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

Abstract

The ability to perform dose recalculation on the anatomy of the day is important in the context of adaptive proton therapy. The objective of this study was to investigate the use of deformable image registration (DIR) and cone beam CT (CBCT) imaging to generate the daily stopping power distribution of the patient. We investigated the deformation of the planning CT scan (pCT) onto daily CBCT images to generate a virtual CT (vCT) using a deformable phantom designed for the head and neck (H & N) region. The phantom was imaged at a planning CT scanner in planning configuration, yielding a pCT and in deformed, treatment day configuration, yielding a reference CT (refCT). The treatment day configuration was additionally scanned at a CBCT scanner. A Morphons DIR algorithm was used to generate a vCT. The accuracy of the vCT was evaluated by comparison to the refCT in terms of corresponding features as identified by an adaptive scale invariant feature transform (aSIFT) algorithm. Additionally, the vCT CT numbers were compared to those of the refCT using both profiles and regions of interest and the volumes and overlap (DICE coefficients) of various phantom structures were compared. The water equivalent thickness (WET) of the vCT, refCT and pCT were also compared to evaluate proton range differences. Proton dose distributions from the same initial fluence were calculated on the refCT, vCT and pCT and compared in terms of proton range. The method was tested on a clinical dataset using a replanning CT scan acquired close in time to a CBCT scan as reference using the WET evaluation. Results from the aSIFT investigation suggest a deformation accuracy of 2-3 mm. The use of the Morphon algorithm did not distort CT number intensity in uniform regions and WET differences between vCT and refCT were of the order of 2% of the proton range. This result was confirmed by proton dose calculations. The patient results were consistent with phantom observations. In conclusion, our phantom study suggests the vCT approach is adequate for proton dose recalculation on the basis of CBCT imaging.

Proton dose calculation on scatter‐corrected CBCT image: Feasibility study for adaptive proton therapy

“…18,20 However, we believe it necessary for this to be independently conducted since the residual HU errors after the scatter correction would have different effects on proton dose calculations compared to photon cases. Moreover, this validation cannot be done in real patient images, as there would be no ground truth of CT ref to compare with daily CBCT images unless CT and CBCT images are acquired at the same patient setup as pointed out by Landry et al 12 Recently, Landry et al 12,13 investigated the feasibility of proton dose calculations on a virtual CT image using deformable image registration between plan CT and daily CBCT images. Even though they demonstrated a promising WEPL error (2% error of the proton range), they encountered residual DIR errors of 2-3 mm which potentially degrade the effectiveness T III.…”

“…However, there have been few studies on CBCT imaging for APT. 12,13 It is expected that the HU accuracy of CBCT is more crucial in APT than it is in ART, because small HU differences may cause range errors and absolute dose errors, which will in turn lead to reduced integrity of the dose calculation. 14 To date, no study has demonstrated proton dose calculation or treatment planning on either raw or scatter-corrected CBCT images.…”

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

“…The method employed here was the same as in Landry et al 19 An automatic rigid translation registration was employed to align the CBCT and pCT, mimicking an online position correction protocol. Prior to rigid registration, the CBCT was cropped to eliminate the conical sections at the inferior end of the FOV.…”

“…[13][14][15][16][17][18] A recent phantom study from our group suggested that the approach may provide stopping power distributions which are equivalent to those obtained by a planning CT with proton range differences below 2%. 19 However, that study was restricted to the evaluation of the stopping power distribution and did not explore the use of the vector field for contour propagation for a more profound analysis.…”

Commissioning an in‐room mobile CT for adaptive proton therapy with a compact proton system

“…Those that do are either limited to one room only with specific adaptations for a CT such as a “track” solution or cone‐beam CT (CBCT). An alternative to mobile helical CT scanners for image‐guided proton therapy is the gantry‐mounted cone‐beam CT (CBCT) 5, 6, 7, 8, 9. Gantry‐mounted CT systems may provide more convenience than mobile scanners.…”