Volume of interest imaging for image guided radiotherapy
Image guidance has become a vital tool for accurate positioning and monitoring of a patient during radiation therapy. However, the current clinical standard is limited to a large static imaging field and constant x-ray tube milliampere-second. In cone beam computed tomography (CBCT), this results in high scatter radiation and increased patient dose. During kilovoltage intrafraction monitoring of the prostate, this gives rise to periods of limited fiducial detectability. This work proposes a novel current-modulated volume-of-interest (VOI) approach to address these deficiencies and to improve the available image quality for image-guided radiation therapy (IGRT) with a considerably lower peripheral dose to the patient. A series of three manuscripts form the foundation of this thesis. The first manuscript entitled, “An investigation of kV CBCT image quality and dose reduction for volume-of-interest imaging using dynamic collimation”, demonstrates the advantages of a dynamic collimator approach for VOI CBCT, in which image quality is improved as imaging aperture decreases for a given dose to the VOI, with significant reductions in dose outside the VOI compared to full-field imaging. The second manuscript, “Volume of Interest CBCT and tube current modulation for image guidance using dynamic kV collimation”, demonstrates how current modulation can be used in combination with VOI to further improve the available image quality for CBCT at a given dose to the VOI or maintain image quality at a reduced dose. The third and final manuscript, “Current modulated volume-of-interest imaging for kilovoltage intrafraction monitoring of the prostate”, demonstrates how current modulated VOI can be used to provide constant intrafraction fiducial detectability for non-ideal imaging conditions (i.e. high kilovoltage attenuation and megavoltage scatter). The associated dosimetric trade-offs required for current modulation are also examined. Overall, this thesis demonstrates that the current modulated VOI approach provides enhanced image quality for guidance in radiation therapy, while sparing tissue extraneous to the image guidance task of unnecessary dose. This is accomplished through quantitative measurement of contrast-to-noise ratio or fiducial position and corresponding Monte Carlo modeled dose distributions.