S Srivastava^1,2*, S Jani^1,2 , D Pinnaduwage^1,2 , X Yan^1,2 , P Metuge^1,2 , D Castaneda² , S Patel² , N Thawani² , S Mutyala² , I Das³ , S Sorensen^1,2 , (1) Barrow Neurological Institute, Phoenix, AZ, (2) University of Arizona Cancer Center \ Dignity Health, St. Joseph's Hospital, Phoenix, AZ, (3) NYU Langone Medical Center, New York, NY,

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  S Srivastava

(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: Dosimetric conformity and organ sparing is integral to the planning and delivery of hypofractionated treatments where even small changes may have significant clinical implications. This study investigates the dosimetric impact of BodyFix positioning system used in SBRT.

Methods: The BodyFix was investigated for attenuation and surface dose differences, as a function of beam energy (6MV, 10MV, 10FFF, 18MV), field size (5x5 and 20x20 cm²), thickness (t=2.5cm, 7.5cm, 12.5cm for attenuation, t=0mm, 1mm, 2mm for skin dose) and gantry angle (g=0°, 25°, 50° for attenuation, g=0° for skin dose ). Eclipse treatment plans were generated on a custom-made phantom setup. Dose was calculated by including and excluding the BodyFix in the body contour. Measurements were taken with a Farmer chamber for attenuation, and parallel plate chamber for skin dose. Measurements were compared with AcurosXB calculations (1x1x1mm³ dose grid). The dosimetric effect of BodyFix was investigated for ten clinical SBRT cases.

Results: Dose attenuation due to 2.5cm of BodyFix, was 3-4% (5x5cm²) and 2-4% (20x20cm²) for all energies. This increased to 4-7% and 4-6% for 12.5cm BodyFix thickness. Attenuation of ~8-10% was found for an oblique gantry angle of 50°, for all field sizes. Measured and calculated data was within ±0.5%. Skin dose increased by 4-5, 2-3, and 1.5-2 fold for 0mm, 1mm and 2mm depth, respectively, across all field sizes. With increasing beam energy the impact on attenuation and skin dose diminished as expected. For clinical plans, including the BodyFix within the body contour, decreased PTV coverage and OAR doses by 3-5%.

Conclusion: Dosimetric impact of any new SBRT fixation device should be investigated before clinical use, as BodyFix can decrease overall dose distribution by ~5% and increase surface dose by ~5fold. A CT scan field-of-view encompassing the entire fixation device is recommended for improved accuracy in treatments.

Keywords

Immobilization, Stereotactic Radiosurgery, Treatment Planning

Taxonomy

TH- External beam- photons: extracranial stereotactic/SBRT

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