TU‐A‐204B‐02: On the Potential of CBCT for Range Verification in Proton Therapy

Abstract

Purpose: We have investigated the potential use of cone beam CT (CBCT) for beam range verifications in proton therapy treatment, in addition to its primary role in geometric targeting. Specifically, we studied the intrinsic imaging variability of a CBCT and its effect on the water equivalent path length (WEPL) calculations, in the context of daily beam range verification/correction required for a recently proposed method of treating prostate using anterior fields. The current approach uses only lateral fields due to the lack of precise range control in patient. Materials and Methods: An anthropomorphic pelvic phantom was scanned using CBCT, in eight sessions on eight different days. In each session, the phantom was scanned twice, first at a standard position as determined by the room lasers, and then with a random shift of one centimeter in lateral directions. The Xio treatment planning system was used to perform the analysis. The average Hounsfield unit (HU) numbers for the water column in the rectal balloon was used to perform a linear calibration of the stopping power ratio, independently for each scan, as supported by the planning system. A number of WEPL values vertically from the anterior skin surface to the anterior surface of the water balloon were calculated on slices covering the region of the prostate, in relevance to a prostate treatment using an anterior field. Results: The HU number in the water column varied significantly even within the same CBCT. The average value also varied from day to day for up to 20 units. However, when these average values are used to calibrate the stopping power ratio, the variations in WEPL values along the anterior beam path are mostly within 2 mm. Conclusions: In‐room CBCT can be used in proton therapy to make online verification of protons range in patients with 2mm accuracy.

Investigating CT to CBCT image registration for head and neck proton therapy as a tool for daily dose recalculation

“…12 It may however be possible to obtain accurate stopping power estimates by performing deformable image registration () between the planning CT (pCT) and CBCT. This has been explored by several groups in the context of adaptive photon therapy for photon dose recalculation and automatic contour propagation.…”

Phantom based evaluation of CT to CBCT image registration for proton therapy dose recalculation

“…While for photon therapy it may be possible to perform CT number to electron density calibration and achieve acceptable dose calculation accuracy for uncorrected CBCT images (Yang et al 2007), this has not been demonstrated for protons where CBCT intensity correction to match CT image intensities is most likely required. Preliminary studies have investigated CT num-ber correction for CBCT images (Bentefour et al 2010). It may be possible to obtain accurate stopping power estimates by performing deformable image registration (DIR) between the planning CT and CBCT.…”

Using CBCT for pretreatment range check in proton therapy: a phantom study for prostate treatment by anterior–posterior beam

“…question is, whether CBCT can be used also for tracking daily variations of the patient waterequivalent path length and, subsequently, could be used as tool for beam range check before proton therapy treatment? (3) Range uncertainty is one of the most difficult problems in particle therapy treatment. (4,5) Such uncertainty translates directly into potential undershooting or overshooting of the Bragg peak in the patient.…”