Influence of Lipiodol Agent on Proton Beam Range in Radiotherapy Planning Using Computed Tomography for Hepatocellular Carcinoma

Purpose: To evaluate the influence of lipiodol on the proton beam range, which has not yet been determined. Methods and Materials: Two computed tomography (CT) data sets were obtained with a T25-flask containing lipiodol and water that was placed above a water phantom. The plan with the lipiodol CT images was performed, and then a verification plan was applied to the water CT images. The actual proton beam ranges in the lipiodol and water were measured under same conditions, and we compared the calculated proton beam range in the treatment planning system with measured values. Results: The calculated distal range in the treatment planning system was 12 cm in water, which was 3.87 cm longer than that in lipiodol (8.13 cm). In contrast, the measured distal range was 12 ± 0.01 cm in water, which was 0.21 ± 0.01 cm longer than that of lipiodol (11.78 ± 0.01 cm). A 3.65 ± 0.01-cm range shift was found in the calculated range compared with the measured range. For 10 hepatocellular carcinoma patients, the distal range in the verification plan with the corrected CT images in which the Hounsfield unit (HU) value of lipiodolized lesion was replaced with the average HU value of the surrounding tissue was 0.61 ± 0.26 cm (range, 0.26-0.99) longer than that in the plan with uncorrected CT images. Conclusions: It could be relevant for the purposes of range calculation of proton beams in the treatment planning system that the HU value of a lipiodolized lesion is replaced by the average HU value of the surrounding normal tissue.