Polf Jerimy C, Avery Stephen, Mackin Dennis S, Beddar Sam
Department of Radiation Oncology, University of Maryland School of Medicine, Baltimore, MD 21201, USA.
Phys Med Biol. 2015 Sep 21;60(18):7085-99. doi: 10.1088/0031-9155/60/18/7085. Epub 2015 Aug 28.
The purpose of this paper is to evaluate the ability of a prototype Compton camera (CC) to measure prompt gamma rays (PG) emitted during delivery of clinical proton pencil beams for prompt gamma imaging (PGI) as a means of providing in vivo verification of the delivered proton radiotherapy beams. A water phantom was irradiated with clinical 114 MeV and 150 MeV proton pencil beams. Up to 500 cGy of dose was delivered per irradiation using clinical beam currents. The prototype CC was placed 15 cm from the beam central axis and PGs from 0.2 MeV up to 6.5 MeV were measured during irradiation. From the measured data (2D) images of the PG emission were reconstructed. (1D) profiles were extracted from the PG images and compared to measured depth dose curves of the delivered proton pencil beams. The CC was able to measure PG emission during delivery of both 114 MeV and 150 MeV proton beams at clinical beam currents. 2D images of the PG emission were reconstructed for single 150 MeV proton pencil beams as well as for a 5 × 5 cm mono-energetic layer of 114 MeV pencil beams. Shifts in the Bragg peak (BP) range were detectable on the 2D images. 1D profiles extracted from the PG images show that the distal falloff of the PG emission profile lined up well with the distal BP falloff. Shifts as small as 3 mm in the beam range could be detected from the 1D PG profiles with an accuracy of 1.5 mm or better. However, with the current CC prototype, a dose of 400 cGy was required to acquire adequate PG signal for 2D PG image reconstruction. It was possible to measure PG interactions with our prototype CC during delivery of proton pencil beams at clinical dose rates. Images of the PG emission could be reconstructed and shifts in the BP range were detectable. Therefore PGI with a CC for in vivo range verification during proton treatment delivery is feasible. However, improvements in the prototype CC detection efficiency and reconstruction algorithms are necessary to make it a clinically viable PGI system.
本文的目的是评估一款原型康普顿相机（CC）测量临床质子笔形束传输过程中发射的瞬发伽马射线（PG）的能力，用于瞬发伽马成像（PGI），以此作为对所传输质子放射治疗束进行体内验证的一种手段。用水模体接受临床114 MeV和150 MeV质子笔形束的照射。每次照射使用临床束流输送高达500 cGy的剂量。将原型CC放置在距束中心轴15厘米处，并在照射期间测量能量从0.2 MeV到6.5 MeV的PG。从测量数据重建PG发射的（二维）图像。从PG图像中提取（一维）轮廓，并与所传输质子笔形束的测量深度剂量曲线进行比较。该CC能够在临床束流条件下测量114 MeV和150 MeV质子束传输过程中的PG发射。针对单个150 MeV质子笔形束以及114 MeV笔形束的5×5厘米单能层重建了PG发射的二维图像。在二维图像上可检测到布拉格峰（BP）范围的移动。从PG图像中提取的一维轮廓表明，PG发射轮廓的远端下降与远端BP下降良好对齐。从一维PG轮廓中可以检测到束范围内小至3毫米的移动，精度可达1.5毫米或更高。然而，对于当前的CC原型，需要400 cGy的剂量才能获取足够的PG信号用于二维PG图像重建。在临床剂量率下传输质子笔形束期间，使用我们的原型CC测量PG相互作用是可行的。可以重建PG发射的图像，并且可检测到BP范围的移动。因此，在质子治疗传输过程中使用CC进行体内射程验证的PGI是可行的。然而，有必要改进原型CC的检测效率和重建算法，使其成为临床上可行的PGI系统。
