Author: Matthew Stephen Andriotty, Taoran Cui, Harold Y Hu, Ke Nie, Tan Phan, Xiao Wang, Ning J. Yue, Chengzhu Zhang 👨🔬
Affiliation: Rutgers Cancer Institute of New Jersey, Basis Scottsdale 🌍
Purpose: Metallic implants are often non-isocentric, and their exact volumes/orientations/shapes are difficult to capture and contour accurately on CT images even if that information is known beforehand. The purpose of this work is to properly calculate the Hounsfield Unit (HU) override for small metal implants—such as gold fiducials—for proton therapy and to demonstrate the effects of HU override on the resulting plan dose as compared to non-overridden or improperly overridden fiducials.
Methods: The HU value for gold fiducial seeds was calculated by first finding the seed contour volume in the treatment planning system (TPS) and using the vendor-provided volume and density information to determine the mass ratio of seed and water within the contour. Then, the relative stopping power of the contour as a whole was calculated as a mass-weighted average of the stopping powers of gold and water for 175-MeV protons, obtained from PSTAR. This stopping power was used to select an appropriate corresponding HU number from the CT calibration curve.
To illustrate the effects of this HU override on dosimetry calculation for proton therapy, dose was calculated for a prostate plan with gold fiducials: once with the fiducial contours overridden using the method described in this work (593 HU), once with no override (mean 1,729 HU), and once with fiducials overridden based on the incorrect assumption that the entire contour volume is gold (10,000 HU).
Results: The plan with fiducials overridden as described in this work achieved a PTV V7000-cGy of 69.894%, compared to 68.35% for no override, and 46.406% for the incorrect 10,000 HU override. The non-overridden and incorrectly overridden fiducials resulted in visible distortions of the isodose lines in and around the target.
Conclusion: Properly calculating the HU number for small metal implants is critical for accurate dose calculation in proton therapy.