Water and tissue equivalence of a new PRESAGE(®) formulation for 3D proton beam dosimetry: a Monte Carlo study.

ABSTRACT

PURPOSE: To evaluate the water and tissue equivalence of a new PRESAGE(®) 3D dosimeter for proton therapy. METHODS: The GEANT4 software toolkit was used to calculate and compare total dose delivered by a proton beam with mean energy 62 MeV in a PRESAGE(®) dosimeter, water, and soft tissue. The dose delivered by primary protons and secondary particles was calculated. Depth-dose profiles and isodose contours of deposited energy were compared for the materials of interest. RESULTS: The proton beam range was found to be ≈27 mm for PRESAGE(®), 29.9 mm for soft tissue, and 30.5 mm for water. This can be attributed to the lower collisional stopping power of water compared to soft tissue and PRESAGE(®). The difference between total dose delivered in PRESAGE(®) and total dose delivered in water or tissue is less than 2% across the entire water∕tissue equivalent range of the proton beam. The largest difference between total dose in PRESAGE(®) and total dose in water is 1.4%, while for soft tissue it is 1.8%. In both cases, this occurs at the distal end of the beam. Nevertheless, the authors find that PRESAGE(®) dosimeter is overall more tissue-equivalent than water-equivalent before the Bragg peak. After the Bragg peak, the differences in the depth doses are found to be due to differences in primary proton energy deposition; PRESAGE(®) and soft tissue stop protons more rapidly than water. The dose delivered by secondary electrons in the PRESAGE(®) differs by less than 1% from that in soft tissue and water. The contribution of secondary particles to the total dose is less than 4% for electrons and ≈1% for protons in all the materials of interest. CONCLUSIONS: These results demonstrate that the new PRESAGE(®) formula may be considered both a tissue- and water-equivalent 3D dosimeter for a 62 MeV proton beam. The results further suggest that tissue-equivalent thickness may provide better dosimetric and geometric accuracy than water-equivalent thickness for 3D dosimetry of this proton beam.