Radiation Oncology, Moffitt Cancer Center, Tampa, FL 33612, USA.
J Appl Clin Med Phys. 2012 Sep 6;13(5):3748. doi: 10.1120/jacmp.v13i5.3748.
The build-up dose in the megavoltage photon beams can be a limiting factor in intensity-modulated radiation therapy (IMRT) treatments. Excessive surface dose can cause patient discomfort and treatment interruptions, while underdosing may lead to tumor repopulation and local failure. Dose in the build-up region was investigated for IMRT delivery with solid brass compensator technique(compensator-based IMRT) and compared with that of multileaf collimator (MLC)-based IMRT. A Varian Trilogy linear accelerator equipped with an MLC was used for beam delivery. A special solid brass step-wise compensator was designed and built for testing purposes. Two step-and-shoot MLC fields were programmed to produce a similar modulated step-wise dose profile. The MLC and compensator dose profiles were measured and adjusted to match at the isocenter depth of 10 cm. Build-up dose in the 1-5 mm depth range was measured with an ultrathin window, fixed volume parallel plate ionization chamber. Monte Carlo simulations were used to model the brass compensator and step-and-shoot MLC fields. The measured and simulated profiles for the two IMRT techniques were matched at the isocenter depth of 10 cm. Different component contributions to the shallow dose, including the MLC scatter, were quantified. Mean spectral energies for the open and filtered beams were calculated. The compensator and MLC profiles at 10 cm depth were matched better than ± 1.5%. The build-up dose was up to 7% lower for compensator IMRT compared to MLC IMRT due to beam hardening in the brass. Low-energy electrons contribute 22% and 15% dose at 1 mm depth for compensator and MLC modalities, respectively. Compensator-based IMRT delivers less dose in the build-up region than MLC-based IMRT does, even though a compensator is closer to the skin than the MLC.
在调强放射治疗（IMRT）中，兆伏光子束的起始剂量可能是一个限制因素。过高的表面剂量会导致患者不适和治疗中断，而过低的剂量则可能导致肿瘤再增殖和局部失败。本研究旨在比较使用实体黄铜补偿器技术（补偿器 IMRT）和多叶准直器（MLC-IMRT）进行调强放射治疗时的起始剂量分布。采用瓦里安 Trilogy 直线加速器配备多叶准直器进行光束传输。设计并制作了一种特殊的实体黄铜逐步补偿器用于测试。编程两个步进式 MLC 野以产生类似的调制阶梯剂量分布。在 10cm 等中心深度处，测量并调整 MLC 和补偿器剂量分布以匹配。使用超薄窗口固定容积平行板电离室测量 1-5mm 深度范围内的起始剂量。使用蒙特卡罗模拟来模拟黄铜补偿器和步进式 MLC 场。在 10cm 等中心深度处，将两种 IMRT 技术的测量和模拟分布进行匹配。量化了浅部剂量的不同成分贡献，包括 MLC 散射。计算了开放和过滤光束的平均光谱能量。补偿器和 MLC 在 10cm 深度处的分布匹配优于±1.5%。由于黄铜的束硬化，补偿器 IMRT 的起始剂量比 MLC-IMRT 低 7%。在 1mm 深度处，补偿器和 MLC 模式下的低能电子分别贡献 22%和 15%的剂量。即使补偿器比 MLC 更接近皮肤，补偿器 IMRT 也能在起始区域输送比 MLC-IMRT 更少的剂量。
