High dose-rate brachytherapy source position quality assurance using radiochromic film

Medical Dosimetry, Vol. 32, No. 1, pp. 13-15, 2007 Copyright © 2007 American Association of Medical Dosimetrists Printed in the USA. All rights reserved 0958-3947/07/$–see front matter

doi:10.1016/j.meddos.2006.10.001

HIGH DOSE-RATE BRACHYTHERAPY SOURCE POSITION QUALITY ASSURANCE USING RADIOCHROMIC FILM M. D. C. EVANS, M.SC., S. DEVIC, PH.D., and E. B. PODGORSAK, PH.D. Medical Physics Department, McGill University Health Centre, Montreal, Quebec, Canada (Received 11 October 2005; accepted 11 October 2005)

Abstract—Traditionally, radiographic film has been used to verify high-dose-rate brachytherapy source position accuracy by co-registering autoradiographic and diagnostic images of the associated applicator. Filmless PACSbased clinics that do not have access to radiographic film and wet developers may have trouble performing this quality assurance test in a simple and practical manner. We describe an alternative method for quality assurance using radiochromic-type film. In addition to being easy and practical to use, radiochromic film has some advantages in comparison with traditional radiographic film when used for HDR brachytherapy quality assurance. © 2007 American Association of Medical Dosimetrists. Key Words: Brachytherapy, Quality assurance, Radiochromic film.

tion cross-section of its organic active layer when irradiated with low-energy photons (below 0.1 MeV). These 2 radiochromic-type films may be used for HDR quality assurance as an alternative to traditional radiographic (light sensitive) films used with wet developer systems.

INTRODUCTION The treatment of selected cancer patients with high-doserate (HDR) remote afterloading devices has been routine for many years. Current HDR remote afterloading units use Iridium-192 (192Ir) with a nominal activity of 370 GBq at the time of source installation. As a consequence of the 74-day half-life of 192Ir, HDR machines typically undergo 4 or more source changes per year. Following a source change, the source strength is verified with a well-type ionization chamber having a calibration coefficient traceable to a standards laboratory. In addition, the quality assurance with regard to the positional accuracy of the source drive mechanism is carried out with radiographic film. The trend toward filmless radiotherapy departments using PACS-based systems for image handling precludes access to traditional film and wet developer systems, and this situation could become problematic with respect to the source positioning quality assurance test. To circumvent this problem, we developed an alternative procedure for quality assurance of the source position using radiochromic-type film. Radiochromic film is a practical replacement for traditional wet-developed film systems, and it has several advantages over the traditional radiographic (light sensitive) film. Two recent radiochromic film models, the XR-QA and EBT, have been introduced by International Specialty Products (Wayne, NJ) and are known commercially as GAFCHROMIC™-type films. While the EBT model was designed for dose measurements in highenergy photon beams (above 1 MeV), the XR-QA model has a sensitive layer containing high atomic number materials, intended to compensate for the lower absorp-

METHODS AND MATERIALS HDR brachytherapy units typically use a radioactive 192Ir source pellet contained in an outer capsule that is attached to a stainless steel cable. The cable is wound around an encoded drum and, in its safe position, the source is housed in a shielded lead container in the body of the afterloading device. During treatment, the drum is rotated so as to feed the source on the steel cable out through a selected channel to a catheter located inside the patient. By varying the source positions and individual source dwell times, a composite dose distribution that conforms to the clinical needs of the patient is deposited in the tumor or target volume. Verification of the positional accuracy of the source drive mechanism is among the many quality assurance tests carried out following a source change and prior to clinical use. Our traditional procedure for this test has been a 2-step process to produce a single co-registered image using a technique that superimposes a source autoradiograph with an overhead radiograph of the dummy sources used clinically inside the applicator.1 In the first step, we use a standard lung catheter (6 French) affixed to an XV-2 light-sensitive radiographic film (Kodak, Rochester, NY) sealed in its light-tight bag. An autoradiograph is obtained by sending the source (Nucletron, source model 096.001) to the nominal tip of the catheter (995 mm) and retracting the source through 5 equal dwell positions having a spacing of 2 cm. In this particular setup, the nominal dwell positions thus are 995, 975, 955, 935, and 915 mm from a reference point on the HDR device. To produce an autoradiograph with a min-

Reprint requests to: Michael Evans, Medical Physics Department, McGill University Montreal General Hospital, 1650 Avenue Cedar, L5-112, Montreal, Quebec H3G 1A4. E-mail: mevans@ medphys.mcgill.ca 13

14

Medical Dosimetry

imal distortion, a 3-mm-thick Lexan plate is placed on top of the test catheter to ensure close contact between the source and the radiographic film sealed in its bag. We have established that a new source having a nominal activity of about 370 GBq (10 Ci) requires about 0.3 seconds per each of the 5 dwell positions to provide sufficient blackening of the radiographic XV-2 film. The second step in this procedure is to disconnect the treatment catheter from the HDR device, while still keeping it fixed to the film bag, and place it under a diagnostic radiography machine. Our brachytherapy suite is equipped with a conventional radiotherapy simulator, which makes this process simple, although the film and catheter could be moved to another location for radiography if sufficient care is taken not to move the relative position of the film and the catheter. Once the film and catheter combination is in place, a steel wire containing tungsten crimps located at 1-cm spacing (commonly referred to as the dummy catheter) is inserted into the treatment catheter. These dummy catheters are routinely used to locate potential source positions within the patient for HDR brachytherapy treatment planning. An overhead image is produced with the spot exposure film option of the simulator and the XV-2 film is then developed using a commercial wet film developer. We have found that by placing the XV-2 film at a source-to-skin distance (SSD) of 100 cm from the source of the radiographic unit and using a technique of 85 kVp and 500 mAs, an acceptable overhead image can be produced. The result of this procedure is a co-registered image showing the radioactive source position as imaged with the autoradiograph superimposed onto the intended source positions as seen with the overhead image of the dummy catheter. Our alternative technique is similar to the traditional technique but it uses radiochromic film instead of the XV-2 (light sensitive) film. Radiochromic film is insensitive to room light and has a high sensitivity to ionizing radiation. It needs no processing and may be kept under ambient conditions without special handling requirements. The EBT model high-sensitivity radiochromic film has been tailored for absorbed dose measurements in high-energy photon beams, and is designed to be used in a dose range from 0.01 to 8.0 Gy.2 In contrast to the traditional silver halide film, which has an atomic composition that preferentially responds to photons in the photoelectric range (H [3%], C [21%], N [7%], O [16%], I [22%], Ag [30%]),3 EBT radiochromic film has an atomic composition that is more tissue equivalent (H [40%], C [42%], N [1%], O [16%], Li [0.3%], Cl [0.3%])2 and less responsive to diagnostic energy ranges. On the other hand, the sensitive layer of the XR-QA radiochromic film has the same composition as that of the EBT model radiochromic film. with higher atomic number halide additives to increase a low-energy photon response via the photoelectric-effect.

Volume 32, Number 1, 2007

The 2-step process described above for the XV-2 film is followed; however, for our alternative method the irradiation technique is not the same for the diagnostic radiograph of the tungsten dummy sources. Because the sensitivity of the EBT film relative to the XV-2 film at diagnostic energies is much lower, we have overcome this by first placing the catheter affixed to the film on a block tray at an SSD of about 45 cm, and by using a technique of 85 kVp and a total of 2500 mAs delivered in 5 successive fractions. Thus, a combination of moving the film closer to the diagnostic machine source and an increase in technique has compensated for the difference in the relative sensitivity between the two films at diagnostic range energies. RESULTS AND DISCUSSION An example of a test film can be seen in Fig. 1a, whereby the autoradiograph consists of discrete dark “blobs” at 2-cm intervals, and the potential source positions are indicated by the dummy catheter with the tungsten

Fig. 1. HDR source position quality assurance using co-registered imaging with: (a) XV-2 radiographic (light sensitive) film, (b) EBT model (radiochromic) film, and (c) XR-QA model (radiochromic) film.

HDR brachytherapy QA using radiochromic film ● M. D. C. EVANS et al.

Table 1. Parameters used to obtain co-registered images in Figure 1 (source activity of about 370 GBq) Autoradiograph Diagnostic Radiograph Film

kVp

mAs

SSD (cm)

Dwell time (sec)

XV-2 (radiographic) EBT (radiochromic) XR-QA (radiochromic)

85 80 75

500 2500 250

100 45 100

0.3 0.3 0.3

crimps at 1-cm intervals. Normally, we expect the center of the autoradiograph to line up with the center of the tungsten crimp to less than 1 mm,4,5 as is the case in Fig. 1a. Autoradiograph irradiation times for the EBT model film are similar to those used for the XV-2 radiographic film. An example for the fusion of the autoradiograph and the overhead radiograph is shown in Fig. 1b for the EBT film. We have also examined the possibility of using the XR-QA model film, which has been developed to have an increased sensitivity to kilovoltage range x-rays. An example of the fusion of the autoradiograph and the overhead radiograph is shown in Fig. 1c for the XR-QA film. Because of its much higher sensitivity in the low-energy region, the XR-QA is ideally suited for HDR brachytherapy positional accuracy quality assurance test. Radiographic and autoradiographic irradiation techniques for the XV-2 (light sensitive) and the XR-QA (radiochromic) films are similar. The imaging techniques for all three film types examined in this work are shown in Table 1. The co-registered images shown in Fig. 1 are qualitatively the same, and all may be used to perform quality assurance on source position accuracy. XV-2 and EBT films are based on a substrate that permits transmitted light, whereas XR-QA film is based on a reflective substrate. All 3 types of film are well suited to qualitative assessment by visual inspection. However, there are in fact some advantages to using radiochromic-type film for this test. Radiochromic film does not require a light-tight bag. Moreover, because the catheter is affixed directly onto the film, there is less likelihood that the catheter position relative to the film will move, thus making it easier to transport the catheter-film combination between irradiations. In addition, the fact that there is no need for a wet developer may be a consideration in filmless departments, or it may simply be useful to have this technique as a backup procedure should a wet developer be unavailable during a HDR brachytherapy source change. The immediate availability of an image with the radiochromic film may also be an advantage when wet developers required for use with the XV-2 film are not conveniently located in the radiotherapy department. When radiochromic film is used for absolute dosimetry, the time from irradiation until analysis is of some importance, and may be typically of the order of 1 day. In this technique, however, the film is only used qualitatively, so that the time to analysis is irrelevant and the film may be visually inspected immediately following irradiation.

15

Radiochromic film may be easily cut into small strips for this particular test, unlike radiographic film, which must be resealed in a darkroom following cutting to avoid light fogging. While both radiographic and radiochromic film are similarly priced, the ability to cut with ease many pieces of radiochromic film from a single sheet in daylight conditions means that many more tests can be performed at a lower cost. In addition, there is a practical lower size limit when using automatic developers and radiographic films, whereas this limit does not exist with self-developing radiochromic films. A potential disadvantage of the EBT film is its relative insensitivity to the kVp range ionizing radiation; however, this is easily solved by a combination of moving the radiochromic film closer to the diagnostic x-ray tube and delivering more radiation. In our experience, this amounts to a 5-fold increase in tube workload, and represents an insignificant increase in the total diagnostic tube workload. Another potential disadvantage of using radiochromic film for reference dosimetry is the time required for it to become fully darkened; however, because this technique uses the radiochromic film qualitatively only, the radiochromic film may be assessed immediately following irradiation. On the other hand, the high sensitivity of XR-QA model GAFCHROMIC film in the low-energy range makes it an ideal alternative for the highdose-rate brachytherapy source position quality assurance. CONCLUSIONS Source position verification for HDR brachytherapy is conveniently accomplished by using radiographic (light sensitive) film to co-register autoradiograph and diagnostic images. The trend toward filmless, PACSbased radiotherapy departments means that the availability of radiographic, light-sensitive film for traditional quality assurance tests may become limited. Radiochromic film may be substituted for traditional radiographic film as a better technique, or at least as a back-up technique, because it does not require post irradiation processing. Moreover, this type of film in fact offers several advantages over traditional radiographic (light sensitive) film requiring wet developer systems. REFERENCES 1. Evans, M.D.C.; Arsenault, C.; Podgorsak, M. Quality assurance for variable-length catheters with an afterloading brachytherapy device. Med. Phys. 20:251–3; 1993. 2. Devic, S.; Seuntjens, J.; Sham, E.; et al. Precise radiochromic film dosimetry using a flat-bed document scanner. Med. Phys. 32:2245– 53; 2005. 3. ICRU. Tissue Substitutes in Radiation Dosimetry and Measurement, ICRU report No. 44. Bethesda, MD: International Commission on Radiation Units and measurements; 1989. 4. AAPM. Comprehensive QA for Radiation Oncology: Report of AAPM Radiation Therapy Committee Task Group 40. New York: American Institute of Physics; 1994. 5. AAPM. Code of Practice for Brachytherapy Physics: Report of AAPM Radiation Therapy Committee Task Group No. 56. New York: American Institute of Physics; 1997.