Cone-Beam Computed Tomography and Deformable Registration-Based “Dose of the Day” Calculations for Adaptive Proton Therapy

Abstract

Purpose: The aim of this work was to evaluate the feasibility of cone-beam computed tomography (CBCT) and deformable image registration (DIR)-based ''dose of the day'' calculations for adaptive proton therapy. Methods: Intensity-modulated radiation therapy (IMRT) and proton therapy plans were designed for 3 head and neck patients that required replanning, and hence had a replan computed tomography (CT). Proton plans were generated for different beam arrangements and optimizations: intensity modulated proton therapy and single-field uniform dose. We used an in-house DIR software implemented at our institution to generate a deformed CT, by warping the planning CT onto the daily CBCT. This CBCT had a similar patient geometry to the replanned CT. Dose distributions on the replanned CT were considered the gold standard for ''dose of the day'' calculations, and were compared with doses on deformed CT (our method) and directly on the calibrated CBCT and rigidly aligned planning CT (alternative methods) in terms of dose difference (DD), by calculating the percentage of voxels whose DD was smaller than 2% of the prescribed dose (DD 2%-pp) and the root mean square of the DD distribution (DD RMS). Results: Using a deformed CT, the DD 2%-pp within the CBCT imaging volume was 93.2% 6 0.7% for IMRT, and 87% 6 3% for proton plans. In a region of higher dose gradient, we found that although DD 2%-pp was 94.3% 6 0.2% for IMRT, in proton plans, it dropped to 74% 6 4%. A larger number of treatment beams and single-field uniform dose optimization appear to make the proton plans less sensitive to DIR errors. For example, within the treated volume, the DD RMS was reduced from 2.6% 6 0.6% of the prescribed dose to 1.0% 6 1.3% of the prescribed dose when using single-field uniform dose optimization. Conclusions: Promising results were found for DIR-and CBCT-based proton dose calculations. Proton dose calculations were, however, more sensitive to registration errors than IMRT doses were, particularly in high dose gradient regions.

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

“…In the last 2 yr, several groups have proposed methods making use of the prior information available from the planning CT scan and deformable image registration to correct CBCT data to allow either WET or dose distribution computation, [59][60][61][62][63][64][65] achieving an accuracy of about 2% relative to planning CT. 62 The in-room CT or corrected CBCT data may also be used for treatment plan adaptation 66 to correct for gross anatomical changes such as weight loss, bowel filling variations or organ motion accompanied by large WET deviations. An example of two CBCT correction methods and potential use of in-room volumetric data for adaptive particle therapy is shown in Fig.…”

Europium‐155 as a source for dual energy cone beam computed tomography in adaptive proton therapy: A simulation study

“…The panel was positioned 500 mm from the centre of rotation of the imaging system. The panel was simulated as a solid slab of CZT with a density of 5.8 g/cm 3 . Photon interactions in the detector panel were considered, including fluorescence.…”

“…Despite the widespread use of CBCT in conventional X‐ray radiotherapy, this technology is only now becoming a standard offering of proton therapy vendors. The most effective utilization of CBCT in a clinical proton therapy environment is a topic receiving increasing interest …”

“…The most effective utilization of CBCT in a clinical proton therapy environment is a topic receiving increasing interest. [3][4][5][6] Due to the dependence of CBCT Hounsfield units (HUs) on the particular scatter environment created by the patient, CBCT image data are not typically used directly for dose calculation. 7,8 Rather, CBCT images are typically only used for geometrical information in online use.…”

Shading correction for cone-beam CT in radiotherapy: validation of dose calculation accuracy using clinical images

“…Various methods have been introduced to enable radiotherapy dose calculations based on CBCT images.These include scatter corrections 5,6 , deformable registration with planning CT [7][8][9] , and uniformity corrections incorporating prior-information either in projection space 10 or reconstructed image space 11 .…”