Investigating deformable image registration and scatter correction for CBCT‐based dose calculation in adaptive IMPT

Abstract

International audiencePurpose:This work aims at investigating intensity corrected cone-beam x-ray computed tomography (CBCT) images for accurate dose calculation in adaptive intensity modulated proton therapy (IMPT) for prostate and head and neck (H&N) cancer. A deformable image registration (DIR)-based method and a scatter correction approach using the image data obtained from DIR as prior are characterized and compared on the basis of the same clinical patient cohort for the first time.Methods:Planning CT (pCT) and daily CBCT data (reconstructed images and measured projections) of four H&N and four prostate cancer patients have been considered in this study. A previously validated Morphons algorithm was used for DIR of the planning CT to the current CBCT image, yielding a so-called virtual CT (vCT). For the first time, this approach was translated from H&N to prostate cancer cases in the scope of proton therapy. The warped pCT images were also used as prior for scatter correction of the CBCT projections for both tumor sites. Single field uniform dose and IMPT (only for H&N cases) treatment plans have been generated with a research version of a commercial planning system. Dose calculations on vCT and scatter corrected CBCT (CBCT cor) were compared by means of the proton range and a gamma-index analysis. For the H&N cases, an additional diagnostic replanning CT (rpCT) acquired within three days of the CBCT served as additional reference. For the prostate patients, a comprehensive contour comparison of CBCT and vCT, using a trained physician’s delineation, was performed.Results:A high agreement of vCT and CBCT cor was found in terms of the proton range and gamma-index analysis. For all patients and indications between 95% and 100% of the proton dose profiles in beam’s eye view showed a range agreement of better than 3 mm. The pass rate in a (2%,2 mm) gamma-comparison was between 96% and 100%. For H&N patients, an equivalent agreement of vCT and CBCT cor to the reference rpCT was observed. However, for the prostate cases, an insufficient accuracy of the vCT contours retrieved from DIR was found, while the CBCT cor contours showed very high agreement to the contours delineated on the raw CBCT.Conclusions:For H&N patients, no considerable differences of vCT and CBCT cor were found. For prostate cases, despite the high dosimetric agreement, the DIR yields incorrect contours, probably due to the more pronounced anatomical changes in the abdomen and the reduced soft-tissue contrast in the CBCT. Using the vCT as prior, these inaccuracies can be overcome and images suitable for accurate delineation and dose calculation in CBCT-based adaptive IMPT can be retrieved from scatter correction of the CBCT projections

ScatterNet: A convolutional neural network for cone‐beam CT intensity correction

“…Due to the considerable interfractional anatomical differences between pCT and daily CBCT in the prostate region, CBCT was used as reference for data evaluation. The accuracy of CBCT has been demonstrated in earlier studies, even in the context of proton therapy 24,25 …”

“…Several papers have investigated intensity correction based on the planning CT, for example, through Monte Carlo simulation of the cone‐beam CT acquisition process, 9,20,21 or through raytracing and filtering 22–24 . These methods have been shown to reproduce the planning CT Hounsfield units (HU) to sufficient accuracy for both photon and proton dose calculation 25 . They are, however, limited by the time it takes to correct the images, which is more than 10 minutes for the raytracing‐based methods, or several hours for the Monte Carlo‐based methods.…”

“…In contrast to the latter work, our study was based solely on measured clinical data in the pelvic region. Low‐frequency deviations of the measured projections, such as the detection of scatter and beam hardening, were estimated by using a prior virtual CT (vCT), retrieved from deformable image registration (DIR) of the planning CT to the CBCT 22–25 . Moreover, we provide for the first time a comprehensive evaluation of the dosimetric accuracy of the ScatterNet CBCT in the scope of photon and proton radiotherapy.…”

Dosimetric effects of anatomical changes during fractionated photon radiation therapy in pancreatic cancer patients

“…Another limitation in our study was the low soft tissue contrast of CBCT; as shown for the prostate,26 this prevents accurate deformable image registration for soft tissues, such as pancreas and surrounding organs. For calculated overall dose distributions, the effect of inaccurate soft tissue registration is very limited due to the limited difference in HU between the different soft tissues.…”

Current state and future applications of radiological image guidance for particle therapy

“…In (e) an in‐room equivalent control CT acquired 1 day after the CBCT was used to calculate the adapted plan and serves as reference. Figure adapted with permission from Kurz et al …”