Verification of an independent dose calculation method for portal-specific QA of proton and carbon ion beams

Objective

To verify the accuracy of an independent dose calculation method, as incorporated into an in-house developed treatment planning system (TPS), for performing quality assurance of dose distributions delivered to a water phantom planned by a clinical TPS.

Methods

A Monte Carlo based track repeating algorithm was incorporated into an in-house treatment planning system for proton and carbon ion beams. Calculations were performed in a flat water phantom for both a traditional pencil beam algorithm and a new Monte Carlo algorithm, and then compared to measurements made at multiple depths with a 2D ionization array for 44 patient portals. The comparisons utilized a Gamma analysis.

Results

A total of 124 measurements were performed for proton and carbon ion patient portals. Using a small Gamma criteria of 2%/2 ​mm, an average of 93% and 97% of measurement points passed for each portal for pencil beam and Monte Carlo calculations, respectively. The passing rate was substantially higher for Monte Carlo calculations than for pencil beam calculations for portals that used a range shifter.

Conclusions

The implemented independent method has been verified against measurements. The high passing rate with small tolerances leads to the possibility of reducing the number of required quality assurance measurements.