Room: Exhibit Hall
Purpose: In this study, the path setting characteristics of a set of small apertures for electron radiotherapy based on Monte Carlo simulation was applied to evaluation of clinical rationale of in vivo measurement.
Methods: A customized cutout set with small circular aperture with diameters ranging 2.0cm, 2.5cm, 3.0cm, 3.5cm, 4cm, 4.5cm, 5cm and 5.5 cm were used for this study. All apertures were mounted on a 6x6 electronic cone from a trilogy LINAC from Varian Medical system. The radiation dose delivered was 100 monitor unit at 6MV electron beam. An electron diode was placed about the solid water phantom surface about the A16 and PTW farmer chambers at 1 cm depth. The chamber and diode readings were collected and the Electron Beam Monte Carlo simulation was applied to compare the physical measurement. The evaluation variable included the normalized difference between in vivo diode reading v.s chamber reading, in vivo diode reading and the Monte carlo simulation of diode volume mean dose, and A16 chamber volume and Farmer chamber volume dose.
Results: The difference between the diode reading between the chamber measurement for 2.0cm aperture was about 35% and 21% for farmer chamber and A16 chamber. The trend line between the Monte Carlo simulation and measurement was similar, the maximum difference was about 9%, which was located at the 2.0cm aperture.The difference was not significant when the aperture diameter was above 4.0cm.
Conclusion: In single point calibration approach, in vivo verification for small aperture electron cutout may exist large discrepancy affecting clinical judgement. Current Monte Carlo simulation could mimic the result, and further improvement of the simulation with smaller aperture could be relied on the investigation of physical reality of calculation resolution, particle histories and smoothing method.