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

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

First Clinical Investigation of Cone Beam Computed Tomography and Deformable Registration for Adaptive Proton Therapy for Lung Cancer

PURPOSE: An adaptive proton therapy workflow using cone beam computed tomography (CBCT) is proposed. It consists of an online evaluation of a fast range-corrected dose distribution based on a virtual CT (vCT) scan. This can be followed by more accurate offline dose recalculation on the vCT scan, which can trigger a rescan CT (rCT) for replanning. METHODS AND MATERIALS: The workflow was tested retrospectively for 20 consecutive lung cancer patients. A diffeomorphic Morphon algorithm was used to generate the lung vCT by deforming the average planning CT onto the CBCT scan. An additional correction step was applied to account for anatomic modifications that cannot be modeled by deformation alone. A set of clinical indicators for replanning were generated according to the water equivalent thickness (WET) and dose statistics and compared with those obtained on the rCT scan. The fast dose approximation consisted of warping the initial planned dose onto the vCT scan according to the changes in WET. The potential under- and over-ranges were assessed as a variation in WET at the target's distal surface. RESULTS: The range-corrected dose from the vCT scan reproduced clinical indicators similar to those of the rCT scan. The workflow performed well under different clinical scenarios, including atelectasis, lung reinflation, and different types of tumor response. Between the vCT and rCT scans, we found a difference in the measured 95% percentile of the over-range distribution of 3.4 ± 2.7 mm. The limitations of the technique consisted of inherent uncertainties in deformable registration and the drawbacks of CBCT imaging. The correction step was adequate when gross errors occurred but could not recover subtle anatomic or density changes in tumors with complex topology. CONCLUSIONS: A proton therapy workflow based on CBCT provided clinical indicators similar to those using rCT for patients with lung cancer with considerable anatomic changes

First acquisitions of realistic Proton Therapy treatments delivered on an anthropomorphic phantom with a prompt gamma camera

Proton Therapy treatments are affected by uncertainties on the penetration depth of the beam within the patient. For this reason, real-time range control is highly desirable to deliver safer treatments. Real-time range control can be performed by imaging prompt gammas emitted along the proton tracks in the patient. Our approach uses a knife-edge slit collimator to obtain a 1-dimensional projection of the beam path on a gamma camera. The energy spectrum of prompt gammas includes energy events up to 10 MeV and the event rate on a 500 cm3 scintillator is tens of MHz. Standard SPECT and PET modules are not suitable for the purpose and a dedicated gamma camera was designed. The camera features a 3 cm thick LYSO crystal segmented in two rows of 20 slabs with a width of 4 mm and a height of 10 cm. The crystal is coupled to arrays of Silicon Photomultipliers, read out by dedicated electronics boards to perform both spectra acquisition at low rates and photon counting at high rates for profile reconstruction. The prototype was aimed at reaching clinical requirements. The camera was tested in the Proton Therapy Center in Prague using an anthropomorphic phantom on which realistic treatment plans were delivered in pencil beam scanning mode. For each layer of the treatment, acquired profiles corresponding to the single spots were compared to simulated profiles and the shift was retrieved. The study demonstrated that the system is actually suitable for patient treatment monitoring

FLASH Mechanisms Track (Oral Presentations) UHDR PROTON BEAM VS. CONVENTIONAL: HYDROGEN PEROXIDE AS FLASH EFFECT SENSOR

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