Room: AAPM ePoster Library
Purpose: modify jaw positions and MLC pattern to permit automatic on-treatment MV/kV detection and monitoring of fiducials for SBRT prostate patients.
Methods: internal program (MLC_MODIFIER, previously described) is used clinically to modify MLC patterns and ensure visibility of fiducials during treatment for MV/kV tracking at given control points (trigger points) during treatment every 20 degrees. This program was modified in preparation of the implementation of jaw tracking, which is a commercial feature available on some linacs that permit dynamic jaw position to track MLC location during VMAT delivery. A consequence of jaw tracking, markers can be blocked by variable jaws. The new MLC_MODIFIER program reads the jaw positions for each control point, and modify it, if necessary, to expose one fiducial. To assess feasibility, 5 patients were retrospectively re-planned with jaw tracking (Jaw_plan) and the new version of MLC_MODIFIER was run to create plans (MV_plans) with MV imaging capabilities. The following parameters were evaluated: trigger point values, jaw position changes (frequency and magnitude), dosimetric consequences and changes in MU.
Results: imaging control points were inserted at the correct trigger locations. The average jaw size was larger, 61.0 vs 58.9 cm2 for MV_plan and Jaw_plan respectively. MLC_MODIFIER adjusted the jaw position in addition to MLC 4% of the time. The PTV mean dose was on average 1.1% higher with the MV_Plans. Once the MV_Plans, were renormalized so the PTV mean dose was the same as the Jaw_plan, the MV_plan met all the clinical planning criteria. MU were lower with the MV_Plans by on average 1.2% due to the renormalization.
Conclusion: is possible to modify both MLC pattern and jaw position to permit automatic on-treatment MV/kV detection and monitoring of fiducials with no impact on plan quality.
Funding Support, Disclosures, and Conflict of Interest: This research was funded in part through the NIH/NCI Cancer Center Support Grant P30 CA008748.