SU‐E‐T‐407: Evaluation of the Stopping Power and Imaging Visibility for Iodine Based Contrast in Proton Therapy

Abstract

Purpose:The purpose of this study was to investigate the difference between calculated and measured relative linear stopping power (RLSP) for contrast medium in proton therapy. Furthermore, the visibility for different concentrations in prostate phantom on orthogonal X‐ray system was evaluatedMethods and Materials:In prostate cancer patient, rectal balloon along with the contrast media was used to visualize the balloon position, thus facilitating the patient setup during each fraction of the uniform scanning proton treatment. There were no fiducial markers implanted in the prostate for this patient. A blue wax phantom with outer dimensions 10cm(H) × 14.5cm(L) × 10cm(W) was made in house. To hold iodine based contrast solution, a rectangular shaped hole with dimensions 7cm(H) × 8cm(L) × 4cm(W) was made inside the phantom. Organically bound 8.5% iodine based Cystografin Dilute contrast agent with molecular formula C11H9I3N2O4.C7H17NO5 was used in this study. Six solutions were prepared; each mixture of water and iodine based contrast agent at different concentrations as 0%, 5%, 10%, 20%, 30% and 50%. During computed tomographic(CT) simulation, solutions were placed together at the isocenter of CT and scanned at 120kVp using the same protocol as for prostate cancer patients. The treatment planning was done in CMS‐XiO system. Multi‐layer‐ion‐chamber (MLIC) was used to measure residual proton range.Results:The 50% concentration contrast solution was used during treatment for better visualization on orthogonal X‐ray image. The measured RLSP for 5%, 10%, 20%, 30% & 50% solutions were 1.005, 1.010, 1.018, 1.023 & 1.033; and similarly calculated RLSP from XiO were 1.090, 1.135, 1.222, 1.299 & 1.448 respectively.Conclusion:The treatment planning system could overestimates the relative stopping power of contrast solution with high concentrations. It is recommended to override the contrast with measured RLSP for high atomic number based contrast solution in treatment planning for proton therapy.