Measuring Skin Dose with a Novel Wide-Area
Dosimetry Film |
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| Janice M. Campbell, MS, William Beaumont Hospital, Troy, MI and David F. Lewis, Ph.D., International Specialty Products, Wayne, NJ |
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| 1. Problem |
| The FDA (1995) has recognized the potential for skin injury from the exposure of patients to x-rays during fluoroscopically guided procedures and advised recording exposure information in the patient's file. Dosimetry methods such as dose-area-product meter, TLD and diodes, capable of measuring doses at skin injury levels, i.e. >2Gy, are unsuitable for mapping patient exposure. Without a measurement tool, patient exposure can only be estimated from factors such as x-ray output, filter factors, time and the geometry of the x-ray source and the patient. |
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| 2. Objective |
| To evaluate GAFCHROMIC® XR, a novel wide-area dosimetry film; to measure and map skin dose with the film in several clinical procedures; to compare the film results with measurements with an ion chamber and; to correlate the results with fluoro time. |
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| 3. Description of the Film |
| GAFCHROMIC® XR dosimetry films were developed to measure low-energy photons (<200keV). The film is formulated to be energy independent from 60-120keV. The Type T film used in this work has the structure shown in Figure 1. |
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| The active layer is sandwiched between transparent, yellow, polyester bases. The active layer is radiochromic. A blue color develops immediately on exposure to ionizing radiation. No darkroom processing is needed. The yellow polyester increases the visual contrast of the color change, protects the active layer from UV exposure and prevents absorption of liquids or damage from handling. |
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| 4. Radiometric Properties |
| The radiometric properties of the film were measured in an exposure system comprised of an x-ray tube/filter housing, a sample table and a Capintec PR-0.6 ion chamber. Exposure in the sample plane were uniform to ±2%. The optical densities of film samples were measured with a Nuclear Associates Radiochromic Densitometer. This densitometer measures in a narrow wavelength band optimized for the spectral absorbance of the film. |
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| 4.1 Sensitometric Response |
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| Figure 2: Sensitometric Response of GAFCHROMIC XR Type T |
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| Film samples were exposed 120kVp x-rays (2mm A1 filtration) at doses ranging from 0.3Gy to 10Gy. The net change in optical density was measured. The results in Figure 2 show that the film has a linear response up to about 3Gy and is almost linear to 10Gy. |
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| 4.2 Energy Dependence |
The energy dependence of the dosimetry film was assessed with 60, 80, 100 and 120kVp x-rays (2mm A1 filtration) at doses between 0.3Gy and 10Gy. The net density changes of the samples were measured. The results are plotted in Figure 3 and show that the responses at 80-120kVp are within 2%, i.e. energy independent. The response at 60kVp is
about 5% lower. |
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| Figure 3: Energy Dependence of GAFCHROMIC XR Type T Dosimetry |
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| 4.3 Dose Fractionation |
| Measurements were made to determine the response of the film to dose fractionation. Two samples were exposed to 5.2Gy of 120kVp x-rays (2mm A1 filtration). One sample received the dose in a single exposure. The other was exposed in four 1.3Gy increments at 30 min. intervals. The net density changes of the samples were measured. The results in Table 1 demonstrate that dose fractionation effects are absent. |
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| Total Dose, Gy |
Number of Dose Fractions |
Number of Measurements |
Average Net Density Change |
| 5.2 |
1 |
5 |
1,847 |
| 5.2 |
4 - @ 30 min. Intervals |
5 |
1,849 |
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Table 1: Response of GAFCHROMIC XR Type T Dosimetry
Film to Dose Fractionation |
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| 4.4 Dose Rate |
Film samples were exposed to 120kVp x-rays (2mm of A1 filtration) at rates of approximately 3.44, 0.35 and 0.03Gy/min. The total dose in each case was 6.9Gy. The density changes of the film samples were measured. The results are given in Table 2. The responses are within about ±1%, demonstrating dose rate independence over the
measured range. |
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| DOSE-RATE Gy/min |
AVERAGE NET ABSORBANCE |
DEVIATION FROM AVERAGE |
| 3.442 |
2.409 |
-0.9% |
| 0.351 |
2.426 |
-0.2% |
| 0.034 |
2.460 |
1.2% |
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| AVERAGE ALL RATES |
2.432 |
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Table 2: Dose Rate Dependence of GAFCHROMIC XR
Type T Dosimetry Film |
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| 5. Clinical Dosimetry Studies |
10"x12" sheets of the dosimetry film was used to measure skin dose during ten interventional procedures. Optical densities were measured with a Nuclear Associates Radiochromic Densitometer. A sheet of the film was placed in a paper cassette and positioned on the table between the patient and the x-ray tube. The placement was based on the nature of the procedure, the patient's anatomy and the x-ray tube position. The use of the film was undetectable by the patient and did not interfere in any way with the fluoroscopic image, or the completion or result of the patient exam. Since the dosimetry film was immediately adjacent to the patient, it effectively recorded the skin dose. Details of the fluoroscopic exam, e.g. anode potential, current, fluoro time, patient position, etc., were recorded for each study.
Observation of a film after the procedure showed distinct visual patterns revealing overlapping exposure fields. The highest density areas were easy to locate visually. The optical density of the film was measured in the densest areas of overlap and other exposed areas. The maximum net optical density for each film was located on the derived dose-response curve (Figure 2) and the dose value was recorded. The results are shown in Table 3. |
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| Procedure |
Fluoro Time (min) |
Maximum Net Optical Density |
Maximum Skin Dose Measured from Film (Gy) |
Maximum Skin Dose Estimated from Ion Chamber Measurements (Gy) |
Difference (Measured - Estimated) / Estimated |
| Electrophysiology / Radiofrequency Ablation |
7.7 |
0.25 |
0.70 |
0.50 |
40% |
| PTCA with 4 grafts |
9.1 |
0.21 |
0.60 |
0.62 |
-3% |
| PTCA with LAD Stent |
9.9 |
0.17 |
0.50 |
0.28 |
78% |
| PTCA with 1 graft |
11.2 |
0.09 |
0.20 |
0.76 |
-74% |
| Biliary Stent |
16.8 |
0.71 |
2.05 |
1.38 |
49% |
| R+L PTCA using BiPlane unit |
19.0 |
0.10 |
0.25 |
0.27 |
-6% |
| PTCA* with CPP |
32.3 |
0.63 |
1.80 |
0.61 |
198% |
| Uterine Artery Embolization |
42.5 |
2.22 |
7.00 |
4.11 |
70% |
| Biventricular Pacemaker insertion |
48.0 |
1.08 |
3.18 |
1.46 |
118% |
| Transjugular Intrahepatic Porto-systemic Stent |
53.9 |
1.54 |
4.60 |
4.43 |
4% |
| Average Maximum Skin Dose from all Procedures |
2.09 |
1.44 |
45% |
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| * PTCA = Percutaneous Transluminal Coronary Angioplasty |
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Table 3: Measured and Estimated Doses for
Ten Interventional Procedures |
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| In the past when an estimate of the patient's skin dose was wanted post-procedure, a separate set of measurements was required. An ion chamber was used to measure the dose rate under conditions similar to the actual clinical procedure. The maximum skin dose could then be calculated. The technique was not very accurate as it did not account for the many variables during a procedure and it could not define the location of the maximum dose. However, since the method had been used previously, it was decided to compare the maximum skin dose from film with the estimated maximum dose from separate in-air ion chamber measurements. |
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| Figure 4: Correlation between Skin Dose Measurements by Film and Estimates from Post-Procedure Ion Chamber Measurements |
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An independent measurement was made for each of the clinical studies. In air measurements were performed with a RadCal Model 9010 ion chamber placed at a known distance from the x-ray source. Exposure rates were measured under multiple beam conditions to approximate clinical procedures. The estimated dose for each procedure was then calculated by correcting the maximum in-air measurements of the corresponding unit for distance and time. These results are presented in Table 3. Figure 4 shows the estimated doses derived from the ion chamber measurements plotted against the doses measured with the film.
While the skin dose depends on the fluoroscopy time, the relationship is complex, not least because of the dynamic nature of interventional procedures. The dependence hinges on several factors including the geometrical relationships between the x-ray source and the patient, field size, dose-rate and the use of cineradiography. To assess the relationship between skin dose and fluoro time, the measured and estimated skin doses (Table 3) were compared with the corresponding fluoro time. The dose values from the films were measurements. Dose values estimated from ion chamber measurements were normalized similarly. The normalized values, plotted versus fluoro time, are shown in Figure 5. |
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Figure 5: Relationship Between Skin Dose Measurements
and Fluoroscopy Time |
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| 6. Clinical Results and Discussion |
It is difficult to assess the dose to a patient's skin from fluoroscopic procedures. While a spot measurement device such as an ion chamber, a TLD or a diode can be used to measure exposure rate for a specific set of x-ray conditions, the conditions are constantly varying during a patient exam. Recreating these conditions to obtain an estimate of patient dose post-procedure is both time-consuming and difficult. Although the film measurements and the post-procedure ion chamber dose estimates exhibit a degree of correlation (R2-value of 0.85, see Fig. 4), the data in Table 3 show that most of the post-treatment estimates significantly underestimate the actual skin doses recorded with the dosimetry film, in one case by almost 198%. Taken overall, the estimated values under-predict the skin dose by 45%.
The difficulty of estimating dose from the fluoro time is revealed in Figure 5. Dose values for film measurements have been normalized as a percentage of the average value and plotted against fluoro time. A trend line has been added. While this shows a gross correlation between the factors, the low R2 value indicates that less than 2/3 of the variability in skin dose can be explained by fluoro time, i.e. fluoro time is not a reliable estimator of skin dose.
With the variability in the positioning of the fluoroscopy beam, it is effectively impossible predict the site of maximum skin exposure and hence to place a TLD, or other spot measurement sensor to record that dose. Even with the knowledge of beam angle and position, it is difficult to accurately predict skin dose at a specific location. However, the use of a wide area dosimetry film removes this uncertainty. Areas of field overlap are very distinguishable and the area of maximum dose is readily located for measurement. |
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| 7. Conclusion |
| GAFCHROMIC® XR dosimetry film is an ideal choice for measuring skin dose during fluoroscopic procedures. It provides health professionals the means for complying with the 1995 FDA Information Paper and the 2001 CRCPD Resolution to monitor and record patient exposure during procedures with the potential to produce radiation-induced injury. The film was non-interfering to the interventionalist and the patient and was easily utilized in multiple clinical situations. Direct skin dose measurements with the film were shown to be preferable to retrospective measurements with an ion chamber and to dose estimation from the fluoro time. The only drawback in clinical use was in positioning the 10" x 12" film sheets. The x-ray field was sometimes located near an edge. 14" x 17" sheets, now available, would have been helpful. |
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| 8. References |
Food and Drug Administration (1995). Important Information for Physicians and Other Health Care Professionals "Recording Information in the Patient's Medical Record that Identifies the Potential for Serious X-Ray Induced Skin Injuries Following Fluoroscopically Guided Procedures" September 15th, 1995. http://www.fda.gov/cdrh/xrayinj.html
Conference of Radiation Control Program Directors (2001). Resolution relating to "Prevention of Unnecessary Radiation Exposure to Patients from Fluoroscopy", May 2, 2001 |
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