Purpose: To introduce a novel surface-based dose mapping method for improved quantitative bladder dosimetric assessment in prostate cancer (PC) radiotherapy.
Methods: Based on the planning and daily pre- and post-fraction MRIs of 12 PC patients, bladder surface models (SM) were generated on manually delineated contours and regionally aligned via surface-based constrained harmonic registration. Subsequently, bladder surface dose models (SDM) were created using face-wise dose sampling. This was followed by localized surface mapping of bladder intrafractional and interfractional dose variation on fractional SDMs. Discrepancies between the received dose, accumulated from daily SDMs, and the planned dose were then assessed on the corresponding SDMs. Complementary to the surface dose mapping, dose-surface-histograms (DSHs) based comparisons were also performed.
Results: The intra-fraction motion substantially altered dose to mid bladder body, but not the bladder surface areas distal to or contiguous with the target. The overall intra-fraction comparisons, after multiple comparison corrections, are significant (< 0.0001) for both the SDMs and DSHs. A similar pattern of dose variations was also detected by inter-fraction comparisons. With surface registration to the common SM, the cumulative bladder dose significantly differs from the planned dose. The discrepancy is evident in the mid-posterior range that corresponds to a mid-to-high dose region. The received DSH significantly differs from the planned DSH after permutation correction (p=0.0122), and the overall surface-based comparison after multiple comparison correction revealed a trend of difference (p=0.0800).
Conclusion: We developed a novel surface-based intra- and inter-dose mapping framework, which was applied to a unique MR dataset for planning and guidance. The framework identified the pattern of inter-fraction dose changes, localized significant intra-fraction dose variations, and quantified planned vs. received dose differences on the bladder surface. The result indicates the importance of adopting the motion-integrated bladder SDM for bladder dose management.