A Novel Analytic Method of Automatically Assessing Beamline Geometric Range Variations for CBCT-Guided Intensity-Modulated Proton Therapy (IMPT) for Localized Prostate Cancer

Abstract

A novel image-processing analytics software was introduced to quantitatively assess the beamline geometric range variations for proton spots around the planning target in daily CBCT-guided prostate IMPT. Three hundred eighty-seven consecutive full-CBCT alignment images of 11 localized prostate cancer IMPT cases were retrospectively analyzed to quantify the range variations of proton beamlines using a home-grown software. Each patient treatment comprises two proton scanning beams. All cases had a history of a plan revision with a re-simulated CT set. With this novel method the patient body surfaces are reconstructed by generating triangle mesh surfaces using a python Delaunay tessellation method. A python ray casting algorithm is utilized to determine the surface offset of the aligned CBCT relative to the planning CT. The proton beamlines are extrapolated from all the beam spots within 10 mm-enlarged planning target volume. The maximal geometric range variation was used to score the selected percentile of the included beamlines of the beam. The accuracy of this analytic software was satisfactorily validated prior to its application to this retrospective investigation. In practice, the proton range uncertainties are routinely estimated from their geometric range variations measured on a setup CBCT. The computation or processing time is about 60 seconds for each CBCT - planning CT analysis on a prostate IMPT positioning. The geometric range variations in the 95th percentile beamlines of 774 fractional beams or 387 aligned CBCT images are 3.29 ± 1.43 mm. As tabulated in table-1, worsened beam range variations of setup CBCTs were reduced by 2.60 ± 0.30 mm in the succeeding 2 fractions post using re-simulated setups. In this serial, an averaged 1.41 ± 0.10 mm range variation reduction was seen by re-simulation throughout the course. The geometric range variations of beamlines in prostate IMPT were comprehensively analyzed, for the first time, using a novel analytic software based on aligned daily CBCTs. The results of this study suggest that the re-simulated setups can be an effective way to reduce beamline range variations for localized prostate IMPT. Incorporation of this process for other IMPT treatment sites, as well as a Gamma-indexed analysis will be added in our immediate future research.