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Validation of the Proton Pristine Bragg Peak in a Compact Synchrotron Treatment System

Y Xu*, H Kooy, N Depauw, J Shin, G Sharp, B Winey, K Jee, B Clasie, Massachusetts General Hospital, Harvard Medical School, Boston, MA

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose:

Commissioning and validation of pristine Bragg peaks (BP) and reference depth doses such as spread-out-Bragg-peaks (SOBP) is essential to achieving accurate dosimetry in proton therapy. BP commissioning involves three steps: energy spread and range as a function of energy measured with a wide detector, normalization as a function of energy measured with a narrow detector, and interpolation to intermediated energies tuned using Monte Carlo (MC).

Methods:

Seventeen BP and four SOBP measurements were performed using a water phantom with BP and Markus parallel plate ion chambers, respectively. BP validation was done by optimizing pseudo-flat dose distributions of SOBPs and checking the measured dose against the expected dose. EBT film was used to check the detector alignment. Energies were measured from 70 to 250 MeV at gantry 270 ° and 250 MeV at gantry 0 °. Ion chambers were set to voltages of 400 V or 200 V to check recombination. Energy contamination was reduced by adjusting the last beamline steering magnet.

Results:

The BP shape was independent of gantry angle and recombination was < 1%. Energy spread, sE, was less than 0.3 MeV. Range at distal 80% dose (R80) and energy polynomial fit resulted in a maximum residual error of 0.5 mmH20. The BP normalization in MC was adjusted = 6% to match measurements. SOBP measurements agree with the MC-generated machine model within 2% except in the entrance region and near the end of range. The SOBP R80 agreed within 0.5 mmH20. Energy contamination reduction adjustments affected the range by < 0.2 mmH20.

Conclusion:

Measured BP from the compact proton synchrotron system was used to tune a MC- derived treatment planning system model. For the SOBP, the model agreed with measurements within 2% except where steep dose gradients existed in the entrance region and near the end of range.

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Keywords

Synchrotron Radiation, Commissioning, Dosimetry

Taxonomy

TH- External Beam- Particle/high LET therapy: Proton therapy – experimental dosimetry

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