SU‐E‐T‐467: The Trick in MLC Calibration to Correct Underdose Region in IMRT Delivery

Abstract

Purpose: By verifying the consistency between the planning dose and the delivered dose for each segment, we figure out the cause of the low dose regions inside the treatment field which only occur in verification images, but not in the treatment plan. Methods: The treatment planning system is ADAC Pinnacle version 9.0 with SmartArc. The IMRT plan is transferred to IMPAC oncology management system version 8.3 to drive the LINACs. Instead of taking an overall two or three dimensional dose distribution, we take the fluence maps for each beam by using Kodak XV films on two ELEKTA Precise LINACs and one ELEKTA Synergy LINAC. Results: Low dose regions are discovered in only Synergy LINAC and always in gun‐to‐target (G‐T) direction. By comparing the fluence maps between treatment plan and verification film segment by segment, we find these low dose regions result from the jaw position, i.e. the MLC directional backup jaw is too close to cause these unplanned underdoses in the treatment volume. We recheck the light‐field and the radiation field again; there is coincidence between the light‐field and the radiation field and just on the correct position. According to the statute, each radiation field edge is to be aligned with the displayed diaphragm within ±1 mm and each light‐field edge is to be aligned with the radiation field edge within ±1 mm. We recalibrate the MLC directional backup jaws and MLCs position to 0.5 mm larger than display and recheck the fluence maps, the low dose regions disappear eventually Conclusions: When doing the radiation field size calibration, one should calibrate the MLC moving directional radiation field size 0.5 mm larger than display each side on ELEKTA machine to prevent the unanticipated low dose regions inside the tumor in IMRT delivery.