Radiotherapy is a prominent cancer treatment modality in medicine, aiming to deliver adequate doses to the target while minimizing harm to healthy tissue. Recent advancements in computer technology, machine engineering, and imaging have facilitated intricate treatment planning and accurate radiation administration. These advancements have allowed for more precise dose distributions to be delivered to cancer patients. However, even small discrepancies in setup or delivery can result in significant dose variations. While treatment planning systems provide 3D dose calculations, there is currently a lack of 3D measurement tools in the clinic to verify the accuracy of dose calculation and delivery. Presently, medical physicists rely on 2D dose plane comparisons with treatment planning calculations using gamma index analyses. However, these results do not directly correlate with clinical dose-volume constraints, and detecting delivery errors using 1D or 2D dosimetry is challenging. The implementation of 3D dosimetry not only ensures the safety of radiation treatment but also facilitates the development of new emerging radiation treatment techniques. This study aims to commission and validate a clinically viable optical scanner for 3D dosimetry and apply the developed system to address current clinical and pre-clinical challenges, thereby advancing our understanding of treatment uncertainties in modern radiotherapy.
The optical CT scanner that was developed comprises four key components: an LED illuminator, an aquarium with matching fluid, a fiber optic taper, and a CCD camera. The LED illuminator emits uniform and parallel red light at a peak wavelength of 625 nm and a full width at half maximum (FWHM) of 20 nm in continuous mode. The aquarium is constructed with transparent acrylic walls and is designed to accommodate the 3D dosimeter PRESAGE, which can be fixed on a rotation stage inside the tank. Clear acrylic has excellent optical clarity and light transmission, with a refractive index of 1.49 that is close to the average refractive index (1.54) of PRESAGE. To match the refractive index of the 3D dosimeters, a matching liquid composed of 90% Octyl Salicylate and 10% Octyl-P-Methoxy Cinnamate is filled in the tank. The fiber optic taper serves two functions: first, it demagnifies the projection images while preserving their shape, and second, it effectively reduces the acceptance angle of the light reaching the CCD camera. The CCD camera used in the system is an Allied Vision model with a resolution of 0.016 mm, capable of acquiring 2D projection images from various angles. The principle of the optical CT scanner follows that of CT imaging, where 2D projection images from different angles are used to reconstruct volumetric 3D dose images using the filtered back projection technique. To validate the dosimetric measurements and assess the uncertainties of the 3D dosimetry system, 21 benchmark experiments, including mechanical, imaging, and dosimetry tests were conducted. Furthermore, the developed system was employed for various applications, including patient-specific IMRT QA, small field dosimetry using kilovoltage and megavoltage beams, as well as end-to-end testing of stereotactic radiosurgery.
A comprehensive analysis assessed uncertainties in each scanner component. Mechanical tests showed maximum uncertainties below 1%. By employing background subtraction and calibration techniques, measurement uncertainty was reduced to <1% in the optimal dose range. Background subtraction resulted in a remarkable 77% reduction in uncertainty by mitigating artifacts, ambient light, and refractive light. Reproducibility was excellent, with mean and standard deviation of dose differences below 0.4% and 1.1%, respectively, in three repeat scans. Dose distribution measurements exhibited strong agreement (passing rates: 98%-100%) between 3D measurements, treatment planning calculations, and EBT3 film dosimetry. Results confirm the optical CT scanner's robustness and accuracy for clinical 3D radiation dosimetry. The study also demonstrates that the developed 3D dosimetry system surpasses the limitations of traditional 2D gamma tests by providing clinicians with more clinically relevant information. This includes measured dose-volume histograms (DVHs) and the evaluation of gamma failing points in 3D space, enabling a comprehensive assessment of individual treatment plans. Furthermore, the study showcased the feasibility of utilizing this system to characterize a radiosurgery platform. It successfully assessed mechanical and dosimetric errors in off-axis delivery and evaluated the accuracy of treatment planning dose calculations, including modeling small fields, out-of-field dose, and multi-leaf collimator (MLC) characteristics. In addition, compelling evidence was presented that the high-resolution 3D dosimeter used in this study is capable of accurate dosimetry for both megavoltage and kilovoltage small fields. Importantly, the dosimeter exhibits no energy or dose rate dependence, further supporting its reliability and suitability for precise dosimetry measurements.
The intricate and three-dimensional nature of dose distributions in modern radiotherapy necessitated the development of 3D dosimetry measurements, particularly for treatments with precise margins, such as SRS and SBRT. The newly developed 3D dosimetry system offers significant enhancements to current QA practices, delivering more clinically relevant comparison results and bolstering patient safety. Furthermore, it can be utilized for independent inspections across multiple institutions or remote dosimetry verification. Beyond its applications in clinical settings, the presented 3D dosimetry system holds the potential to expedite the development and utilization of novel radiation platforms.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/r174-wd40 |
| Date | January 2024 |
| Creators | Wang, Yi-Fang |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
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