Objective To quantify the detectability of implanted markers in volume-modulated arc therapy for prostate cancer and to identify the best implant location.

Methods (1) Markers were implanted with different fixed coordinate combinations in pelvic prostheses in proper order under four modes. (2) Twenty-five past volume-modulated arc therapy plans for prostate cancer were selected. In accordance with the experimental design, the pelvic prostheses implanted with the markers were irradiated for 25 times with single dose of about 60 Gy, and MV-level electronic portal image device images were obtained at each control point. (3) The mean value of detectability score ( \overlineD) of the implanted markers in each coordinate combination after 25 times of irradiation was calculated for every time point by using MV-level images and a self-developed algorithm to identify the best implant location. The optimal implant location parameters a and b were calculated via curve-fitting analysis.

Results Group-level results: Among all group-level coordinate combinations, the \overlineD of group B10-Ⅰ ((10.0,3.0,0), (−10.0,3.0,0), (10.0,−3.0,0), (−10.0,−3.0,0); unit: mm) under mode B was the highest (0.2652). Mode-level results: Among all modes, the mean \overlineD of mode D ((0,3b,0), (a,b,a), (−a,b,−a), (0,−3b,0); a and b are parameters, unit: mm) was the highest (0.2489). System-level results: The \overlineD of all combinations under the four modes were ideal when a=9.6 mm and b=4.6 mm.

Conclusion This study successfully quantified the detectability of implanted markers, and the best implant location from the group level to the model level and then to the system level was obtained.