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SU‐E‐T‐623: Delivery of 3D Conformal Proton‐Therapy Fields at Extended Source‐ To‐Axis Distances
Author(s) -
Kryck E,
Slopsema R
Publication year - 2014
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4888959
Subject(s) - isocenter , ionization chamber , percentage depth dose curve , imaging phantom , bragg peak , dosimetry , monitor unit , proton therapy , physics , optics , flatness (cosmology) , materials science , proton , nuclear medicine , ionization , beam (structure) , ion , nuclear physics , medicine , cosmology , quantum mechanics
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 1.1×10‐3 g/cm2 per centimeter shift in SSD due to proton energy loss in air, very close to the theoretically calculated value of 1.06×10‐3 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.