British Journal of Radiology (2004) 77, 871-875
© 2004 British Institute of Radiology
doi: 10.1259/bjr/39627007
Suitability of resin-coated photographic paper for skin dose measurement during fluoroscopically-guided X-ray procedures
E Guibelalde, PhD
1
L González, PhD
1 and
E Vañó, PhD
1,2
1 Medical Physics Group, Department of Radiology, University Complutense of Madrid, 28040 Madrid and 2 Medical Physics Service, San Carlos Hospital, 28040 Madrid, Spain
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Abstract
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The need for mapping skin doses during fluoroscopically-guided X-ray procedures has been described by a number of institutions and experts. Different large photographic or X-ray films placed on the patient's skin have been found to be useful for recording doses up to 1.0–2.0 Gy – depending on the film – and up to 15 Gy using radiochromic films. Though the upper limit of the film sensitivity is seldom exceeded during interventional procedures, the main disadvantage of the X-ray films is still the excessive sensitivity for long, high dose procedures. Radiochromic films show poor definition for doses below 0.5 Gy and are expensive. The goal of the present paper is to analyse the possibilities of using common resin-coated photographic paper for this purpose. Sensitometric curves obtained with different paper types processed in conventional X-ray film automatic processors demonstrate that some of them can be used with better results than X-ray films at a very low cost. Doses from about 10 mGy to near 3.0 Gy can be measured with good accuracy using a variety of glossy photographic papers.
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Introduction
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Among all organs at risk for patient deterministic injuries in interventional radiology and in interventional cardiology, the skin has the highest probability of damage, ranging from transient erythema to skin necrosis. Reports of those injuries are fully documented in the scientific literature [1]. International institutions and organizations with competence in this field have published documents and guidelines for patient dose assessment and dose reduction [2–4] and they have encouraged research into new methods for patient dose monitoring.
Some direct and indirect methods for estimating the local maximum skin dose (MSD) have been proposed by different authors [5]. Indirect methods are based on acquisition and processing of X-ray generator technical factors (kilovoltage peak values, tube current) and procedure factors (source-to-skin distance and projection angle, fluoroscopy time, number of frames, etc.) [6]. Direct methods use different types of dosemeters and detectors, and several of them give real-time monitoring of skin dose [6, 7]. Thermoluminescent dosemeters (TLDs) are commonly used in contact with the patient for direct dose measurements. As a method for determining MSD, the main drawback of using small TLDs is the practical impossibility of ensuring the correct position when different X-ray beam orientations are employed. Techniques for using TLDs in arrays partially solve this limitation but the complexity of handling and cost limit severely this method [8].
Mapping of skin doses is useful to determine the probability of a possible injury and its extent, to detect areas of overlapping irradiation and the possibility to obtain a permanent register of the most exposed areas. This is essential for the follow-up of patients with multiple fluoroscopy interventions [9].
To fulfil this task, large films with slow X-ray response can be used. Early uses of slow films for skin dose determinations can be found in the literature for different fluoroscopic examinations [10, 11]. The radiograph can be scanned, making it possible to obtain quick and detailed information about the most irradiated areas on the patient. Slow film is a superior maximum dose determination method as long as the MSD is below the saturation dose of the film. The measurement accuracy of MSD with slow film is about as good as the accuracy of local skin dose measurement with TLDs. Because the field areas are displayed on the film, the information content of dosimetry is extremely rich. Nearly all patient dose quantities can be derived from this information. The quality of film is well controlled, but the development process may vary between hospitals causing some variation in the useful dose range. Because the dose information at saturated areas of the film is less precise, it is useful to have a backup method for MSD estimation in hospitals where slow film is the primary method.
Several types of films can be used, including laser printer, duplicating or fine grain positive films [12]. Recently, some manufacturers have developed radiochromic films especially designed for measuring and mapping skin exposure during fluoroscopically-guided procedures in the range 0.1 Gy to 15 Gy (e.g. GAFCHROMIC® XR Type R FILM, manufactured by the Advanced Materials Group of International Specialty Products, NJ). According to Giles and Murphy [13], this film may prove to be a valuable tool for monitoring patient skin dose in interventional procedures over the range 0.2 Gy to at least 10 Gy. The main drawback is, however, the poor definition for doses below 0.5 Gy and the excessive cost (around \#8364;50 per sheet), quite expensive to be used on a routine basis.
Different large X-ray films placed on the patient's skin have been found to be useful for recording doses up to 1.0–2.0 Gy and for documenting different dose distributions. Thus, different protocols and different ways of managing radiation fields in daily practice (e.g. use of collimation and wedge filters, radiation field overlapping and so on) can be registered [14, 15]. It is very important to note that the threshold for dermal injuries is approximately 2 Gy and the effects may begin a few hours after the exposure [3]. Thus, availability of simple methods for routine estimations of peak entrance doses around that threshold value is important. In the event of doses above the threshold, the next step should be to enroll the patient in a clinical follow-up [3]. With this approach, cheap and quick methods to estimate doses with reasonable accuracy around 2 Gy will be preferred. In support of this, it must be noticed that doses >4 Gy are not likely to be found in normal practice. Thus, in a recent survey by Boer et al [6], peak entrance dose was estimated in 322 cardiac interventions with a mean value of 0.475 Gy; 13.5% of the patients received a maximum dose >2 Gy and only 1.2% of the patients received a dose >4 Gy at some parts of the skin. Also, in our experience, doses greater than or equal to 1.5 Gy are seldom achieved (only 16% of 200 monitored patients), at the interventional cardiology laboratories of the San Carlos University Hospital. The aim of the work was to investigate ways of extending the range of doses for using films at a reasonable cost in daily practice. Although doses above 4 Gy are rare, these are the doses we need to be aware of if possible. For that reason, suitability of using resin coated photographic paper has been investigated. The survey was performed using six different papers from one manufacturer. Different X-ray fluoroscopy techniques were used. Photographic paper has the advantage of its very low cost (around \#8364;1–2 per sheet) and the possibility of using standard X-ray film automatic processors.
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Materials and method
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The interventional cardiology service at the hospital where the survey was carried out has four interventional laboratories performing more than 4500 fluoroscopically guided cardiac procedures per year. The facilities are equipped with C-arm angiographic X-ray units designed for interventional work (Integris 5000, 3000 and Optimus M-200 models by Philips Medical Systems, Best, The Netherlands). Quality assurance programs include skin dose evaluations to a sample of selected patients and included in this evaluation sample are the most complex procedures (about 2–4 patients per week). MSDs are evaluated using Kodak X-Omat V or Kodak EDR2 films (Eastman Kodak, Rochester, NY) [14, 15]. Films are placed on the table, underneath the patient, for an undercouch tube position, and situated as close as possible to the expected most irradiated area. The same methodology is used with photographic paper.
Calibration and response analysis was performed on various types of photographic papers from Ilford (Ilford Imaging UK Limited, Cheshire), although similar photographic paper can be obtained from other manufacturers. ILFORD RC de LUXE black and white (B&W) photographic paper with medium weight (190 g m–2) and types IS1, IS2, IS3 and ILFORD Multigrade IV were checked (www.Ilford.com). Glossy and pearl surfaces have been analysed. The maximum dimensions commercially available for these papers are 30.5 cm x 40.6 cm. A paper for colour photography (Kodak Supra III professional) was also investigated. Comparisons with radiochromic film GAFCHROMIC XR Type R Film and with radiotherapy verification KODAK EDR2 film (www.kodak.com) were made.
A Kodak RPX-OMAT, model M6B, 90 s automatic processor, under daily sensitometric quality control, was used at the facility exclusively to process films from patient dosimetry, adjusted at the replenishment rate compatible with a low work load and with the developer at the minimum adjustable temperature (31.8°C±0.3°C) to minimize photographic paper sensitivity (in order to avoid saturation and to increase dynamic range). Kodak RP X-OMAT developer and Kodak RP X-OMAT LO fixer were employed. It is important to notice that these are not the standard conditions for processing B&W paper. The precise developing time depends on the type of paper and manufacturer. The recommended developing time for most papers is approximately 30 s at 25°C; however, longer developing times are relatively uncritical according to manufacturer's technical information (www.ispcorp.com).
The X-ray equipment used to calibrate photographic papers and films was a General Electric MPG 50 generator with a GE MSN 742/200 tube (GE, Milwaukee, USA). The tube potential was fixed at 80 kVp, at which the measured half value layer (HVL) was 3.0 mm Al. Radiation output, exposure time and tube potential accuracy and reproducibility were better than 2%.
The inverse-square law was applied to obtain the different experimental points of the characteristic curve under fluoroscopy. Focus-to-film distance was varied from 45 cm to 100 cm. Doses were measured with a RADCAL 2025 radiation meter with an external 20 x 6–60 chamber (RadCal Co., Monrovia, CA). For each exposure value, three different sheets of paper were irradiated to estimate grey level deviations. The photographic paper sensitivity for different photon energies was tested with the same methodology by exposing films at different tube potentials from 60 kVp to 110 kVp. Doses and tube potentials were measured using calibrated instruments.
A HP tabletop scanner ScanJet 7400 (Hewlett Packard Co., Palo Alto, CA) and photographic edition software, Photo Paint 10 by Corel (Corel Corporation, Ottawa, ON Canada), were employed to determine grey levels.
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Results and discussion
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Figure 1
shows dose–response curves for the photographic papers included in the survey. Grey level, expressed as the pixel value ranged from 0 (black) to 255 (white), is presented versus dose on a log scale. Since the aim of this figure is just to select the most adequate paper for X-ray dosimetry, only three experimental points per paper have been plotted for the sake of convenience. Photographic paper is usually graded in types ranged 1 to 4. Curves show that sensitivity for X-ray exposures increases with type from 1 to 4. For our purposes, type 1 is obviously the most adequate because of its lowest sensitivity. Also, glossy coating has wider dynamic range than pearl coating (dotted lines). Note that the behaviour of the colour paper KODAK supra III is not good enough to consider further evaluations with such colour paper.
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Figure 1. Log dose vs grey level pixel value (range from 0 (black) to 255 (white)) is presented for different photographic papers.
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Figure 2 shows the grey pixel value versus dose characteristic curve for paper ILFORD IS1 glossy coated. We present this pixel content vs dose curve instead of the usual log sensitometric curves to draw attention to the advantages and limitations of the paper for X-ray dosimetry. Each exposure was repeated three times in different papers from different batches to analyse batch to batch possible differences. In all cases, pixel dispersion was less than or equal to ±2 grey pixel value (in a scale from 0 to 255). The paper saturates at about 3500 mGy, which represents an advance with respect to previous studies using X-ray films [15]. Doses in the range from 10 mGy to 1500 mGy can be measured with reasonable accuracy using Ilford glossy photographic paper IS1, from the shape of the curve it can be estimated that an uncertainty of 1% in the grey pixel value measurement represents about 10% error in dose estimation.
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Figure 2. Grey level pixel value versus dose for the selected paper (ILFORD IS1 glossy coated) processed at 31.8°C in an X-ray film automatic processor.
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To ensure that photographic paper can be employed for patient dosimetry, we have also checked the energy and beam quality response of paper. Variations in tube potential from 60 kVp to 110 kVp show no significant changes in the characteristic curve of this photo paper.
One important point is the stability of processor conditions. As mentioned before, standard temperature for developing paper is around 4–6°C lower than the usual temperature of most automatic processors in conventional X-ray installations. In our laboratory conditions, the processor was set to the minimum adjustable temperature (31.8°C). In Figure 3, the marked variation in paper sensitivity when using different developing conditions is shown. Data correspond to two sensitometric strips processed at two extreme developing conditions: one strip has been obtained with developer at ambient temperature (about 23°C), very close to the manufacturer's recommended conditions but easily influenced by environmental fluctuations. The other strip is developed at 36°C, quite a standard temperature for 90 s X-ray film automatic processors. Note, in this case, that the paper sensitivity increases, thus saturation is obtained at lower doses than those given above. Owing to this high dependence, it is strongly recommended that each paper sheet dosimetry is accompanied by a sensitometric strip in order to check the stability of the processor.
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Figure 3. Light sensitometry for ILFORD IS1 glossy coated paper at two different temperatures: developer at ambient temperature; developer at 36°C.
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Some comparisons between using photographic paper and other large slow film detectors for patient dosimetry in interventional cardiology are now discussed.
The same irradiation fields were registered using different films for patients undergoing coronary interventions. A radiochromic plate GAFCHROMIC® XR Type R FILM, a radiotherapy verification film Kodak EDR2 and a ILFORD glossy photographic paper IS1 were placed over the patient table and under the patient's back. Fields with doses around 100 mGy give uncertainties of 20% for radiochromic plate whereas using IILFORD paper uncertainties are about 5%. The reason is that radiochromic plate generates very low contrast for low doses. This illustrates an advantage of photographic paper versus radiochromic plate to visualize irradiated areas with low skin dose.
Figure 4 shows a comparison between EDR film by Kodak and ILFORD glossy photographic paper IS1. EDR film shows two saturated fields, labelled (A) and (B), where dose calculation is not possible (in these areas skin doses are greater than or equal to 1400 mGy, according to the calibration curve presented in a previous paper [15]). However, the same irradiated fields registered with photo paper still allow estimation of doses using the calibration curve of Figure 2
. In fact, skin dose at area (A) is estimated to be 1600 mGy±200 mGy and skin dose at area (B) 1900 mGy±200 mGy.
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Figure 4. Same irradiation field registered for a patient undergoing a coronary intervention using (a) ILFORD glossy photographic paper IS1 and (b) Kodak EDR film. EDR shows two saturated fields, (A) and (B) not yet saturated using photographic paper.
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B&W photographic paper handling must be under brown safety light; thus it must be manipulated in dark rooms, as for conventional X-ray films. One inconvenience is that boxes contain sheets of paper that are not individually packed. According to manufacturers, it would certainly be possible to pack individual sheets of paper in much the same way as for X-ray film. However, obtaining black envelopes and oversize boxes and packing sheets into envelopes and boxes is a manual operation, so this would make the product more expensive. Thus most potential users prefer the inconvenience of transferring sheets into envelopes before use themselves.
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Conclusions
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Photographic paper could be a very cheap and suitable option for regular monitoring of skin patient doses in interventional radiology and interventional cardiology. Photographic paper could be a preferred option to radiochromic plates or other slow films for estimating skin doses up to 2000 mGy. The main disadvantage of photographic paper is the need for darkroom handling before and after the irradiation and the inconvenience of reducing the processor temperature to reduce paper sensitivity. In a busy X-ray department this is not always possible so it would be necessary to dedicate a processor (or alternatively to do manual development) exclusively for this task.
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Footnotes
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This study was partially supported by the European Commission contract N. FIGM-CT- 2000-00061 (contract DIMOND III). 
Received for publication October 6, 2003.
Revision received April 23, 2004.
Accepted for publication June 16, 2004.
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Abstract
Introduction
