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VMAT CSI: Getting the Junction Right

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N Clements

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


SU-I-GPD-T-430 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Craniospinal irradiation (CSI) presents a challenge to radiotherapy planning given the extended anatomy that requires treatment. 3D CRT plans generally involve jaw-defined feathered junctions between brain and spine fields to minimize hot/cold spots in the target. One challenge of VMAT CSI is achieving linearly-ramping dose profiles across arc overlap regions; desirable for robustness against misalignments. The literature reports using control structures and convoluted optimization strategies to achieve this; while this study sought out simple and adaptable field/beam parameters to generate linearly-ramping dose profiles across junctions.

Methods: Plans were generated using Eclipse AAA (v11.0.31) on a pediatric (8yr F) dataset previously immobilized for and treated with 3DCRT. A 360° brain arc and 360° spine arc were initially used to optimize a single junction of varying length. The work then extended to these configurations: 3 arcs separated longitudinally, 2 brain arcs and a spine arc, 2 brain arcs plus 2 upper and 2 lower spine arcs. Longitudinal misalignments of ±1,2mm in 3DCRT and VMAT plans were simulated in Eclipse to compare robustness of VMAT junctions.

Results: Linearly-ramping junctions were generated for all configurations using the following optimal parameters: using only the outer 10 mm wide MLC leaves in the junction, offsetting MLC leaves by 5mm, y-jaws positioned 2mm across MLC leaf, and collimator angles up to ±5°. Shortest and longest overlaps of 5mm, and 85mm resulted in junction dose variations of ±20%, and ±4%, respectively. Simulated 2mm misalignments of 3DCRT and VMAT junctions increased junction dose by 30%, and 5%, respectively.

Conclusion: Judicious positioning of isocentres, jaws, and collimators created linearly-ramping junctions for various beam arrangements. Longer overlaps both reduce dose variation across junctions and increase robustness to misalignments. The adaptable nature of these strategies increases VMAT CSI planning efficiency without the need for elaborate measures as described in the literature.

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

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