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Commissioning of Craniospinal Irradiation with VMAT Using a Novel Method to Produce Arc Junctions with Linear Dose Gradients

C Bojechko

C Bojechko*, N Clements , Tom Baker Cancer Centre, Calgary, AB


SU-K-FS1-8 (Sunday, July 30, 2017) 4:00 PM - 6:00 PM Room: Four Seasons 1

Purpose: Craniospinal-irradiation (CSI) has long been a challenging treatment due to the necessity for field junctions. For 3D-CRT treatments even minor misalignment can lead to cold/hot spots on the order of 30-40% due to the matching of sharp penumbra regions. Recently overlapping VMAT arcs have been used create more gradual junctions making use of a planning structure set to force stepwise gradients across the junction. We have developed a method to create overlapping arcs with a linear dose gradient without the need for multiple structures. We integrated this technique in the commissioning of clinical CSI treatments, creating high quality, safe treatments. New planning techniques as well as immobilization techniques were developed in the commissioning of the treatment.

Methods: Commissioning consisted of a planning study, phantom measurements and immobilization considerations. Plans were made for patients with a CT data set of the whole brain and spine, previously treated with CSI. Plans were generated using Eclipse AAA (v11.0.31) millennium 120 MLC leaves. The VMAT plans were optimized to produce junctions robust to misalignments while providing the best target coverage and normal tissue sparing. Measurements of the junction region were made with the Octavius SRS detector.

Results: It was found that 3 isocenters was able to produce the best overall dose distribution for different patient sizes. Arc overlaps of 8 cm produced the most robust junction dose profiles. Phantom measurements of the junction found that shifts of up to 3 mm resulted in a maximum underdose of 10%. Organ at risk doses were reduced by the optimization of the placement of the junction region and the use of avoidance sectors.

Conclusion: We commissioned a CSI technique using VMAT that is able to provide treatments robust to field misalignments while providing better target coverage, homogeneity and better sparing of organs at risk.

Funding Support, Disclosures, and Conflict of Interest: Cancer Control Alberta Enhanced Care and Innovative Project Funding

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