Abstract
Most of the treatment units, both new and old models, are equipped with a megavoltage portal imager but its use for volumetric imaging is limited. This is mainly due to the poor image quality produced by the high‐energy treatment beam (>6 MV). A linac at our center is equipped with a prototype 2.5 MV imaging beam. This study evaluates the feasibility of low‐dose megavoltage cone‐beam imaging with the 2.5 MV beam and a thick cesium iodide detector, which is a high‐efficiency imager. Basic imaging properties such as spatial resolution and modulation transfer function were assessed for the 2.5 MV prototype imaging system. For image quality and imaging dose, a series of megavoltage cone‐beam scans were acquired for the head, thorax, and pelvis of an anthropomorphic phantom and were compared to kilovoltage cone‐beam and 6X megavoltage cone‐beam images. To demonstrate the advantage of MV imaging, a phantom with metallic inserts was scanned and the image quality was compared to CT and kilovoltage cone‐beam scans. With a lower energy beam and higher detector efficiency, the 2.5 MV imaging system generally yields better image quality than does the 6 MV imaging system with the conventional MV imager. In particular, with the anthropomorphic phantom studies, the contrast to noise of bone to tissue is generally improved in the 2.5 MV images compared to 6 MV. With an image quality sufficient for bony alignment, the imaging dose for 2.5 MV cone‐beam images is 2.4−3.4 MU compared to 26 MU in 6 MV cone‐beam scans for the head, thorax, and pelvis regions of the phantom. Unlike kilovoltage cone‐beam, the 2.5 MV imaging system does not suffer from high‐Z image artifacts. This can be very useful for treatment planning in cases where high‐Z prostheses are present.PACS number(s): 87.57.Q‐
Highlights
236 Tang et al.: Low-dose 2.5X megavoltage cone-beam imaging (MVCB) tissues as well
Since most linacs are already equipped with a MV portal imager, one would expect that MVCB can be readily implemented in the clinic with minimal additional cost or modification to the treatment units
MVCB has yet to be widely implemented, in part because high-energy photons from the therapeutic MV beams often produce images with poor tissue contrast and the imaging doses are relatively high
Summary
236 Tang et al.: Low-dose 2.5X MVCB tissues as well. More importantly, these kV imaging systems are fully integrated to treatment units and are readily available. KV image quality can be significantly perturbed by high-Z materials, especially for patients with metallic implants such as dental fillings, hip prosthesis, and prostate seeds These high-Z image artifacts are greatly reduced in MV X-ray imaging. MVCB has yet to be widely implemented, in part because high-energy photons from the therapeutic MV beams (typically 6 MV) often produce images with poor tissue contrast and the imaging doses are relatively high. While this can be resolved by lowering the energy of the MV beam, it can be mechanically challenging to produce a stable and sufficiently low-energy MV beam that attains similar imaging quality to kV X-rays for current waveguide designs. This beamline configuration subsequently improves the image contrast as demonstrated by several previous studies.[10,12,13,14,15]
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