WE‐C‐BRB‐06: Clinical Impact of Uncertainties in the Mean Excitation Energy of Human Tissues during Proton Therapy

Purpose: Uncertainties in the estimated mean excitation energies (I‐value) needed forcalculating proton stopping power can be in the order of 10–15%, which introduces afundamental limitation in the accuracy of proton range determination. Previous efforts havequantified shifts in proton depth dose distributions due to I‐value uncertainties in homogenoustissue phantoms. This study is the first to quantify the clinical impact of I‐value uncertainties onproton dose distributions within patient geometries. Methods: A previously developed Geant4 based Monte Carlo code was used to simulate aproton treatment plan for prostate cancer with varying tissue I‐values. A total of five cases weresimulated using nominal I‐values as well as I‐values modified by ±5% and ±10% of the nominalvalues. Dose volume histograms were generated for the GTV, CTV, PTV and relevant organs‐at‐risk (OARs). Results: Modification of tissue I‐values impacted both the proton range and SOBP width. D90range shifts up to 4 mm from the nominal range were recorded whereas D80 range shiftsreached up to 2 mm. For an increase in I‐value of 10% of the nominal value, the increase inrange and SOBP width resulted in a 1.4% decrease in the CTV mean dose. Inversely, decreasing the I‐value by 10% increased the CTV mean dose by 0.8%. The difference in themean dose to the OARs was relatively small except for the rectum that differedby up to 5%. Conclusions: This study demonstrated that the impact of I‐value uncertainties on patient dosedistributions. Clearly, sub‐millimeter precision in proton therapy would necessitate reduction in I‐value uncertainties to ensure an efficacious clinical outcome.