 2.3.
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Organ doses and doctor's eye dose
In most of the patients the doses to the right eye and right lobe of the thyroid gland are higher than the doses to the left eye and left lobe of the thyroid. This was expected because the X-ray tube is always on the right side of the patient's head. In most of the cases eye doses are low, but for one patient the dose was 0.5 Gy. The threshold for detectable opacity is 1 or 2 Gy but a recent paper by Vano et al [12], showed that detectable opacity can be induced at even lower doses.
The doctor's left eye average dose was found to be 0.13 mGy. In the hospital where the study was performed the number of CE procedures that a neuroradiologist performs is less than three per week. The dose does not reach 3/10 of the annual dose limit, i.e. classified worker, even if one case per day (250 each year) with such a dose is performed. There is a good relationship between the total DAP and the doctor's eye dose which means that if the total DAP is known then the doctor's left eye dose can be estimated.
Comparison with published results
In Table 6 the results from this study and from other published studies are shown. Bergeron et al [2] in a study of 8 patients reported a mean DAP of 116 Gy cm2 and a mean ESD of 0.615 Gy and a maximum ESD of 1.335 Gy by using azimuthal arrays of TLDs about the head. The ESD is close to that of the present study but not for the DAP. O'Dea et al [5] reported for a study of 522 patients high mean values of ESD for both planes compared with this study by use of an automated dosimetry system. McParland [4] has calculated the ESD from DAP values and reported a low mean value of ESD which is close to that reported by Vano et al [8]. Norbash et al [7] have reported three different values of ESDs by using different filtration and by use of tube rotation in order to reduce the ESD.
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Berthelsen and Cederblad [9] reported a maximum doctor's eye dose of 0.21 mGy while this study presents a mean doctor's eye dose of 0.13 mGy and a maximum eye dose of 0.47 mGy.
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Conclusions |
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TopAbstractIntroductionMethods and materialsResultsDiscussionConclusionsReferences |
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For one patient the irradiated area having received a dose in the range 2 Gy to 3.4 Gy was found to be 58 cm2. For the PA plane the ESD was within the range 0.12–2.81 Gy with an average value of 0.77 Gy while for the LAT plane the ESD was within the range 0.03–3.4 Gy with an average value of 0.78 Gy. Seven patients have received a dose above 1 Gy and one received a dose above 2 Gy, which is the threshold for transient erythema, and one received a dose above 3 Gy, which is the threshold for temporary epilation. The average DAP for the PA plane was 48 Gy cm2 and for the LAT plane 58 Gy cm2. There was a good correlation between the total DAP and the ESD for each plane. The equations derived may be used to estimate the skin dose when the total DAP for each plane separately is known. There was poor correlation between the total DAP and the fluoroscopy time but good correlation between the total DAP and the number of images. The doses to the right eye and to the right thyroid gland were, in most cases, higher than those to the left eye and left thyroid gland. In most of the patients the doses were kept below the thresholds for causing radiation-induced skin injuries. The dose to the doctor's left eye was kept below 3/10 of dose limit. There was a good correlation between the total DAP and the doctor's left eye dose.
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Acknowledgments |
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Received for publication August 16, 2002. Revision received April 14, 2003. Accepted for publication May 21, 2003.
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TopAbstractIntroductionMethods and materialsResultsDiscussionConclusionsReferences |
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