Ciangaru George, Yang James N, Oliver Patrick J, Bues Martin, Zhu Mengping, Nakagawa Fumio, Chiba Hitoshi, Nakamura Shin, Yoshino Hirofumi, Umezawa Mosumi, Smith Alfred R
Proton Therapy Center, Department of Radiation Physics, The University of Texas M.D. Anderson Cancer Center, Houston, Texas, U.S.A.
Power Systems Division, Hitachi America, Houston, Texas, U.S.A.
J Appl Clin Med Phys. 2007 Oct 24;8(4):65-75. doi: 10.1120/jacmp.v8i4.2671.
We present a technique--based on the Lutz, Winston, and Maleki test used in stereotactic linear accelerator radiosurgery--for verifying whether proton beams are being delivered within the required spatial coincidence with the gantry mechanical isocenter. Our procedure uses a proton beam that is collimated by a circular aperture at its central axis and is then intercepted by a small steel sphere rigidly supported by the patient couch. A laser tracker measurement system and a correction algorithm for couch position assures precise positioning of the steel sphere at the mechanical isocenter of the gantry. A film-based radiation dosimetry technique, chosen for the good spatial resolution it achieves, records the proton dose distribution for optical image analysis. The optical image obtained presents a circular high-dose region surrounding a lower-dose area corresponding to the proton beam absorption by the steel sphere, thereby providing a measure of the beam alignment with the mechanical isocenter. We found the self-developing Gafchromic EBT film (International Specialty Products, Wayne, NJ) and commercial Epson 10000 XL flatbed scanner (Epson America, Long Beach, CA) to be accurate and efficient tools. The positions of the gantry mechanical and proton beam isocenters, as recorded on film, were clearly identifiable within the scanning resolution used for routine alignment testing (0.17 mm per pixel). The mean displacement of the collimated proton beam from the gantry mechanical isocenter was 0.22 +/- 0.1 mm for the gantry positions tested, which was well within the maximum deviation of 0.50 mm accepted at the Proton Therapy Center in Houston.
我们提出了一种基于立体定向直线加速器放射外科中使用的Lutz、Winston和Maleki测试的技术，用于验证质子束是否在与机架机械等中心所需的空间重合范围内传输。我们的程序使用一束质子束，该质子束在其中心轴处由圆形孔径准直，然后被刚性支撑在患者治疗床上的小钢球拦截。激光跟踪测量系统和治疗床位置校正算法确保钢球精确位于机架的机械等中心。选择基于胶片的辐射剂量测定技术，因其具有良好的空间分辨率，用于记录质子剂量分布以进行光学图像分析。获得的光学图像呈现出一个围绕较低剂量区域的圆形高剂量区域，该较低剂量区域对应于钢球对质子束的吸收，从而提供了质子束与机械等中心对准情况的一种度量。我们发现自行研制的Gafchromic EBT胶片（国际特种产品公司，新泽西州韦恩）和商用爱普生10000 XL平板扫描仪（爱普生美国公司加利福尼亚州长滩）是准确且高效的工具。在用于常规对准测试的扫描分辨率（每像素0.17毫米）内，胶片上记录的机架机械等中心和质子束等中心的位置清晰可辨。对于所测试的机架位置，准直质子束与机架机械等中心的平均位移为0.22±0.1毫米，这完全在休斯顿质子治疗中心所接受的最大偏差0.50毫米范围内。
