Purpose: Electron applicator collisions with the patient are difficult to discern in most treatment planning systems (TPS) which do not display a model of the applicator. Collisions discovered during a pre-treatment plan check or worse, during the first treatment must be re-planned, causing additional work and costly patient treatment delays. We developed a TPS independent program that accurately predicts collisions and determines the proximity of the body and bolus to the applicator using DICOM files exported from any TPS.
Methods: External dimensions and position of all electron applicators relative to isocenter for a Varian linac were physically measured. A MATLAB script was developed that creates and renders 3D representations of the applicator, body contour, and bolus contour extracted from the plan and structure DICOM files. The script calculates either the overlap or minimum distance between the applicator and body+bolus contours. The script was retrospectively tested on 9 electron plans (n=3 shoulder, n=1 scalp, n=5 breast) from 8 patients planned in either Eclipse (n=4) or RayStation (n=4), and prospectively utilized during an initial plan check.
Results: Retrospectively, the software correctly identified 2 plans for one patient where the applicator intersected with the body, which was discovered during the first day of treatment. For the remaining 7 plans, the shortest distance between the applicator and body/bolus contour were calculated and rendered, and accurately predicted no collisions (n=6, dist.>3cm; n=1 dist.<3cm). Prospectively, the software was utilized during an initial breast-boost plan check and predicted an overlap at the armpit preventing a patient delay. This predicted collision was confirmed by the therapists by simulating the original boost-setup before treating with a corrected plan.
Conclusion: The developed software provides quantitative measurements and accurate visualizations of the proximity of the electron applicator to the external body and bolus contours for any TPS.