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Delivery of 3D Conformal Proton-Therapy Fields at Extended Source-To-Axis Distances

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E Kryck

E Kryck*, R Slopsema , University of Florida Proton Therapy Institute, Jacksonville, Florida


SU-E-T-623 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To evaluate the dosimetric properties of proton dose distributions delivered at extended source-to-skin distances (SSD).

Methods: Radiation was delivered with a gantry-mounted proton double scattering system (Proteus 230, IBA). This system has a maximum field diameter of 24 cm at isocenter and a nominal source-to-axis distance of 230 cm. Dose was measured at nominal SSD as well as at -10, +10, +25, +50, +75, and +100 cm for several range and modulation width combinations. Depth dose distributions were measured with a multi-layer ionization chamber (MLIC), and lateral dose distributions with a 2D ionization chamber array as well as with a diode in a water phantom.

Results: The maximum field diameter was found to increase from 24.0 cm at nominal SSD to 29.1 cm and 33.3 cm at +50 cm and +100 cm respectively. Field flatness remained below 3% for all SSD. Tilt in the spread-out Bragg peak depth dose distribution increased with SSD up to 0.4 %-per-g/cm2 at +100 cm. The measured range decreased with 1.1x10-3 g/cm2 per centimeter shift in SSD due to proton energy loss in air, very close to the theoretically calculated value of 1.06x10-3 g/cm3. The output and dose rate decrease with the inverse of the SSD squared as expected.

Conclusion: Extending the SSD up to 100 cm increases the maximum field diameter from 24.0 cm to 33.3 cm while the dose uniformity remains acceptable.

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