Dosimetrical characteristics of a cubic-block-piled compensator for intensity-modulated radiation therapy.

Abstract

We examined the dose distributions generated by Pinnacle3 for IMRT plan with the cubic-block-piled compensator as the intensity modulator for photon beams of 4 MV and 10 MV. This TPS uses an algorithm in which only the physical density of the absorber is required for calculation of the characteristics of the modulator. The intensity modulator consists of cubic blocks (attenuator) of a tungsten alloy, as well as cubic blocks composed of polyethylene resin foam, which fill the spaces between the attenuator and the PMMA boards used as the platform for the modulator. By measuring the transmission for various thicknesses of the attenuator and deriving values for the total physical density of the modulator, we determined the optimal effective density by comparing the curves fitted for the actual transmission data with the transmission calculated by Pinnacle3. Using these effective densities, the accuracy of Pinnacle3 was examined for the dose profiles of specific geometrical patterns. The levels of consistency between the measurements and calculations were within a tolerance of 3% of the dose difference and had a 3 mm distance-to-agreement for the ladder, stairstep, and pyramid-shaped test patterns. In this modulator assembly, there was leakage from the pinhole slits between the corners of the cubic blocks. This leakage was about 1% and the influence on dose distribution was not crucial. However, for the ladder pattern, the calculated doses exceeded the measured doses by a maximum of 8% for the region in which the absorber was absent. In the TPS, in which physical density was the only user-controllable parameter, we used the effective density of the absorber deduced from the effective mass attenuation coefficient. We conclude that the intensity modulation compensator system together with a piled cubic attenuator is clinically applicable with an acceptable tolerance level.