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SU‐FF‐T‐335: Dose Uncertainty Due to Motion‐Induced Depth Changes in Energy‐Stacked Proton Beams
Author(s) -
Slopsema R
Publication year - 2009
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 - 10.1118/1.3181816
Subject(s) - sobp , proton therapy , imaging phantom , ionization chamber , percentage depth dose curve , dosimetry , bragg peak , amplitude , proton , dose profile , strips , energy (signal processing) , range (aeronautics) , physics , materials science , nuclear medicine , optics , computational physics , ionization , nuclear physics , ion , medicine , composite material , quantum mechanics
Purpose: To measure the dose uncertainty due to motion‐induced depth changes in energy‐stacked proton fields. Method and Materials: In energy‐stacked proton beams the spread‐out Bragg peak (SOBP) is generated by delivering the energy layers sequentially. In the presence of motion the depth of a point can change between the different energy layers, generating uncertainty compared to the planned dose. The magnitude of the uncertainty depends on the time‐structure of the dose delivery, and the frequency and amplitude of the depth variations. We add wax strips to the outside of a 4D Dynamic Thorax Phantom (CIRS). The strips are 1.5cm wide and 0.5 or 1.0cm thick. A cc04 ionization chamber (IBA Dosimetry) is moved perpendicularly behind the strips, at various amplitude and frequency. The proton field is delivered using both energy‐stacking and ‘quasi‐instantaneous’ for comparison. For each combination of motion and field delivery the dose is measured ten times to determine the statistical variation. A Matlab program determines the expected error in dose based on the time‐structure of both the motion and dose delivery. Results: Motion at 5.0cm depth behind 0.5cm strips, at a frequency of 0.25Hz, in a field with a range of 8.0g/cm2, modulation width of 6.0g/cm2, dose rate of 2Gy/min, total dose of 50cGy, gives a maximum error in dose of 7.4%. Decreasing the dose rate to 1Gy/min reduces the error to 5.3%. Conclusion: Motion‐induced variations in depth can lead to clinically significant dose uncertainties.