Investigation of Prostate Deformation and Associated Dosimetric Implications in IMRT of the Prostate
E Mayyas1*, J Kim2, S Kumar3, N Wen4, M Elshaikh5, I Chetty6, (1) Henry Ford Health System, Detroit, MI, (2) HENRY FORD HEALTH SYSTEM, Detroit, MI, (3) Henry Ford Health Systems, Detroit, MI, (4) Henry Ford Health System, West Bloomfield, MI, (5) Henry Ford Health System, Detroit, MI, (6) Henry Ford Health System, Detroit, MITU-E-108-6 Tuesday 2:00PM - 3:50PM Room: 108
Purpose: Prostate deformation is generally considered a secondary correction and is consequently often ignored in planning margin design. In this study we estimated prostate deformation and investigate the associated dosimetric impact on target coverage.
Methods: A cohort of ten prostate cancer patients were retrospectively selected for the study, each with three fiducial markers implanted in the prostate. All CBCT images were registered to respective planning CT images using the Velocity AI deformable image registration (B-spline-based) software. For cases with accurate prostate registrations (defined by mean marker misalignments of <1.5 mm), deformation-vector-fields (DVFs) of the prostate and seminal vesicles (SVs) were quantified using deformation-volume histograms. For the case with the largest prostate deformation, target coverage degradation was analyzed in each of three treatment plans with PTV margins of 10, 5, and 3 mm.
Results: Deformation of the prostate and SVs was most significant in the anterior-posterior (AP) direction. Maximum prostate deformation of > 10mm, 5mm, and 3mm occurred in 0.5%, 10.0%, and 68.5% of cases, respectively. Based on the deformation-volume histograms, DVF magnitudes greater than 5mm and 3mm occurred in 2% and 27% of cases, respectively. In general, SV deformation was larger than that of the prostate. For the case with the largest prostate deformation, prostate coverage (D95%) was reduced by 0.5%, 9.0%, and 17.0% for 10mm, 5mm, and 3mm margin plans respectively.
Conclusion: These preliminary results are suggestive that prostate deformation over a course of fractionated prostate radiotherapy may not be insignificant, and thereby that proper margin design may be necessary to account for deformation. A consequence of these results is that use of tight planning margins of (e.g. less than 5 mm) must be viewed with severe caution.