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Practical aspects for the evaluation of skin doses in interventional cardiology using a new slow film

¹ Medical Physics Group, Department of Radiology, University Complutense of Madrid, 28040 Madrid, ² Medical Physics Service, San Carlos Hospital, 28040 Madrid and ³ Diagnostic Radiology Service, San Carlos Hospital, 28040 Madrid, Spain

	Abstract

Top Abstract Introduction Materials and method Results and discussion Conclusions References

 
Mapping skin doses in complex fluoroscopy interventions is useful to determine the probability of a possible injury, to detect areas of overlapping irradiation fields and to obtain a permanent register of the most exposed patient skin areas. To fulfil this task, large films with slow X-ray response can be used. Recently, Kodak has introduced a new radiotherapy verification film, named EDR2 (Extended Dose Range). The aim of this paper is to analyse the possibilities of using this new film for estimating skin dose distributions in interventions with potentially higher doses, such as complex percutaneous transluminal coronary angioplasty (PTCA), intravascular brachytherapy procedures (IVB) or cardiac ablations. The EDR2 film by Kodak is an improved option to be used in interventional cardiology to obtain maps of patient skin doses and to estimate maximum skin doses up to 1400 mGy. Film kVp dependence is negligible and the processor conditions can be standardized to obtain skin dose estimations. The linear range for accurate dose measurements is from 50 mGy to 500 mGy.

	Introduction

Top Abstract Introduction Materials and method Results and discussion Conclusions References

 
Reports of patient skin injuries in interventional radiology and in interventional cardiology are fully documented in the scientific literature [1]. Research into new methods for patient dose monitoring, especially that focused on preventing deterministic effects, should be encouraged. Hence, display of the maximum local skin dose (MSD) at the operator console would be desirable. Unfortunately, there is no technical means currently available for routine assessment of this quantity. Some direct and indirect methods for estimating the local MSD have been proposed by different authors [2]. For direct measurements, different detectors such as thermoluminescent dosimeters or scintillation dosimeters can be placed in contact with the patient. All these devices in contact with the patient's skin have the problem of the great uncertainties involved when different X-ray beam orientations are used and the practical impossibility of obtaining distributions of such irradiated areas at a reasonable cost.

Mapping skin doses is useful to determine the probability of a possible injury and its extent, to detect areas of overlapping radiation fields and to provide the possibility of obtaining a permanent register of the most exposed patient skin areas. This is essential for the follow-up of patients with multiple fluoroscopy interventions, e.g. multiple percutaneous transluminal coronary angioplasties (PTCAs) due to restenosis [3]. To fulfil this task, large films with slow X-ray response can be used. Several types are available: laser printer; duplicating; fine grain positive films; or radiochromic films [4, 5]. One good alternative is the use of slow films such as those for radiotherapy. In a previous paper [6], the authors found that the verification film Kodak X Omat V film (Eastman Kodak, Rochester, NY) is adequate, when correctly calibrated and placed close to the skin, for estimation of skin doses. The procedure is valid for interventions, such as coronary angiography or other cardiac and vascular interventions when the MSD does not exceed 500 mGy. Unfortunately, long and complex procedures usually require a wider measurement range. Recently, Kodak has introduced a new radiotherapy verification film with low radiation sensitivity, named EDR2 (Extended Dose Range). The aim of this paper is to analyse the possibilities of this new film for estimating skin dose distributions in interventions with potentially higher doses, such as complex PTCA, intravascular brachytherapy procedures (IVB) or cardiac ablations.

	Materials and method

Top Abstract Introduction Materials and method Results and discussion Conclusions References

 
The Interventional Cardiology Service at the hospital in which the slow film method has been adopted collaborates within the framework of the research program sponsored by the European Commission named DIMOND III (Digital Imaging: Measures for Optimizing Radiological Information Content and Dose). The service has four interventional laboratories performing more than 4500 diagnostic and interventional 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). Since 1992, laboratories have been subject to quality assurance programs including regular patient dose evaluations. Maximum skin dose estimations with slow films are performed on all patients with a priori complex PTCAs, PTCAs with IVB and cardiac ablations. The "slow film method" [6] combined with a set of 5–7 thermolumiscent dosimeters attached to the film was used to measure MSD. Films were placed on the table underneath the patient for an undercoach tube position and centered as close as possible to the most irradiated area of the patient. All slow film images generated at the cardiology service are converted into digital DICOM images by a Lumiscan 75 (Eastman Kodak, Rochester, NY) laser scanner and analysed on line with the help of the Osiris viewer and associated image analysis tools.

Lumiscan 75 laser film digitizer
The Lumiscan 75 is a tabletop, laser film digitizer designed by Lumisys Inc. (now acquired by Eastman Kodak Company) especially for high-resolution medical imaging requirements. According to manufacturer's indications, the density resolution and precision is a linear function from 0.001 to 3.6 optical density (OD). The unit accommodates film sizes from 8'' x 10'' to 14'' x 17'' and resolutions of up to 2 K x 2.5 K x 12 bits (line pair resolution of 2.8 lp mm^-1). This unit is used in the present work to obtain digital images of the patient skin dose maps for further analysis and dose estimations. Figure 1 shows the OD versus grey level calibration that ensures that no significant uncertainties are introduced for dose estimations when the digitalized film is employed.

View larger version (16K): [in this window] [in a new window]   Figure 1. Experimental determination of the optical density versus pixel value for a Lumiscan 75 digitizer showing a linear behaviour.

The films calibrated and analysed in the present work are Kodak X-Omat V film and new Kodak EDR2 film:

• Kodak 33 cm x 41 cm, X-Omat V film (Eastman Kodak Co., Rochester, NY). This film is designed, according to manufacturer's indications, for verifying the orientation and for approximating patient doses in radiation therapy procedures, recording doses linearly from 0.4 Gy (producing a net density of 1.00) to 2 Gy (producing a net density of 3.00) for Co-60 photon energies. The film packaging does not need darkroom loading and may be safely handled under standard safelight filters for unloading. Automatic processing is recommended. Uncertainty in skin dose estimates has been reported to be less than 10% over the range of 20–200 mGy and around 30% on the toe and on the shoulder of the curve (up to 0.5 Gy) for X-ray fluoroscopic beam qualities, according to data previously published [6].
  
• Kodak, 35 cm x 43 cm EDR2 film (Eastman Kodak Co., Rochester, NY). Intended for direct exposure applications, EDR2 film also belongs to the line of Kodak Ready-Pack products. According to the manufacturer's technical information (www.kodak.com) this film is a convenient medium for calibration and monitoring exposures; excellent for relative dosimetry in radiotherapy and with appropriate calibration, the film may be used for absolute dosimetry. The EDR2 film is relatively insensitive to X-ray energies and the response extends to very high exposures. EDR2 film saturates in direct exposure at approximately 7 Gy for the radiotherapy beam qualities. Exact dose responses are a function of facility-dependent factors including processing conditions (time, temperature, equipment, chemistry), the density sampling (digitizer equipment and calibration) and exposure monitoring equipment. The aim of this paper is to determine the exact response relationship for fluoroscopic beam quality conditions.
  

A Kodak RPX-OMAT, model M6B, 90-s processor was used at the facility exclusively to process slow films, 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 film sensitivity. Kodak RPX-OMAT developer and Kodak RPX-OMAT LO fixer were employed. The processor is under a daily sensitometric quality control.

The X-ray equipment used to obtain a densitometric pattern on the film was a General Electric MPG 50 generator (GE Medical Systems, Waukesha, WI) with a GE MSN 742/200 tube. The measured half value layer was 3.0 mm Al at 80 kVp. Radiation output, exposure time and tube potential accuracy and reproducibility were better than 2%.

Individually calibrated Lithium Fluoride TLD-100 dosimeters from Harshaw (Harshaw, Thermoelectron Co., Berkshire, UK) and a RadCal 2025 radiation meter (RadCal Co., Monrovia, CA) with an external 20 x 6-60 chamber were used to measure film entrance doses.

OD readings were obtained with a digital densitometer Victoreen 07-424 previously calibrated with an ANSI sensitometric strip (Victoreen, Cleveland, OH).

Method for determining the shape of the characteristic curve
To obtain the different experimental points on the characteristic curve the inverse square distance method was employed using both radiographic X-ray and fluoroscopy beams. The tube potential was fixed at 80 kVp (±2%) and an abdomen configuration of an ANSI phantom [7], with 15 cm of polymethyl methacrylate (PMMA) and inserts of 1 mm and 2 mm Al layers, was placed below the slow film to include the contribution from patient backscatter radiation to the film characteristic curve and the speed determination. The radiation field was collimated to obtain an irradiation field size at the film of 11.5 cm x 11.5 cm. Focus to film distances were varied from 45 cm to 100 cm. Additional points on the characteristic curve were obtained by varying the number of radiographic exposures (and the exposure time under fluoroscopy). The film was processed after 2 h using a Kodak M6B automatic processor.

Method for the speed determination
A calibrated external ionization chamber was placed in contact with the film to measure doses. The same geometry, tube potential, irradiation fields and backscatter condition as for the characteristic curve methodology were selected. With an appropriate X-ray radiographic technique, the focus to film distance was varied until a net OD of 2.0 at the film was obtained. The procedure was repeated under X-ray fluoroscopy. Since a possible shift of film response with kV is only expected for the absolute value of the film speed and not for the shape of the characteristic curve, the film sensitivity kV dependence (from 60 kVp to 110 kVp) has been checked using the same methodology as described for speed determination. In this work the speed index was defined as:

where D_s is the entrance dose in air with backscatter that produces a net OD of 2.0 on the direct radiographic slow film.

	Results and discussion

Top Abstract Introduction Materials and method Results and discussion Conclusions References

 
Figure 2 shows the X Omat V characteristic curve used in previous work [6] and the new EDR2 film curve. Similar film processing conditions were used for both studies. We present the dose vs. OD curves instead of the usual OD vs log E characteristic curves to point out the advantages and limitations of the films for X-ray dosimetry. It can be seen that X-Omat V film saturates at about 700 mGy. Doses up to 500 mGy could probably be evaluated with this film. However, the optimum linear fit to obtain doses from the response curve is just 20–200 mGy. EDR2 saturates at about 1400 mGy with possible dose estimations up to 1000 mGy. With EDR2 film, doses estimated from optical readings between 3 and 3.5 could generate uncertainties from 1000 mGy to 2000 mGy. The linear range for accurate dose measurements is from 50 mGy to 500 mGy.

View larger version (24K): [in this window] [in a new window]   Figure 2. Dose vs. optical density curves for Kodak EDR2 and X-OMAT-V films showing the linear ranges and saturation points.

Obviously, dose response curves could change from batch to batch and be strongly influenced by processing conditions, thus periodic revision of the calibration curve is advisable to minimize dose uncertainties. Figure 3 shows the variation of speed index with developer temperature. As expected, sensitivity increases with temperature, so it is advisable to work at the lowest possible controlled temperature. Variations of the developer temperature of 0.5°C will change the film speed by about 2%. Dose curves presented in Figure 2 have been obtained at 31.8°C

View larger version (16K): [in this window] [in a new window]   Figure 3. Variation of film speed index with developer temperature: Kodak EDR2 film.

In Figures 4 and 5 some practical differences when using the X-Omat V and the EDR2 film for patient dosimetry of complex PTCAs (with IVB) are shown. IVB usually produces saturated films as long fluoroscopy times and high field concentration are used. Figure 4 shows the irradiation fields registered with the X-Omat V film for a patient undergoing a coronary intervention with a dose area product of 109 Gy cm², measured with a transmission chamber during the intervention. Figure 5 shows the radiation fields registered with the EDR2 film after a similar intervention with a higher dose area product (238 Gy cm²). The dose map in the first case shows large areas of film saturation (about 300 cm²) in which doses could be higher than 700 mGy. The second case shows no saturation except in an area of 60 cm², according to the EDR2 characteristic curve that means maximum skin doses over 1400 mGy.

View larger version (80K): [in this window] [in a new window]   Figure 4. Irradiation fields registered with the X-Omat V film for a patient undergoing a coronary intervention with a dose–area product (DAP) of 109 Gy cm2. ROI, region of interest; IVB, intravascular brachytherapy; PTCA, percutaneous transluminal coronary angioplasty; MSD, maximum skin dose.

View larger version (68K): [in this window] [in a new window]   Figure 5. Irradiation fields registered with the Kodak EDR2 film for a patient undergoing a coronary intervention with a dose–area product (DAP) of 238 Gy cm2. PTCA, percutaneous transluminal coronary angioplasty; MSD, maximum skin dose; IVB, intravascular brachytherapy.

It would be desirable to establish a follow-up programme for patients whose slow film pattern shows saturation densities and a large concentration of fields. At the cardiology service where this protocol is being implemented, approximately 1% of the images show these characteristics. For those patients, their identification number and resulting slow film image is registered at a database, so that the cardiologist can actually avoid accumulation of doses at the same skin areas, especially for patients with repeated procedures.

	Conclusions

Top Abstract Introduction Materials and method Results and discussion Conclusions References

 
The Kodak EDR2 film is an improved option to be used in interventional cardiology to obtain maps of patient skin doses and to estimate MSD up to 1400 mGy. Film kVp dependence is negligible and the processor conditions can be standardized to obtain skin dose estimations. The linear range for accurate dose measurements is from 50 mGy to 500 mGy.

The global cost of the procedure (about \#8364;4 per film) is low enough with respect to the cost of the interventional procedure to consider the implementation of this methodology in interventional radiology and interventional cardiology services.

	Footnotes

 
This study was partially supported by the European Commission contract N. FIGM-CT-2000-00061 (contract DIMOND III).

Received for publication August 6, 2002. Revision received . Accepted for publication March 10, 2003.

	References

Top Abstract Introduction Materials and method Results and discussion Conclusions References

 

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