A comprehensive evaluation of the accuracy of CBCT and deformable registration based dose calculation in lung proton therapy

Abstract

A comprehensive evaluation of the accuracy of CBCT and deformable registration based dose calculation 4 in lung proton therapy 5 The uncertainties in water equivalent thickness (WET) and accuracy of dose estimation using a virtual CT 30 (vCT), generated from deforming the planning CT (pCT) onto the daily cone-beam CT (CBCT), were 31 comprehensively evaluated in the context of lung malignancies and passive scattering proton therapy. The 32 validation methodology utilized multiple CBCT datasets to generate the vCTs of twenty lung cancer 33 patients. A correction step was applied to the vCTs to account for anatomical modifications that could not 34 be modeled by deformation alone. The CBCT datasets included a regular CBCT (rCBCT) and synthetic 35CBCTs created from the rCBCT and rescan CT (rCT), which minimized the variation in setup between the 36 vCT and the gold-standard image (i.e., rCT). The uncertainty in WET was defined as the voxelwise 37 2 difference in WET between vCT and rCT, and calculated in 3D (planning target volume, PTV) and 2D 1 (distal and proximal surfaces). The uncertainty in WET based dose warping was defined as the difference 2 between the warped dose and a forward dose recalculation on the rCT. The overall root mean square (RMS) 3 uncertainty in WET was 3.6±1.8, 2.2±1.4 and 3.3±1.8 mm for the distal surface, proximal surface and PTV, 4 respectively. For the warped dose, the RMS uncertainty of the voxelwise dose difference was 6±2% of the 5 maximum dose (%mD), using a 20% cut-off. The rCBCT resulted in higher uncertainties due to setup 6 variability with the rCT; the uncertainties reported with the two synthetic CBCTs were similar. The vCT 7 followed by a correction step was found to be an accurate alternative to rCT.

“Patient‐specific validation of deformable image registration in radiation therapy: Overview and caveats”

“…Similar results were also found for virtual CT derived from megavoltage CT scans . The evaluation of a water equivalent thickness (WET) uncertainty as a measure of the impact of DIR inaccuracies on dose calculation was also recently proposed for the use of virtual CT in passive scattering proton therapy of lung tumors . The WET is the thickness of water needed to cause a proton beam to lose the same amount of energy as in a given thickness of a different medium.…”

“…The WET is the thickness of water needed to cause a proton beam to lose the same amount of energy as in a given thickness of a different medium. When comparing tumor margins of lung treatment plans as defined on the virtual and re‐planning CTs, the resulting root mean squared uncertainty in WET was 3.3 ± 1.8 mm . The WET metric is limited to particle therapy applications, but can provide a quantitative evaluation in these kind of treatments.…”

Feasibility study of range‐based registration using daily cone beam CT for intensity‐modulated proton therapy

“…The advantage of range‐based registration over adaptive planning for IMPT is its fast calculation for the patient on couch. Adaptive planning can better improve the dose coverage but often takes longer time from gathering the information to generating a new plan, including the computation time on the deformable registration of the CBCT to planning CT . The new plan may need quality assurance.…”

A robotic C-arm cone beam CT system for image-guided proton therapy: design and performance

“…For example, the SAD/SID is 200/300 cm for Varian ProBeam systems and 288.4/347 cm for IBA Proteus systems. 12,13,19 Although the scatter effect theoretically becomes less severe as the cone angle decreases and the patient-to-imager distance increases, as in many gantry-mounted systems, the sensitivity to mechanical accuracy and stability also plays an important part in the resultant image quality. The larger SID of gantry-mounted CBCT systems may also place a greater burden on the X-ray tube if the same photon flux to the detector is expected as in the C-arm CBCT system with a smaller SID; even in the absence of an anti-scatter grid, the mAs exposure is typically much higher.…”

“…This CBCT system design is different from the mobile C-arm CBCT system used in interventional radiology and surgery, 9,10 the existing robotic C-arm imaging systems used in some particle therapy centres, 11 and the 360° gantry-mounted CBCT systems. 12,13 The use of standalone robot arms on ceiling rails provides flexibility in imaging locations (at or off the treatment isocentre) in the room. Having the imaging equipment decoupled from the proton gantry and nozzle makes it easier to upgrade the system as technology advances than would otherwise be the case.…”