  Objective: To develop and validate an independent Monet Carlo dose
calculation engine to support for software verification of treatment planning
systems and quality assurance workflow. Method: GATE Monte Carlo toolkit was
employed to simulate a fixed horizontal active scan-based proton beam delivery.
Within the nozzle, two primary and secondary dose monitors have been designed
allowing to compare the accuracy of dose estimation from MC simulation with
respect to physical quality assurance measurements. The developed beam model
was validated against a series of commissioning measurements using pinpoint
chambers and 2D array ionization chambers in terms of lateral profiles and
depth dose distributions. Furthermore, beam delivery module and treatment
planning has been validated against the literature deploying various clinical
test cases of AAPM TG-119 and a prostate patient. Result: MC simulation showed
an excellent agreement with measurements in the lateral depth-dose parameters
and SOBP characteristics within maximum relative error of 0.95% in range, 3.4%
in entrance to peak ratio, 2.3% in mean point to point, and 0.852% in peak
location. Mean relative absolute difference between MC simulation and the
measurement in terms of absorbed dose in SOBP region was $0.93\% \pm 0.88\%$.
Clinical phantom study showed a good agreement compared to a commercial
treatment planning system (relative error for TG-119 PTV-D${}{95}$
$\mathrm{\sim}$ 1.8%; and for prostate PTV-D$_{95}$ $\mathrm{\sim}$ -0.6%).
Conclusion: The results confirm the capability of GATE simulation as a reliable
surrogate for verifying TPS dose maps prior to patient treatment.
