Room: Exhibit Hall
Purpose: As the technology advanced we are able, now to develop treatment plans using 6X Flattening Filter Free (FFF) for RapidArc. We noticed that the dose rate has high fluctuations in value during the treatment delivery for 6X FFF. We started monitoring the dose delivered using portal dosimetry.Our goal is to develope arc treatment plans when using dose rates above 600 MU/min.
Methods: A retrospective study was performed on 25 patients treated using a Varian TrueBeam machine.The SBRT plans were made using RapidArc technique in Varianâ€™s Eclipse. For 25 treatment plans, we measured the dose using the Electronic Portal Imaging Dosimetry (EPID) and the diode detectors via Mapcheck. We also recorded the dose rate variance during the treatment delivery and the dose conformality index. We evaluated the biological effective dose (BED), normal tissue complications probability (NTCP) and tumor control probability (TCP).
Results: For every treatment delivered the dose rate varies between 100 MU/min and the prescribed rate (600 to 1400 MU/min).The dosimetric measurements are correlating very well with the dose rate variance. There is also a correlation between the dose rate, the width of the treated fields and the arcâ€™s size. The larger the field the higher the dose rate variance. We also noticed difficulties in passing the complex RapidArc QA measuring plans consistency with the range of the dose rate variation.Biological effectiveness is also affected by the dose rate variance in a percentage between 0.75% and 2.73%.
Conclusion: We found optimum dose rate for RapidArc with a value of 622MU/min that allow a minimum dose rate variance of 36MU/min during the treatment delivery and an optimum field size that in combination with the correct dose rate will generate plans that will deliver the calculated dose rate with a minimum variance and will not depend to the arcâ€™s degrees range.