Evaluation of Deformable Image Registration and Dose Accumulation Using Histogram Matching Algorithm between kVCT and MVCT with Helical Tomotherapy

Purpose: To evaluate the accuracy of deformable image registration (DIR) between the planning kVCT (pCT) and the daily MVCT combined with the histogram matching (HM) algorithm, and evaluate the deformable dose accumulation using a suggested method for adaptive radiotherapy with Helical Tomotharapy (HT). Methods: For five prostate cancer patients (76 Gy/38 Fr) treated with HT in our institution, seven MVCT series (a total of 35 series) acquired weekly were investigated. First, to minimize the effect of different HU values between pCT and MVCT, this image-processing method adjusts HU values between pCT and MVCT images by using image cumulative histograms of HU values, generating an HM-MVCT. Then, the DIR of the pCT to the HM-MVCT was performed, generating a deformed pCT. Finally, deformable dose accumulation was performed toward the pCT image. Results: The accuracy of DIR was significantly improved by using the HM algorithm, compared with non-HM method for several structures (p ±0.05, 0.83 ±0.06, and 0.90 ± 0.04 for the CTV, rectum, and bladder, respectively, while that of the HM method was 0.81 ±0.06, 0.81 ±0.04, and 0.92 ±0.06, respectively. For the deformable dose accumulation, some difference was observed between the two methods, particularly for the small calculated regions, such as rectum V60 and V70. Conclusion: Adapting the HM method can improve the accuracy of DIR. Furthermore, dose calculation using the deformed pCT using HM methods can be an effective tool for adaptive radiotherapy.

Feasibility of a Direct-Conversion Method from Magnetic Susceptibility to Relative Electron Density for Radiation Therapy Treatment Planning

Recently, several institutions have been developing magnetic resonance imaging (MRI)-guided radiotherapy treatment systems. In this study, we examine whether it is possible to perform radiation therapy planning (RTP) using a magnetic susceptibility map obtained using MRI. The head of a healthy volunteer was scanned using dual-energy computed tomography (CT) and MRI. A T2-star-weighted 3D gradient echo-based sequence (GRE) with images taken at four different echo times was acquired using the MRI scanner. The CT images were converted to relative electron density (rED) using a predefined ΔCT-rED conversion table. ΔCT was derived using the energy-subtraction method. The rED map was obtained from a single-linear relationship with the ΔCT-rED conversion table, whereas the magnetic susceptibility map was obtained from quantitative susceptibility mapping (QSM) via MRI. Subsequently, to obtain the relationship between the magnetic susceptibility and the rED, the rED map was rigidly aligned to the susceptibility map and resampled at the susceptibility map’s resolution. Finally, the magnetic susceptibility rED conversion table was obtained via voxel-by-voxel mapping between the two maps. No strong relationship between magnetic susceptibility and rED was obtained in the healthy volunteer’s head or in this study. The coefficient correlation between these parameters was 0.0145. Magnetic susceptibility values may be not able to convert to rED using our proposed method in healthy volunteer’s head. In contrast to the magnetic-susceptibility values obtained from the QSM algorithm, which were strongly affected by calcification and iron content, the rED or CT number was not considerably affected by such materials.

Audio-Visual Biofeedback for Respiratory Motion Management: Comparison of the Reproducibility of Breath-Holding between Visual and Audio Guidance

Purpose: To compare the reproducibility of breath-holding and comfort between two biofeedback guidance methods (audio and visual) in a respiratory monitoring system. Method and Materials: An audio-feedback respiratory monitoring device, which was modified from a visual-feedback self-respiration monitoring system (Abches, APEX Medical Inc., Tokyo, Japan) previously developed by the authors’ group, was constructed. Twenty patients (13 men, 7 women;mean age, 68.5 years;range, 54 - 85 years) with tumors in the thorax or abdominal region were enrolled in the present study. Computed tomography images were acquired from all patients three times using the two (i.e., audio and visual) respiration monitoring devices. To evaluate the reproducibility of breath-holding, the distance between an anatomical landmark and the tumor position was measured. Furthermore, patients were asked which guidance method they preferred (visual or audio) for comfortable breath-holding. Results: The two guidance methods improved the reproducibility of breath-holding compared with free-breathing, and no significant overall differences between two methods were observed (the mean displacements of the landmark-tumor distance were 2.60 ±1.38 mm and 2.35 ±1.63 mm, for visual and audio guidance, respectively). In five patients, the magnitude of position displacement in the series of three computed tomography images under audio guidance was twice as large as the other under visual guidance. Audio guidance was preferred to visual guidance by 65% (13 of 20) of the patients. However, the reproducibility of breath-holding did not always correspond with patient preference. Conclusion: There were some individual differences in the reproducibility of the visual and audio guidance methods. More appropriate, individualized guidance methods for each patient would improve the reproducibility of breath-holding in respiratory motion management.