Stopping-power ratio of body tissues with updated effective energies and elemental I values for treatment planning of proton therapy and ion beam therapy with helium, carbon, oxygen, and neon ions

The stopping-power ratio (SPR) of body tissues relative to water depends on the particle energy and mean excitation energy ( I value) of the tissues. Effective energies to minimize the range error in proton therapy and ion beam therapy with helium, carbon, oxygen, and neon ions and elemental I values have been updated in recent studies. We investigated the effects of these updates on SPR estimation for computed tomography-based treatment planning. The updates led to an increase of up to 0.5% in the SPRs of soft tissues, whereas they led to a decrease of up to 1.9% in the SPRs of bone tissues compared with the current clinical settings. For 44 proton beams planned for 15 randomly sampled patients, the mean water-equivalent target depth change was − 0.2 mm with a standard deviation of 0.2 mm. The maximum change was − 0.6 mm, which we consider to be insignificant in clinical practice.