Feasibility of tungsten functional paper in electron grid therapy: a Monte Carlo study

Electron grid therapy is expected to be a valid treatment for bulky superficial tumors. It is difficult, however, to fit irradiation fields to bulky superficial tumor shapes for conventional electron grid therapy with a cerrobend grid collimator. In this study, we investigated whether a grid collimator using tungsten functional paper (TFP), with its radiation shielding ability, could be used for electron grid therapy. Dose distributions were measured using 9 MeV electron grid beams from a cerrobend grid collimator. For the simulation study, the same grid irradiation fields were shaped using a TFP grid collimator (thicknesses of 0.15, 0.3, 0.6, 0.9, and 1.2 cm) by laying them on a phantom. We then determined the dose distributions using Monte Carlo calculations and compared the cerrobend and TFP electron grid beams regarding dose distributions, including the depths of the maximum dose (dmax), 90% dose (d90), and 80% dose (d80), and the ratios of the doses in the areas with and without shielding (valley to peak ratios). The equivalent dosimetric thickness was obtained with the TFP grid collimator that was equivalent to the dose distribution of the cerrobend grid collimator. For the cerrobend electron grid beams, the dmax, d90, and d80 were 1.0, 2.1, and 2.5 cm, respectively, and the valley to peak ratios at those depths were 0.48, 0.66, and 0.73, respectively. The equivalent dosimetric thickness of TFP was 0.52 cm. The dmax, d90, and d80 for the 0.52 cm thick TFP electron grid beams were 1.1, 1.9, and 2.3 cm, respectively, and the valley to peak ratios at those depths were 0.49, 0.63, and 0.71, respectively. The TFP grid collimator flexibly delivered excellent dose distributions by simply attaching it to the patient's skin. It could thus be used for electron grid therapy instead of the cerrobend grid collimator.