Liu, L., Prasad, S., Bassano, J.H., Keshler, B., Hahn, S. (2004). A dwell position verification method for high dose rate brachytherapy. Journal of Applied Clinical Medical Physics, Volume 5, Number 1, pages 1-5.

    The purpose of the study is to verify the dwell position of the radioactive source during high dose rate (HDR) brachytherapy treatments. The authors presented two methods of dwell position verification using fluoroscopic images. The methods are done in a test phantom and in an interstitial implant. Both methods used a mobile C-arm fluoroscopic machine to capture fluoroscopic image of the check cable when it reaches the most distal dwell position during the treatment. The captured image is compared with the treatment planning image and are displayed side-by-side on a dual monitor relay station at the HDR treatment console. The comparison of the two images played a significant course of action for the authors to check if there is a discrepancy between the position of the check cable and the intended first dwell position on the planning image to avoid the possibility of treating the wrong volume.

    This study mentioned other different methods of verifying and recording the dwell positions in HDR brachytherapy treatments. Such methods are done by several authors like Sheikh-Baheri and Munro (1998) who evaluated the possibility of monitoring the high activity of Iridium-192 source using x-ray fluoroscopic images. The two suggested that the use of a large air gap between the patient and the x-ray image intensifier, a well designed anti-scatter grid to suppress the spurious signals generated by the Iridium-192 gamma rays, and a high current x-ray fluoroscopy technique can make the real time monitoring of source position feasible. Other method is done by Duan et al. (2001) which used a pinhole camera to capture the autoradiographic image of the active Iridium-192 source. Unfortunately, Sheikh-Baheri and Munro produced poor quality images while Duan et al. produced images that contained no patient anatomic information.

    The authors used the check cable, instead of radioactive Iridium-192 source, to produce a fluoroscopic image free of noise signals. Difference in verification images with the previous study are observed. The acquired images are clear and the distance scale indicated by the dummy seeds can be easily identified by the naked eye. The outcome of the images taken are very important for them to identify if there is a difference between the position of the check cable and the intended first dwell position on the planning image.

    An Ethernet ready C-arm workstation is used to allow the direct transfer of fluoroscopic images to a treatment planning computer, which has dual monitors to allow the images to be displayed side-by-side for image comparison. The HDR treatment machine used has a high activity Iridium-192 source attached to the end of a drive cable, an indexing system for 20 channels, and a check cable. In this check cable, the travel time between the most distal dwell positions in successive catheters is about 7 seconds.  It has been reported that this amount of time is sufficient for the performer to have a quick visual inspection of the verification image and to save it to the workstation. After all verification images are taken, they interrupted the HDR treatment sequence and conducted a detailed evaluation of all the images. The authors observed that ideally the C-arm workstation software could make a composite image of all the individual verification images that would provide the verification of the positions for all the catheters in a short time. Unfortunately, the C-arm workstation used does not provide that kind of functionality. Thus they might have been finished the process patiently.  Based on my experienced, this method is empirical and can be done easily when all the required technical features of a machine are met. Otherwise, the whole procedure will take a lot of time.  In this case, you may think of another approach to do these things easily.

    In results and discussion, it has been shown for the implant with two catheters in a phantom that the verification image of the check cable at first dwell position and the first dwell position of the treatment planning image, taken in the AP orientation, has a variation of around 1 cm or larger.  In this regard,  I noticed that this verification method is used to estimate the effect of the deviation of the actual dwell position from the planning dwell position to the actual treatment volume. For endobronchial verification, I suggest that the verification method should be performed only by the  trained personnel. Especially in those cases where you have to determine the length of the tubes you need for the actual treatment procedure. Otherwise, prescribed dose on the tumor might not be attained. I can also tell that one major advantage of HDR is that we are sure that the prescribed dose to the tumor is quite accurate because the patient and implant position is the same as when the treatment plan is devised. The treatment planning dosimetry will take less than an hour while the treatment time will only take 15-30 minutes so there is little opportunity for the implant to move and deposit the radiation dose where it is not intended.

    The author pointed out that dwell position verification is not meant to replace catheter's depth measurement that require millimeter accuracy. Rather, the purpose of the verification is to catch any gross errors in the determination of the dwell positions during the treatment planning process. For intracavitary implants, I am thinking that this dwell position verification is no longer needed since the catheters are already embedded in the patient. Moreover, this verification method is limited to the first dwell position for each implant catheter. The last dwell position is not able to be verified since the check cable does not dwell at this position. The author suggested to reprogram the HDR treatment console in such a way that the last dwell position in the catheter is kept and a separate check cable run sends the cable to this position.  Though he also mentioned that this approach requires creating a separate treatment control file, and it is not a direct quality assurance of the actual control file used for the patient treatment delivery. In addition to that, I am thinking that this method is only applicable to treatment techniques where the applicator could not be properly positioned and fastened safely. Fletcher, Henschke, and vaginal cylinder applicators are the examples where movements are possible when use. The paper is helpful for the physicist to think of ways on how to be careful of their work since they are dealing with radiation. Quality assurance and quality control for each catheter should be a part of the safety operating procedure of every cancer centers to ensure the safety of the patient and the benefit through radiation should out waste the risk of undergoing a radiation treatment procedure.

1. Sheikh-Bagheri D, Munro P. A Monte Carlo study of verification imaging in high dose rate brachytherapy. Med. Phys. 1998; 25, 404-414.
2. Duan J, Macey DJ, Pareed PN, Brezovich IA. Real time monitoring and verification of in vivo high dose rate brachytherapy using a pinhole camera. Med. Phys. 2001; 28, 167-173.