Purpose
To investigate dosimetric impact of intra-fraction prostate motion for photon and proton plans.
Methods
Gross directional displacement (4.3mm Inf and 4.8mm Post) was applied to offset the isocentre on a case study (74Gy in 37#, prostate alone) to simulate the impact on VMAT photon (2-Arc) and proton (two lateral-oblique SFUD with 3% range uncertainty) plans using the Eclipse treatment planning system (v13.6). The offset values were inferred from the trends of shifts derived from the real-time intra-fraction prostate motion tracking using a 4D transperineal ultrasound system previously published (n=55, 1744 fractions) [Pang et al.,2018]. Both treatment plans had dosimetric impact simulated based on the applied geometrical offsets and subsequently compared with the pre-offset scenario for CTV ( prostate) coverage and doses to the OARs (bladder and rectum). 
Results
D95 of the prostate was reduced from 74.4Gy to 73.9Gy (-0.7%) and from 74.3Gy (RBE) to 71.1Gy (RBE) (-4.3%) for the photon and proton plans, respectively with the simulated offset. However, D95 of prostate for the proton plan achieved 74.1Gy (RBE) when plan uncertainties of the offset were incorporated. Mean dose to the bladder decreased by 5.5Gy and 1.6Gy (RBE); mean rectum dose increased by 6Gy in the photon and decreased by 1.1Gy (RBE) proton plans with offset. 
Conclusion
2-Arc VMAT photon plan is less susceptible to the impact of intra-fraction prostate motion compared to 2-field SFUD proton plan. Incorporating intra-fraction motion uncertainties within the plan uncertainty parameters should be considered despite daily online image guidance and setup corrections in proton treatment.

Dosimetric impact of residual prostate motion alone based on real-time acoustic tracking information.