Carbon-Ion Pencil Beam Scanning Treatment With Gated Markerless Tumor Tracking: An Analysis of Positional Accuracy.

PURPOSE: Having implemented amplitude-based respiratory gating for scanned carbon-ion beam therapy, we sought to evaluate its effect on positional accuracy and throughput. METHODS AND MATERIALS: A total of 10 patients with tumors of the lung and liver participated in the first clinical trials at our center. Treatment planning was conducted with 4-dimensional computed tomography (4DCT) under free-breathing conditions. The planning target volume (PTV) was calculated by adding a 2- to 3-mm setup margin outside the clinical target volume (CTV) within the gating window. The treatment beam was on when the CTV was within the PTV. Tumor position was detected in real time with a markerless tumor tracking system using paired x-ray fluoroscopic imaging units. RESULTS: The patient setup error (mean ± SD) was 1.1 ± 1.2 mm/0.6 ± 0.4°. The mean internal gating accuracy (95% confidence interval [CI]) was 0.5 mm. If external gating had been applied to this treatment, the mean gating accuracy (95% CI) would have been 4.1 mm. The fluoroscopic radiation doses (mean ± SD) were 23.7 ± 21.8 mGy per beam and less than 487.5 mGy total throughout the treatment course. The setup, preparation, and irradiation times (mean ± SD) were 8.9 ± 8.2 min, 9.5 ± 4.6 min, and 4.0 ± 2.4 min, respectively. The treatment room occupation time was 36.7 ± 67.5 min. CONCLUSIONS: Internal gating had a much higher accuracy than external gating. By the addition of a setup margin of 2 to 3 mm, internal gating positional error was less than 2.2 mm at 95% CI.

Digital reconstructed radiography with multiple color image overlay for image-guided radiotherapy.

Registration of patient anatomical structures to the reference position is a basic part of the patient set-up procedure. Registration of anatomical structures between the site of beam entrance on the patient surface and the distal target position is particularly important. Here, to improve patient positional accuracy during set-up for particle beam treatment, we propose a new visualization methodology using digitally reconstructed radiographs (DRRs), overlaid DRRs, and evaluation of overlaid DRR images in clinical cases. The overlaid method overlays two DRR images in different colors by dividing the CT image into two CT sections at the distal edge of the target along the treatment beam direction. Since our hospital uses fixed beam ports, the treatment beam angles for this study were set at 0 and 90 degrees. The DRR calculation direction was from the X-ray tube to the imaging device, and set to 180/270 degrees and 135/225 degrees, based on the installation of our X-ray imaging system. Original and overlaid DRRs were calculated using CT data for two patients, one with a parotid gland tumor and the other with prostate cancer. The original and overlaid DRR images were compared. Since the overlaid DRR image was completely separated into two regions when the DRR calculation angle was the same as the treatment beam angle, the overlaid DRR visualization technique was able to provide rich information for aiding recognition of the relationship between anatomical structures and the target position. This method will also be useful in patient set-up procedures for fixed irradiation ports.

First clinical experience in carbon ion scanning beam therapy: retrospective analysis of patient positional accuracy.

Our institute has constructed a new treatment facility for carbon ion scanning beam therapy. The first clinical trials were successfully completed at the end of November 2011. To evaluate patient setup accuracy, positional errors between the reference Computed Tomography (CT) scan and final patient setup images were calculated using 2D-3D registration software. Eleven patients with tumors of the head and neck, prostate and pelvis receiving carbon ion scanning beam treatment participated. The patient setup process takes orthogonal X-ray flat panel detector (FPD) images and the therapists adjust the patient table position in six degrees of freedom to register the reference position by manual or auto- (or both) registration functions. We calculated residual positional errors with the 2D-3D auto-registration function using the final patient setup orthogonal FPD images and treatment planning CT data. Residual error averaged over all patients in each fraction decreased from the initial to the last treatment fraction [1.09 mm/0.76° (averaged in the 1st and 2nd fractions) to 0.77 mm/0.61° (averaged in the 15th and 16th fractions)]. 2D-3D registration calculation time was 8.0 s on average throughout the treatment course. Residual errors in translation and rotation averaged over all patients as a function of date decreased with the passage of time (1.6 mm/1.2° in May 2011 to 0.4 mm/0.2° in December 2011). This retrospective residual positional error analysis shows that the accuracy of patient setup during the first clinical trials of carbon ion beam scanning therapy was good and improved with increasing therapist experience.