Author: Clifford Ghee Ann Chua, Calvin Wei Yang Koh, Kah Seng Lew, Zubin Master, Sung Yong Park, Hong Qi Tan, Andrew Wibawa 👨🔬
Affiliation: National Cancer Centre Singapore 🌍
Purpose: To investigate the impact of spot position deviation on reference dosimetry measurements using different delivery patterns and to determine the optimal delivery pattern for reference dosimetry measurements.
Methods: A total of 10 different energies (70.2MeV, 81.8MeV, 94.6MeV, 116.4MeV, 142.4MeV, 150.2MeV, 168.0MeV, 187.5MeV, 208.3MeV and 228.7MeV) were delivered using 7 different delivery patterns; Y fast scan (8MU), Y fast scan (50MU), X fast scan (8MU), X fast scan (50MU) , Y fast scan 3 repainting (50MU), Y fast scan 5 repainting (50MU) and random scan (50MU) and was repeated 5 times each for reference dosimetry measurements. During each delivery, the log files were retrieved, and the delivered parameters were use for dose calculation using a simplified pencil beam algorithm in MATLAB. The spot positions in each log file were also replaced with the planned positions and dose ratio for each delivery was calculated. A ratio close to unity indicates that there is no difference between the two calculated doses.
Results: Across the energy range, we can see that as the energy of the protons increases, the dose ratio decreases. Among delivery patterns, Y fast scan (50MU) has the highest dose ratio of 0.979±0.004, while X fast scan (8MU) has the lowest ratio of 0.948 0.002. In terms of measurement uncertainty across the different delivery pattern, Y fast scan with 5 repeating (50MU) and X fast scan (8MU) has the lowest uncertainty at 0.002.
Conclusion: This work demonstrates that there is a difference when different delivery patterns are used for reference dose measurement. Although using X fast scan (8MU) has the lowest measurement uncertainty, it has a dose ratio of 0.948 which is not as ideal. Using Y fast scan (50MU) is then the most optimal delivery pattern as the dose ratio of 0.979 which is the closest to unity.