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Using High-Definition Multileaf Collimator (HDMLC) for Long Volume Treatment with Multiple Isocenter VMAT (RapidARC)

C Venencia

C Venencia*, Y Schworer , E Garrigo , F Munoz , S Zunino , Instituto de Radioterapia - Fundacion Marie Curie, Cordoba, ARGENTINA


SU-I-GPD-T-510 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: The use of HDMLC provides the highest target dose conformation and OAR spares, especially for SRS and SBRT treatments. However, its maximum IMRT field size of 32cmx22cm requires the use of multiple isocenter when long volume needs to be treated. Dose distribution at field’s junctions and dosimetric impact of patient position uncertainties could be a concern. The objective of this work was to evaluate the dose distribution on long volume treatments using multiple isocenter RapidARC.

Methods: A 6MV beam produced by a Novalis TX (BrainLAB-Varian) equipped with HDMLC and Eclipse v13.6 TPS were used. A solid water phantom with multiple slab and ion chamber insert was used. Four RapidARC plans, with two and three isocenters (single and double arcs per isocenter) were created for the treatment a long square volume. Mean dose at the ion chamber volume was calculated. Coronal 2D dose distribution was measured using film dosimetry (EDR2). Phantom was positioned using infrared spheres. Longitudinal errors of ±5mm between isocenters were purposely introduced in one of the plans (Plan 5). Comparison between measured versus calculated dose distribution was made using RITv6.5 software by gamma evaluation (3%/3mm). Treatment planning for cervix cancer was performed and compared with IMRT using 1cm leaf width MLC. Treatment planning for cranio-spinal irradiation (CSI) was performed and compared with conventional technique.

Results: Absolute dose variation was less than 1.2%. Gamma evaluation showed an agreement better than 99%. Gamma evaluation for Plan 5 showed a reduction of the agreement to less than 80% in the region corresponding to field junction. Treatment planning of cervix and CSI achieved all clinical requirements.

Conclusion: RapidARC plans with HDMLC can be used for the treatment of long volume without any planning compromise and length limitation. The distance between isocenters is crucial and patient position must be defined before treatment.

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