Authors (including presenting author) :
Ngai HY(1), Lee WY(1), Wong WC(1), Law ML(1), Ngar YK(1)
Affiliation :
(1) Department of Clinical Oncology, Tuen Mun Hospital
Introduction :
Brachytherapy is a clinical procedure where a sealed radioactive source is placed next to or inside a patient’s body for treating tumors. For high-dose-rate brachytherapy, an afterloader is used to deliver the high-activity radiation source to designated positions for radiotherapy instead of manual insertion. Daily quality assurance (QA) of the afterloader regarding the accuracy of the delivered positions and dwell times is required prior to treatment according to the Task Group No. 56 report issued by the American Association of Physicists in Medicine. QA of source position is usually performed using radiographic/radiochromic films or a source-position check ruler. The positional accuracy has a tolerance of 1 mm. However, evaluation of the accuracy by eyeballing the source position relative to a reference is prone to human error. Temporal accuracy can be checked using stop watches where the accuracy is limited by human reaction time as well. In order to overcome the above sources of error, and to eliminate the use of costly and soon-to-be-obsoleted films, we have developed an afterloader QA system utilizing RavenQA device (LAP Laser, Germany), which is a fluorescence screen-based digital camera, and a dedicated software program.
Objectives :
We aim to eliminate the use of films in brachytherapy pre-treatment QA. The target is to develop a QA system using RavenQA with in-house phantom and image-analysis software for an automatic and quantitative measurement of both the source position and dwell time. The results are stored digitally and electronically in a computer.
Methodology :
A phantom was made from a 3-mm thick acrylic board which can fit tightly on RavenQA’s imaging panel. The acrylic board was embedded with 16 catheters for source delivery. Ten metal beads were seeded beneath each catheter acting as the reference dwell positions. The phantom was imaged by RavenQA with the help of an external X-ray irradiation and the coordinates of each reference position as indicated by a metal bead were recorded after the correction of geometric effects. During QA, all catheters were connected to the microSelectron afterloader (Elekta, Sweden) and the source was delivered to each designated dwell position for at least 1 second. RavenQA was configured to image the source at a rate of 5 images per second. An image-analysis software was developed in Python programming language to process the image stream recorded by RavenQA. First, all images were stacked and the maximum intensity projection (MIP) was calculated to enhance the image contrast. A median filter was applied to the MIP image to remove the salt-and-pepper noises. Second, all the source dwell positions were identified from the MIP image at once by the method of Laplacian of Gaussian. Then, the coordinates of each dwell position were calculated by the means of center of mass, and subtracted by the reference coordinates to obtain the source positional error. Finally, the dwell time at each position was obtained by counting the number of images fully illuminated by the source at that position. Partially illuminated images were also accounted in the estimation of dwell times by a linear scaling with brightness.
Result & Outcome :
We have successfully developed a filmless brachytherapy pre-treatment QA system using a radiation camera with phosphorus screen and in-house phantom and image-analysis software. The system has a positional resolution of 0.25 mm and a temporal resolution of 0.2 s, which are sufficient for the routine QA tasks. The system could automatically format the QA result in a report with statistics on the afterloader positional and temporal accuracies. The afterloader can be calibrated when necessary according to the quantitative result. The system is more accurate, more cost effective, and more efficient compared to the traditional QA technique using films and stop watches.
