Room: AAPM ePoster Library
The application of 6-degree treatment couch in LINAC-based stereotactic radiosurgery (SRS) allows higher treatment accuracy, as well as improved target coverage and dose conformity. Despite a tight tolerance clinically implemented per AAPM TG-142, couch and patient rotational uncertainties may still inevitably lead to suboptimal or even unacceptable dose distributions and thus compromise clinical outcomes. These undesired uncertainties become especially concerning when treating multiple sparsely distributed targets. In this study, we developed a novel methodology to determine the optimal isocenter to minimize errors caused by rotational uncertainties.
An optimization scheme was developed and evaluated as follows. A polyhedral mask was first constructed from a target contour. To simulate variations caused by the rotational uncertainties, the mask was then rotated by multiple times with rotation angles set as ±0.5/0 degree around a point of interest. A genetic global optimization maximized the average 3D dice coefficient between the rotated and the original masks and therefore determined the optimal isocenter. Furthermore, two series of randomly rotated masks were generated with rotational angles varying between -0.5 and 0.5 degree about the optimal isocenter and the center of mass (COM), respectively. The average dice coefficients were compared with a student t-test for statistical analysis.
Seven patients treated for at least three brain metastases were identified. The optimal isocenter location for each patient was determined, with the median distance of 6.3 cm from the COM (range: 2.4-7.8 cm). The average dice coefficient of 1000 random rotations about the optimal isocenter was significantly larger than that about the COM. (Mean: 0.89 vs. 0.85; p=0.01)
The developed methodology to determine the optimal isocenter allows significant reduction of the errors caused by couch rotational uncertainties. Using the optimized isocenter for LINAC-based SRS when treating multiple brain metastases is especially meaningful to ensure accurate treatment delivery.