Hironori Kojima

Accuracy of Dose Delivery in Multiple Breath-Hold Segmented Volumetric Modulated Arc Therapy: A Static Phantom Study

Purpose. Accuracy of dose delivery in multiple breath-hold segmented volumetric modulated arc therapy (VMAT) was evaluated in comparison to noninterrupted VMAT using a static phantom. Material and Methods. Five VMAT plans were evaluated. A Synergy linear accelerator (Elekta AB, Stockholm, Sweden) was employed. A VMAT delivery sequence was divided into multiple segments according to each of the predefined breath-hold periods (10, 15, 20, 30, and 40 seconds). The segmented VMAT delivery was compared to noninterrupted VMAT delivery in terms of the isocenter dose and pass rates of a dose difference of 1% with a dose threshold of 10% of the maximum dose on a central coronal plane using a two-dimensional dosimeter, MatriXX Evolution (IBA Dosimetry, Schwarzenbruck, Germany). Results. Means of the isocenter dose differences were 0.5%, 0.2%, 0.2%, 0.0%, and 0.0% for the beam-on-times between interrupts of 10, 15, 20, 30, and 40 seconds, respectively. Means of the pass rates were 85%, 99.9%, 100%, 100%, and 100% in the same order as the above. Conclusion. Our static phantom study indicated that the multiple breath-hold segmented VMAT maintains stable and accurate dose delivery when the beam-on-time between interrupts is 15 seconds or greater.

Prototype imaging protocols for monitoring the efficacy of iodine-131 ablation in differentiated thyroid cancer.

Whole-body and single photon emission tomography (SPET) images during sodium iodide-131 (Na131I) ablation are useful to confirm the efficacy of ablation using 131I imaging. However, there have been no attempts to improve the quality of 131I imaging. We therefore investigated imaging protocols for 131I imaging in differentiated thyroid cancer (DTC). Phantoms containing 131I were used to simulate extra-thyroid beds and thyroid beds. To simulate extra-thyroid beds, a phantom containing 0.19, 0.37, 0.74 or 1.85 MBq was placed in the acquisition center. To simulate the thyroid beds, four phantoms were applied as normal thyroid tissue, and four phantoms containing 0.19, 0.37, 0.74 and 1.85 MBq were arranged around normal thyroid tissue as a cancer. Whole-body imaging was performed at different table speeds, and SPET data acquired with various pixel sizes were reconstructed using a filtered backed projection (FBP) and ordered-subsets expectation maximization with 3-dimensional (OSEM-3D) algorithm. We measured full width at half maximum (FWHM) and % coefficient of variation (%CV). Patients were then examined based on the results of phantom studies. In extrathyroid beds, slower table speed in whole-body imaging improved %CV, but had little effect on FWHM. For SPET imaging OSEM-3D produced high-resolution and low-noise images, and FWHM and %CV improved with smaller pixel size, as compared with FBP. In the thyroid beds, only the 1.85 MBq phantom could be confirmed on whole-body imaging. Images by SPET had high FWHM and low %CV when the smaller pixel size and OSEM-3D were applied. Accumulation of ≤1.85 MBq was detected with a smaller pixel size of ≤4.8 mm and OSEM-3D. For Na131I ablation imaging, slower scan speed is suitable for whole-body imaging and smaller pixel size and OSEM-3D is appropriate for SPET imaging. In conclusion, we confirmed Na131I accumulation in thyroid beds using slower scan speed (≤15 cm/min) on whole-body imaging, and then accurate identification of Na131I accumulation using SPET and CT fusion imaging with smaller pixel size (≤4.8 mm) and OSEM-3D.

Creation and application of three-dimensional computer-graphic animations for introduction to radiological physics and technology

Physics-related subjects are important in the educational fields of radiological physics and technology. However, conventional teaching tools, for example texts, equations, and two-dimensional figures, are not very effective in attracting the interest of students. Therefore, we have created several multimedia educational materials covering radiological physics and technology. Each educational presentation includes several segments of high-quality computer-graphic animations designed to attract students’ interest. We used personal computers (PCs) and commercial software to create and compile these. Undergraduate and graduate students and teachers and related professionals contributed to the design and creation of the educational materials as part of student research. The educational materials can be displayed on a PC monitor and manipulated with popular free software. Opinion surveys conducted in undergraduate courses at Kitasato University support the effectiveness of our educational tools in helping students gain a better understanding of the subjects offered and in raising their interest.

Development of Optical Computed Tomography for Evaluation of the Absorbed Dose of the Dyed Gel Dosimeter

Optical computed tomography (optical CT) is a reading device of the dyed gel dosimeters. We are developing the optical CT for the evaluation of three dimensional radiation absorbed dose distribution in the dyed gel dosimeters. We made dyed gel with leuco crystal violet and the dyed gel will be contained in vials. The dyed gel dosimeters were irradiated with 10 MV X-ray beam at 100–2000 MU. The optical CT we developed was consists of a liquid crystal monitor VL-176SE (FUJITSU, Japan) as a light source and a camera uEye XS (iDS, Germany). The dyed gel dosimeter was rotated by a step of every 0.9° with the stepper motor ST-42BYH 1004-5013 (MERCURY MOTOR, China) in a water tank and be taken 400 projections per rotation. Volume data was reconstructed from the projection images with the image processing software Plastimatch. The correlation between the absorbed dose and signal values of the dyed gel dosimeters in the reconstructed image was analyzed. The developed optical CT could reconstructed the images of the dyed gel dosimeters and the signal values of the dyed gel dosimeters in the reconstructed images had linear response related to the dose up to 20 Gy.

Evaluation of Deformable Image Registration Between High Dose Rate Brachytherapy and Intensity Modulated Radiation Therapy for Prostate Cancer

High risk prostate cancer is treated with a combination of intensity-modulated radiation therapy (IMRT) and high dose rate brachytherapy (HDR-BT). Deformable image registration (DIR) techniques used for dose accumulation sums dose distributions. The accuracy of DIR would get worse when density of an organ in a pair of registering two images differs greatly each other. Needles and contrast medium are used in HDR-BT. In this study, the effect of needles and contrast medium for DIR accuracy was evaluated. Six patients with prostate cancer were enrolled, who were treated with the combination of HDR-BT and IMRT. In the HDR-BT plan, needleless image (NI) and needleless and no-contrast medium image (NCI) were created from the original HDR-BT plan image (OI) to investigate the influence of needles and contrast medium. Both DIR and rigid registration (RR) were performed on the OI, NIs and NCIs by using MIM Maestro ver. 6.7.6 (MIM software Inc, Cleveland, USA) and after that the dose distribution of HDR-BT (used as the reference image) and IMRT were accumulated. The Dice Similarity coefficient (DSC) between DIR and RR were analyzed and compared each other. The mean DSC values of the prostate with DIR on OI, NI and NCI were 0.51, 0.57 and 0.73, respectively. The DSC with DIR on NCI was higher than DSC with DIR on OI and NI. The DSC values improved by removing the contrast medium.

An uncertainty metric to evaluate deformation vector fields for dose accumulation in radiotherapy

Background and purpose In adaptive radiotherapy, deformable image registration (DIR) is used to propagate delineations of tumors and organs into a new therapy plan and to calculate the accumulated total dose. Many DIR accuracy metrics have been proposed. An alternative proposed here could be a local uncertainty (LU) metric for DIR results. Materials and methods The LU represented the uncertainty of each DIR position and was focused on deformation evaluation in uniformly-dense regions. Four cases demonstrated LU calculations: two head and neck cancer cases, a lung cancer case, and a prostate cancer case. Each underwent two CT examinations for radiotherapy planning. Results LU maps were calculated from each DIR of the clinical cases. Reduced fat regions had LUs of 4.6 ± 0.9 mm, 4.8 ± 1.0 mm, and 4.5 ± 0.7 mm, while the shrunken left parotid gland had a LU of 4.1 ± 0.8 mm and the shrunken lung tumor had a LU of 3.7 ± 0.7 mm. The bowels in the pelvic region had a LU of 10.2 ± 3.7 mm. LU histograms for the cases were similar and 99% of the voxels had a LU < 3 mm. Conclusions LU is a new uncertainty metric for DIR that was demonstrated for clinical cases. It had a tolerance of <3 mm.

SU-F-J-78: Characterization of Deformable Image Registration for the Pelvic Region Based On Prostate Shifting, Image Noise and the Existence of Implanted Fiducial Markers

PURPOSE To assess the impact of prostate shifting, image noise and the existence of fiducial markers on deformable image registration (DIR) performance in the pelvic region using original digital phantoms. METHODS A digital phantom of the pelvic region of a prostate cancer patient was created using contoured anatomy as a base image set. Two simulated fiducial markers were implanted at the apex and base of the prostate. With and without markers, the prostate was shifted by 6.00 and 10.0 mm superior, 5.63 and 9.38 mm posterior, and 5.63 and 9.38 mm left, respectively. We also added image noise with a standard deviation (SD) of 9, 21, or 28. A phantom with a noise SD of 9 and zero prostate shift was defined as the target, and the other phantoms were defined as the sources. DIR was performed using MIM Maestro (MIM Software Inc., Cleveland, OH, USA) on the digital phantoms. A dice similarity coefficient (DSC) was used as the index of correspondence level of the prostates on the target images and the results of the DIR. Also, we calculated the motion distance of the fiducial markers. RESULTS The DSC decreased with increasing prostate distance. The degree of correspondence of the fiducial markers and the DIR performance were worse when the bladder volume changed after the prostate shifted in the superior direction than when it shifted in the other directions. Changes in the image noise and the existence of fiducial markers produced minimal impact on the DIR results. CONCLUSION We found that DIR was not affected by image noise, and the reproduction of prostate position by maintaining the bladder volume was important to preserve DIR performance. Further, the existence of two fiducial markers had no impact on DIR performance. This work was supported by Japan Society for the Promotion of Science (JSPS) KAKEN Grant-in-Aid for Scientific Research (C) Grant number 26460724.