Abstract
Novel imaging modalities can improve the estimation of patient elemental compositions for particle treatment planning. The mean excitation energy (I-value) is a main contributor to the proton range uncertainty. To minimize their impact on beam range errors and quantify their uncertainties, the currently used I-values proposed in 1982 are revisited. The study aims at proposing a new set of optimized elemental I-values for use with the Bragg additivity rule (BAR) and establishing uncertainties on the optimized I-values and the BAR.We optimize elemental I-values for the use in compounds based on measured material I-values. We gain a new set of elemental I-values and corresponding uncertainties, based on the experimental uncertainties and our uncertainty model. We evaluate uncertainties on I-values and relative stopping powers (RSP) of 70 human tissues, taking into account statistical correlations between tissues and water. The effect of new I-values on proton beam ranges is quantified using Monte Carlo simulations.Our elemental I-values describe measured material I-values with higher accuracy than ICRU-recommended I-values (RMSE: 6.17% (ICRU), 5.19% (this work)). Our uncertainty model estimates an uncertainty component from the BAR to 4.42%. Using our elemental I-values, we calculate the I-value of water as 78.73 ± 2.89 eV, being consistent with ICRU 90 (78 ± 2 eV). We observe uncertainties on tissue I-values between 1.82-3.38 eV, and RSP uncertainties between 0.002%–0.44%. With transport simulations of a proton beam in human tissues, we observe range uncertainties between 0.31% and 0.47%, as compared to current estimates of 1.5%.We propose a set of elemental I-values well suited for human tissues in combination with the BAR. Our model establishes uncertainties on elemental I-values and the BAR, enabling to quantify uncertainties on tissue I-values, RSP as well as particle range. This work is particularly relevant for Monte Carlo simulations where the interaction probabilities are reconstructed from elemental compositions.
Highlights
Radiotherapy with protons has become the preferred treatment technique for various indications
V(r) = (K − 1N×N ) V (K − 1N×N )T + u2BAR1N×N . (12) To solve for u2BAR, we find the value such that the sum of the residuals squared normalized to their variance equals its number of degrees of freedom, that is:
Using the ICRU 37 recommended elemental I-values suggested by Berger and Seltzer, we observe root mean square (RMS) errors of 1.02% and 6.17% when using the Bragg additivity rule (BAR) to predict the underlying experimental I-values
Summary
Radiotherapy with protons has become the preferred treatment technique for various indications. The main advantage of radiotherapy with protons lies in their favorable dose deposition pattern as compared to photons. An important part of treatment planning with protons or heavier ions is the prediction of the position of this Bragg peak within the patient. In clinical practice, this position is predicted based on a single-energy CT (SECT) scan of the patient. The acquired CT numbers are converted into the needed quantities. These quantities can be relative stopping powers (RSPs) or elemental compositions for analytical (Schneider et al 1996) or Monte Carlo (MC)
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