The effects of mega-voltage CT scan parameters on offline adaptive radiation therapy.

TomoTherapy involves image-guided radiation therapy (IGRT) using Mega-voltage CT (MVCT) for each treatment session. The acquired MVCT images can be utilized for the retrospective assessment of dose distribution. The TomoTherapy provides 18 distinct imaging conditions that can be selected based on a combination of algorithms, acquisition pitch, and slice interval. We investigated the accuracy of dose calculation and deformable image registration (DIR) depending on MVCT scan parameters and their effects on adaptive radiation therapy (ART). We acquired image values for density calibration tables (IVDTs) under 18 different MVCT conditions and compared them. The planning CT (pCT) was performed using a thoracic phantom, and an esophageal intensity-modulated radiation therapy (IMRT) plan was created. MVCT images of the thoracic phantom were acquired under each of the 18 conditions, and dose recalculation was performed. DIR was performed on the MVCT images acquired under each condition. The accuracy of DIR, depending on the MVCT scan parameters, was compared using the mean distance to agreement (MDA) and Dice similarity coefficient (DSC). The dose distribution calculated on the MVCT images was deformed using deformed vector fields (DVF). No significant differences were observed in the results of the 18 IVDTs. The esophageal IMRT plan also showed a small dose difference. Regarding verifying the DIR accuracy, the MDA increased, and the DSC decreased as the acquisition pitch and slice interval increased. The difference between the dose distributions after dose mapping was comparable to that before DIR. The MVCT scan parameters had little effect on ART.

[Development of an automated method for analysis of Winston-Lutz test results using digital radiography and photostimulable storage phosphor].

Stereotactic radiosurgery (SRS) and radiotherapy (SRT) are intricate techniques that deliver a highly precise radiation dose to a localized target, usually a tumor. At our hospital, we perform SRS and SRT on brain tumors using a linear accelerator (linac) mounted with an external micro multi-leaf system. The Task Group TG-142 Report by the American Association of Physicists in Medicine recommends the coincidence of the radiation and mechanical isocenter to be within ±1 mm. The Winston-Lutz test is commonly used to verify the linac isocenter position: it has the advantages of being a simple method that uses a film or electronic portal imaging device (EPID). However, the film method requires a higher radiation dose, which makes it more time-consuming than the EPID method, and the results are highly dependent on the skills of the observer. The EPID method has certain advantages over the film method, but it has low resolution and can only be used for a few combinations of gantry and couch angles. This prompted us to develop an in-house-designed radiation receptor system based on digital radiography, using a photostimulable storage phosphor and automated analysis algorithm for Winston-Lutz test images using a template-matching technique based on cross-correlation coefficients. Our proposed method shows a maximum average absolute error of 0.222 mm (less than 2 pixels) for 0.5 mm and 1.0 mm displacement from the isocenter toward the inline and crossline directions. Our proposed method is thus potentially useful for verifying the Linac isocenter position with a small error and good reproducibility, as demonstrated by improved accuracy of evaluation.

Automated estimation of number of implanted iodine-125 seeds for prostate brachytherapy based on two-view analysis of pelvic radiographs.

Digital pelvic radiographs are used to identify the locations of implanted iodine-125 seeds and their numbers after insertion. However, it is difficult and laborious to visually identify and count all implanted seeds on the pelvic radiographs within a short time. Therefore, our purpose in this research was to develop an automated method for estimation of the number of implanted seeds based on two-view analysis of pelvic radiographs. First, the images of the seed candidates on the pelvic image were enhanced using a difference of Gaussian filter, and were identified by binarizing the enhanced image with a threshold value determined by multiple-gray level thresholding. Second, a simple rule-base method using ten image features was applied for false positive removal. Third, the candidates for the likely number of a multiply overlapping seed region, which may include one or more seeds, were estimated by a seed area histogram analysis and calculation of the probability of the likely number of overlapping seeds. As a result, the proposed method detected 99.9% of implanted seeds with 0.71 false positives per image on average in a test for training cases, and 99.2% with 0.32 false positives in a validation test. Moreover, the number of implanted seeds was estimated correctly at an overall recognition rate of 100% in the validation test using the proposed method. Therefore, the verification time for the number of implanted seeds could be reduced by the provision of several candidates for the likely number of seeds.

[Investigation of Region of Interest (ROI) for measurement of slice thickness in Computed Tomography (CT)].

We evaluated an appropriate region of interest (ROI) size for the measurement of full width at half maximum (FWHM) in the bead method (0.1 mm and 0.5 mm diameter; lead) and the microdisk method (0.05 mm thickness and 1.0 mm diameter; tungsten) using multislice computed tomography (CT). The FWHM of preset slice thicknesses 0.625 mm, 1.25 mm, 5.0 mm and 7.5 mm were measured by varying helical pitch, location of measurement [center and off-center of scan field of view (SFOV)] and ROI size, and they were compared with the tolerance stated in the Japanese Industrial Standards (JIS). It was conlcuded that the appropriate ROI size was influenced by preset slice thickness in this study. At the center of SFOV, measurements of FWHM were enabled within the tolerance of the JIS with small variations in all preset slice thicknesses if the ROI sizes were set between 0.4 times and equal to the size of the bead or microdisk indicating the maximum CT value in the series of CT images. At the off-center of SFOV, the tendency of increasing FWHM was confirmed, but it was shown that variations of the off-center in thicker slice thickness were larger regardless of helical pitch when the orbital synchronized helical scan technique was not used.

[Development of automatic analyses for star-shot images using computed radiography (CR)].

Recent progress in radiation therapy has been greatly enhanced in many facilities by the development of new machines for treatment, improved computer technology for radiotherapy treatment planning systems (RTPs), increased accuracy of radiation therapy such as stereotactic irradiation, and intensity-modulated radiation therapy (IMRT). Quality control (QC) of the isocenter, which has consisted of gantry rotation and limiting the radiation field, is important for greater accuracy of these radiation therapy technologies. Star-shot analyses using computed radiography (CR) for evaluation of the isocenter were employed in this study. Devices to support CR were created, and a method of automatically analyzing images obtained by the star-shot technique, which calculated the error (distance) from the isocenter and the incident beam angle, were developed. In terms of the accuracy of our method, the average maximum error was 0.33 mm (less than 2 pixels: 0.35 mm), the average absolute error and incident beam angle errors were 0.3 mm and 0.4 degrees at maximum and at one standard deviation (SD), respectively. In this study, the processing times were 16 sec at minimum, 152 sec at maximum, 18 sec at most frequencies, and 23.6 sec on average. In conclusion, it was considered that our newly developed method for analyzing star-shot images using CR enabled immediate, quantitative evaluation of the isocenter.