gms | German Medical Science

Purpose: The manual beam control method dependent on the subject analysis of therapist, however, has been used in this treatment of ocular tumor using the proton although the movement of patient's eyeball may cause the several tens of percentages in dose error, therefore it is needed to develop techniques for eyeball tracking and gating. The purpose of this study was, therefore, to develop a gating system in which the movement of patient's eyeball is tracked and analyzed quantitatively and the proton beam is released and stopped automatically in the case of excessive movement of patient's eyeball over the allowed range during the treatment of ocular tumor using proton.

Material and method: The basic concepts of eyeball tracking and gating are as the Figure 1 [Fig. 1]. The template image is created using the images taken from the patient's treatment position planned before treatment. The created template image is compared with the images taken using the CCD camera in real-time during treatment in order to check whether the location of eyeball follows the planned one. The beam is stopped automatically when the location of eyeball is found to be out of tolerance range.

The real time image of patient's eyeball is taken using the CCD camera and using Lavbiew 8.6 and Vision Assistant 8.6 (National Instrument, USA), and Eye Tracking and Gating Program was developed using the Pattern Matching Method. The techniques for collection of images and generation of trigger signal were developd using the NIPCI-1411image acquisition board (National Instrument, USA). The eyeball phantom which is made for the purpose of assessing the performance of automatic release and stop of proton gating was used along with the 1D-moving phantom in order to measure the dose distribution in accordance with the movement of eyeball. The allowed range of eyeball were set to 0.5 mm, and the movement distance of 1D moving phantom were set as 1 cm. The hardware for the connection between proton therapy and gating program was developed and linked to therapy equipment.

Result and discussion: The template imaged used treatment was acquired in template window using image capture. If eyeball moves out from the tolerance range, the generation of trigger signal is stopped along with the turning on of red alert lamp and the irradiation of proton beam is blocked automatically, whereas it resumes automatically when the eyeball return to the allowed range, as shown in Figure 2 [Fig. 2]. The Figure 3 [Fig. 3] is result of measurement of dose distribution of proton beam when the eye ball did not move, when the eyeball move without gating, and when the eyeball movement was gated with the 0.5 mm of allowable variation range, respectively.

Conclusion: The eyeball tracking in real time was possible using pattern matching method when the patient's eye ball move, and based on it, the gating of proton beam was also impossible, during the treatment of ocular tumor using proton, and the accuracy of the tracking was under 0.05 mm. This study is the first attempt of proton therapy using gating as far as we know, although the treatment of ocular tumor using proton has been used on many sites over the world.