On the measurement of scaling factors in the RW3 plastic phantom during high energy electron beam dosimetry.

Abstract

Ionometric electron dosimetry inside water-equivalent plastic phantoms demands special considerations including determination of depth scaling and fluence scaling factors (cpl and hpl) to shift from in-phantom measurements to those relevant to water. This study evaluates these scaling factors for RW3 slab phantom and also introduces a new coefficient, k(RW3), for direct conversion from RW3 measurements to water without involving scaling factors. The RW3 solid phantom developed by the PTW Company was used and the corresponding scaling factors including cpl, hpl, and k(RW3) were measured for conventional electron energies of 4, 6, 9, 12, and 16MeV. Separate measurements were performed in water and the RW3 slab phantom using the Advanced Markus chamber. The validity of the reported scaling factors was confirmed by comparing the direct and indirect percentage depth dose (PDD) measurements in water and in the RW3 phantom. The cpl values for the RW3 phantom were respectively equal to 0.915, 0.927, 0.934, 0.937, and 0.937 for 4, 6, 9, 12, and 16MeV electron energies. The hpl and k(RW3) values were dependent on the depth of investigation and electron energy. Application of the cpl-hpl factors and k(RW3) coefficients to measured data inside the RW3 can reliably reproduce the measured PDD curves in water. The mean difference between the PDDs measured directly and indirectly in water and in the RW3 phantom was less than 1.2% in both approaches for PDD conversion (cpl-hpl coupling and the use of k(RW3)). The measured scaling factors and k(RW3) coefficients are sufficiently relevant to mimic water-based dosimetry results through indirect measurements inside the RW3 slab phantom. Nevertheless, employing k(RW3) is more straightforward than the cpl-hpl approach because it does not involve scaling and it is also less time-consuming.