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Peripheral Dose Comparison Between Static and Dynamic Jaw Tracking On a High Definition MLC System

C Chuang

A Perez-Andujar , J Cheung , C Chuang*, University of California San Francisco, San Francisco, CA


SU-F-T-307 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

To investigate the effect of dynamic and static jaw tracking on patient peripheral doses.

Materials and Methods:
A patient plan with a large sacral metastasis (volume 800cm3, prescription 600cGyx5) was selected for this study. The plan was created using 2-field RapidArc with jaw tracking enabled (Eclipse, V11.0.31). These fields were then exported and edited in MATLAB with static jaw positions using the control point with the largest field size for each respective arc, but preserving the optimized leaf sequences for delivery. These fields were imported back into Eclipse for dose calculation and comparison and copied to a Rando phantom for delivery analysis. Points were chosen in the phantom at depth and on the phantom surface at locations outside the primary radiation field, at distances of 12cm, 20cm, and 30cm from the isocenter. Measurements were acquired with OSLDs placed at these positions in the phantom with both the dynamic and static jaw deliveries for comparison. Surface measurements included an additional 1cm bolus over the OSLDs to ensure electron equilibrium.

The static jaw deliveries resulted in cumulative jaw-defined field sizes of 17.3% and 17.4% greater area than the dynamic jaw deliveries for each arc. The static jaw plan resulted in very small differences in calculated dose in the treatment planning system ranging from 0-16cGy. The measured dose differences were larger than calculated, but the differences in absolute dose were small. The measured dose differences at depth (surface) between the two deliveries showed an increase for the static jaw delivery of 2.2%(11.4%), 15.6%(20.0%), and 12.7%(12.7%) for distances of 12cm, 20cm, and 30cm, respectively. Eclipse calculates a difference of 0-3.1% for all of these points. The largest absolute dose difference between all points was 6.2cGy.

While we demonstrated larger than expected differences in peripheral dose, the absolute dose differences were small.

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