ESTRO 2023 - Abstract Book

S832

Monday 15 May 2023

ESTRO 2023

Ten patients treated for prostate cancer were selected. Treatment plans are generated for HF (20 x 3.0 Gy) and UHF (5 x 7.25 Gy). An isotropic CTV-PTV margin of 5mm is clinically used. For every patient, 20 systematic errors are generated and dose is recalculated. The random error is sampled by the number of fractions The EUD was calculated for prostate ( α / β = 1.2 en α / β = 3.0), rectum ( α / β = 3.0) and bladder ( α / β = 3.0 and α / β = 2.0). These EUD calculations are performed for every instance (i.e. both fractionation schedules and several error scenarios). For the OARs, the difference ( Δ EUDmax) between maximum EUD (Gy) and nominal EUD (Gy) is reported. For the CTV, the difference ( Δ EUDmin) between minimum EUD (Gy) and nominal EUD (Gy) is reported. The Δ EUD (Gy) is plotted as a box and whisker plot as a function of the systematic ( Σ ) and random error ( σ ), shown for 10 patients. Differences between the mean Δ EUD of both arms are tested with a paired t-test. Figure 1 shows Δ EUDmin for the CTV (i.e. prostate). When using α / β = 3, the minimum EUD in the CTV decreases more rapidly with setup error in the UHF arm, compared to the HF arm. The difference in mean Δ EUD between the arms is significant (p = 0.0006 and 0.0010, for α / β = 1.2 and α / β = 3 respectively). Although the difference between the groups is significant, the decrease in minimal EUD is small in the α / β = 1.2 scenario. Organs at Risk In Figure 2 the Δ EUDmax is calculated for rectum ( α / β = 3) and bladder ( α / β = 2 and α / β = 3). In the UHF arm, the maximum Δ EUD for the rectum increases more rapidly than in de HF arm, with increasing setup error. The difference in mean Δ EUD between the arms is significant for rectum (p = 0.0002) and bladder (p=0.001 for both α / β ratio’s). Results CTV coverage

Conclusion In UHF, the delivered dose is more sensitive for systematic and random errors than in HF treatments. This can result in ‘unlucky patients’ with larger maximum dose on the OARs. The lower dose on the CTV due to UHF, will have less impact because of the favorable α / β ratio of the prostate. The “unlucky patients” with higher OAR dose due to UHF might benefit from a more robust planning strategy.