| Summary: | 博士 === 國立清華大學 === 生醫工程與環境科學系 === 105 === Stereotactic radiosurgery (SRS) is a well-established technique in the treatment of intracranial lesions, generally leading to better preservation of brain functions and therefore a better quality of life for the patient. Linear accelerators (linacs), meanwhile, have become ubiquitous pieces of equipment in radiotherapy around the world, and the combined use of SRS and linac technology is entry increasingly popular option in stereotactic brain radiotherapy. Dramatic recent improvements in both treatment techniques and linac accuracy performance have enhanced the reliability of SRS, which is now routinely used. This thesis reports the results of three completed studies for linac-based SRS. Based on these findings, the thesis proposes new clinical guidelines and perspectives for linac-based SRS treatment, registration technique and development of the detector for quality assurance.
There are several available forms of linac-based SRS, and the goal of first study in thesis was therefore to identify which of these techniques is optimal when the target is located adjacent to the brainstem. We collected the records of 17 patients with lesions close to the brainstem who had previously been treated with single-fraction radiosurgery. In all, five different lesion catalogues were collected, and the patients were divided into two groups: one consisting of seven individuals with a target-to-brainstem distance of less than 0.5cm, and the other of 10 patients with a target-to-brainstem distance of between 0.5cm and 1cm. Comparisons were then made among the following three types of linac-based radiosurgery: dynamic conformal arcs (DCA), intensity -modulated radiosurgery (IMRS), and volumetric modulated arc radiotherapy (VMAT). All three techniques included multiple non-coplanar beams or arcs, with or without intensity-modulated delivery. Dosimetric metrics were used to evaluate plan quality, with particular attention paid to gradient indexes. This study found that the optimal SRS technique when the tumor was located adjacent to the brainstem was VMAT.
The regular functions of CT-MRI registration include delineation of targets and organs-at-risk (OARs) in radiosurgery planning. The question of whether deformable image registration (DIR) could be applied to SRS in its place remains a subject of debate. Accordingly, study in second part collected data regarding 16 patients who had undergone single-fraction SRS treatment. All their lesions were located close to the brainstem. CT-MRI image sets were registered by both rigid image registration (RIR) and DIR algorithms. The contours of the OARs were drawn individually on the rigid and deformable CT-MRI image sets by qualified radiation oncologists and dosimetrists, and the evaluation metrics included volume overlapping (VO), Dice similarity coefficient (DSC), and dose. A modified-demons deformable algorithm (VARIAN SmartAdapt) was used for evaluation in this study, which verified that software accuracy and concluded reliability of DIR in SRS.
The third and final study investigated utility used in SRS quality assurance, along with the dosimetric characteristics and daily-output consistency of an amorphous-silicon (aSi) electronic portal imaging device (EPID). Specifically, the portal dose prediction function of an aSi-1000 EPID was used to test that panel for linearity, field size, dose rate and source-to-detector distance (SDD). The percentage variance of daily output also was measured using two alternative types of dosimeter: EBT3 film, and a multiple-channels output-check device. A total of 165 daily output readings were taken using each of the three dosimeters between July 2014 and February 2015, and the results suggested that the quality of daily-output monitoring via this aSi EPID was reasonably consistent with that of the commercial daily-output device, and therefore aSi EPIDs should be considered seriously as a replacement for complex film-based procedures.
Taken as a whole, the results of three studies in this thesis provide important new evidence and new perspectives on image registration, delivery techniques and quality assurance for linac-based SRS treatment. As such, the thesis contributed important implications for doctors, patients, medical technicians and hospital administrators around the world, which are discussed in depth.
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