Poster — Thur Eve — 65: A dosimetric comparison of isocentric and non-isocentric coplanar SBRT VMAT plans for peripheral lung tumours

Authors

  • Conroy L,

    1. Department of Medical Physics, Tom Baker Cancer Centre, Calgary, Alberta
    2. Department of Physics and Astronomy, University of Calgary, Calgary, Alberta
    3. Central Alberta Cancer Centre, Red Deer, Alberta
    4. Department of Oncology, University of Alberta, Edmonton, Alberta
    5. Department of Oncology, University of Calgary, Calgary, Alberta
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  • Liu HW,

    1. Department of Medical Physics, Tom Baker Cancer Centre, Calgary, Alberta
    2. Department of Physics and Astronomy, University of Calgary, Calgary, Alberta
    3. Central Alberta Cancer Centre, Red Deer, Alberta
    4. Department of Oncology, University of Alberta, Edmonton, Alberta
    5. Department of Oncology, University of Calgary, Calgary, Alberta
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  • Lau H,

    1. Department of Medical Physics, Tom Baker Cancer Centre, Calgary, Alberta
    2. Department of Physics and Astronomy, University of Calgary, Calgary, Alberta
    3. Central Alberta Cancer Centre, Red Deer, Alberta
    4. Department of Oncology, University of Alberta, Edmonton, Alberta
    5. Department of Oncology, University of Calgary, Calgary, Alberta
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  • Smith WL

    1. Department of Medical Physics, Tom Baker Cancer Centre, Calgary, Alberta
    2. Department of Physics and Astronomy, University of Calgary, Calgary, Alberta
    3. Central Alberta Cancer Centre, Red Deer, Alberta
    4. Department of Oncology, University of Alberta, Edmonton, Alberta
    5. Department of Oncology, University of Calgary, Calgary, Alberta
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Abstract

Volumetric modulated arc therapy (VMAT) delivers lung sterotactic body radiotherapy (SBRT) in shorter treatment time and less monitor units with comparable coverage and organ at risk sparing compared to conventional SBRT treatments. Isocentric VMAT treatment of peripheral lung tumours occasionally requires couch shifts that can inhibit 360° gantry rotation, resulting in additional imaging shifts for each treatment session, and increased potential for involuntary in-fraction motion. Here, we investigate whether non-isocentric VMAT plans can achieve comparable plan quality to isocentric plans for peripheral lung tumours. Three patient plans were selected with targets displaced > 8.5 cm (range: 8.8 – 9.9 cm) laterally from patient midline. For each patient, a plan with isocentre placed within the target volume (isocentric plan) was created and optimized. The same optimization parameters were then used to create a plan with the isocentre at patient midline (non-isocentric plan). Plan quality was evaluated and compared based on planning target volume (PTV) coverage, high dose spillage, dose homogeneity, intermediate dose spillage, dose fall-off gradient, and organ at risk contraints. Non-isocentric plans of equivalent plan quality to isocentric plans were achieved for all patients by optimizing collimator rotations. Field isocentres can be placed at patient midline, as opposed to inside the target volume, with no significant degradation in VMAT plan quality for lateral tumour displacements up to 10 cm. Non-isocentric treatment of peripheral lung tumours could result in decreased overall treatment session time and eliminate the need for imaging shifts prior to VMAT treatment.

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