Ionometric electron dosimetry inside water-equivalent plastic phantoms demands special considerations including determination of depth scaling and uence scaling factors (c pl and h pl ) to shift from in-phantom measurements to those relevant to water. This study evaluates these scaling factors for RW3 slab phantom and also introduce a new coe cient, 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 c pl , h pl , and k(RW3) were measured for conventional electron energies of 4, 6, 9, 12, and 16 MeV. Separate measurements were performed in water and in the RW3 slab phantom using the Advanced Markus chamber. The validity of the reported scaling factors was con rmed by comparing the direct and indirect percentage depth dose (PDD) measurement in water and in the RW3 phantom.The c pl 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 16 MeV electron energies. The h pl and k(RW3) values were dependent on the investigation depth and on the electron energy. Application of the c pl -h pl factors and of the k(RW3) coe cients 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 1.5% in both approaches for PDD conversion (c pl -h pl coupling and the use of k(RW3)).The scaling factors measured and the k(RW3) coe cients are su ciently relevant to mimic water-based dosimetry results through indirect measurements inside the RW3 slab phantom. Nevertheless, employing k(RW3) is more straightforward than the c pl -h pl approach because it does not involve scaling and it is also less time-consuming.
