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Absolute Monte-Carlo Dose Calculations Using the Numberof Protons in Spot-Scanning Proton Beams

A Pourmoghaddas

A Pourmoghaddas*, W Yao , R Lukose , T Merchant , J Farr , V Moskvin , St. Jude Children's Research Hospital, Memphis, TN


SU-I-GPD-T-124 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Monte-Carlo (MC) dose calculations in proton radiotherapy is an accurate method for treatment planning verification. The goal of this study is to determine the proton flux for each beam energy at our synchrotron based facility to guide absolute MC-based dose calculations.

Methods: The absorbed dose at reference depth (2 cm) in a water phantom was measured using a Bragg-Peak Ionization chamber (PTW) for 96 pristine nominal energies ranging from 69.4 - 221.3 MeV. Phase-space data corresponding to each irradiation were generated by using TOPAS and the passed onto FLUKA for dose scoring. The MC-based absorbed dose per primary particle was calculated and divided by physical measurement given the delivered monitor units (MU) to return number of protons per MU for each energy. To verify, the delivery of a 10cmx10cm spread-out Bragg peak (SOBP) field commonly used to check the machine output was simulated with 4 million primary protons, using the MC framework mentioned above. The MC dose values were compared with pinpoint ionization chamber (PTW) readings for multiple points along the depth profile.

Results: The number of protons per MU range from 9x108 to 2x109 for proton energies ranging from 69.4 - 221.3 MeV. For the SOBP test-case, the difference in dose between MC vs measurement was 2.3% at the mid-point of the SOBP and varied from 0.5%-1.6% on the proximal edge and from 1.8%-8% on the distal edge of the SOBP. Validation for heterogeneous medium and intensity modulated field is underway.

Conclusion: Estimation of the primary proton flux per MU is feasible for use with spot-scanning proton beams and can be used to guide absolute MC dose calculations.

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