Program Information
The Interplay Effect When Treating Moving Tumors Using High Dose Rate and Increased MLC and Gantry Rotation Speeds
T Netherton , Y Li , P Nitsch , P Balter , S Gao , A Klopp , L Court*, UT MD Anderson Cancer Center, Houston, TX
Presentations
TH-CD-205-9 (Thursday, August 3, 2017) 10:00 AM - 12:00 PM Room: 205
Purpose: In most modulated radiation therapy techniques, the interplay between respiratory motion and moving MLC results in dose variations that have been shown to average out after several treatment fractions. There are, however, conditions that can potentially exacerbate the interplay effect, including high dose rate and fast MLC motion. We are now testing a prototype 6MV-FFF linac that has a high dose rate, and significantly faster MLC and gantry rotation speed than conventional linacs (5cm/sec and 6rpm, respectively). We investigated the impact of these on the interplay effect for VMAT treatments.
Methods: Twenty micro-MOSFETs were inserted into a tumor model in an anthropomorphic breathing phantom. The phantom was programmed to move with a sinā¶ waveform, 5second period, and peak-to-peak motion of 1 or 2 cm. VMAT plans (200cGy/fraction to the target, with 2cm margin) were created for the new linac with 1-6 arcs. Each plan was treated 5 times, with each irradiation starting at a random point in the respiratory cycle.
Results: Treatment times were 37-165 seconds. Total MUs were similar for all plans: 391-429MU. The maximum dose deviation (standard deviation) for a single point was 12%, found for 1cm motion, 1-arc plan. The average dose deviation for this scenario (across all points) was 3.3%. Plans with 2+ arcs all had similar dose deviations (average deviation 1.6-1.9% for 1cm motion and 1.9-2.4% for 2cm motion). For comparison, the measured dose deviation for a conformal (open field) plan was 1.3%, indicating the noise level of the mosefets.
Conclusion: The interplay between respiratory motion and MLC/gantry motion caused daily deviations in the target dose, but these were generally reduced when more VMAT arcs were used. We suggest that single-arc VMAT plans should be avoided when this prototype linac (with its increased MLC and gantry speeds) is used to treat lung tumors.
Funding Support, Disclosures, and Conflict of Interest: This work was funded by Varian Medical Systems
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