WE-E-BRB-02: Evaluation of Analytical Proton Dose Predictions with a Lung-Like Plastic Phantom

Former studies have shown that in homogeneities in the path of therapeutic proton beams can lead to a degradation of the distal edge of the Bragg peak. These studies mostly investigated bone-air interfaces. This study focuses on distal edge degradation caused by finely structured soft tissue - air interfaces, which can be found in lung tissue.

A randomly filled voxelized lung-like phantom was designed and produced using rapid prototyping methods. The results of transmission measurements on this phantom were used to validate Monte Carlo (MC) calculations, which were then used as gold standard to calculate doses in several lung equivalent geometries (phantoms). The results were compared to the results of analytical dose calculation engines.

Transmission measurements showed that the distal falloff width (from 90 % of the peak dose to 10 %) in water increased from 3.32 mm by 117 % to 7.19 mm for an initial proton energy of 140 MeV, and from 5.95 mm to 9.03 mm (52 %) for 200 MeV. The peak dose in the degraded beam was only 70 % (for 140 MeV) and 84 % (for 200 MeV) of the value observed in non-degraded beams. These findings were in contrast to the results obtained with analytical dose computation engines, but are in agreement with MC calculations.

If not predicted correctly, Distal Edge Degradation in lung cancer therapy can lead to severe under-dosage of the target region and unwanted dose in organs at risk distal to the Bragg peak. Therefore clinically used dose calculation algorithms have to be extended to take lateral in homogeneities into account.