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Preliminary Measurements Towards Radiation Therapy Involving Dynamic Couch Trajectories

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J Mullins

J Mullins*, J Seuntjens, F DeBlois, A Syme, McGill University, Montreal, QC

TU-E-108-4 Tuesday 2:00PM - 3:50PM Room: 108

Purpose: To assess the geometrical accuracy of treatment deliveries involving dynamic couch trajectories and to evaluate the accuracy of DMLC dose calculations in Eclipse at nonstandard SSDs.

Methods: Eleven DMLC IMRT plans were recalculated as verification plans in Eclipse (AAA 10.0.28) using the collapsed gantry method, at SSDs between 85-135 cm. Ion chamber and film measurements were performed to compare planned and delivered dose. DLG and MLC transmission were also measured as a function of SSD. Simple circular couch trajectories that simulated isocentric treatment at an SAD of 95 cm were evaluated using film and ion chamber measurements. Trajectories were implemented using Varian TrueBeam Developers Mode.

Results: The relative agreement between Eclipse-calculated dose and ion chamber measurements decreased systematically by an average of 1.5±0.3 % between 85 and 135 cm SSD. Film measurements were consistent with the ion chamber results. Following corrections for the geometric projection of the DLG, no dependence on SSD was observed, and the dependence of MLC transmission on SSD was negligible. These results are both consistent with Eclipse calculations. The standard error of ion chamber current measurements was 0.19 % for the dynamic couch trajectory at 95 cm SAD, compared to 0.17 % for static couch position. From the film measurements, the FWHM of the field delivered with dynamic couch motion was 94.6 % of the FWHM with static couch position, matching the predicted geometrical minification.

Conclusion: Experiments were performed to investigate aspects of radiation therapy involving dynamic couch trajectories. The relative agreement between calculated and measured dose was found to systematically decrease with SSD, while DLG and MLC transmission were independent of SSD. Isocenter positional accuracy with dynamic couch motion was demonstrated through the consistency of ion chamber current measurements and observation of the expected decrease in field size at a shortened SAD.

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