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Excessive IMRT Patient Dose Due to the MLC Misalignment and Its Determination Using Archeck From Sun Nuclear

E Lief

E Lief*, P Jeffe , J Restrepo , A Cheuk , VA Medical Center, Bronx, NY


SU-I-GPD-T-272 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: 1) To quantify potential excessive patient dose due to MLC misalignment in sliding window IMRT treatments, 2) To choose a method of measuring this dose in the patient-specific QA, 3) To suggest using MLC tuning to eliminate this discrepancy.

Methods: IMRT treatment of large volumes, such as pelvis, using 7-9 static sliding window fields. The total number of MU in 14-18 split segments was about 2000, even for a standard 180 cGy prescription. Patient-specific QA was performed first with MapCheck, where all the fields were directed down, and then with ArcCheck with the fields at their nominal angles. The measurements were repeated after the MLC cleanup with motors, driveshafts, t-nuts, smart cards, leaf wipers and softpots changed.

Results: Measurements with MapCheck showed required 95% compliance to our standard criteria 3%/3mm, while similar criteria in ArcCheck typically failed 25% measurement points by exceeding the calculated value by 2-5%. This can result in dose excess to the patient of the order of 3%. After the MLC tune up, QA on both devices successfully passed. ArcCheck was instrumental in uncovering those discrepancies because the measured dose was spread onto a larger surface with less overlap, and cumulative discrepancies in low-dose regions added up to unacceptable level.

Conclusion: Highly modulated large IMRT fields with sliding window tend to deliver a large number of monitor units with a potential of excessive dose to the patient of about 3%, due to MLC misalignment. This discrepancy can be better measured by ArcCheck, where the dose is spread over a larger surface. The MLC tuneup procedure can significantly reduce the disagreement to an acceptable level.

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