Medical Physics Unit, Oman.
Phys Med. 2011 Jul;27(3):163-8. doi: 10.1016/j.ejmp.2010.10.003. Epub 2010 Nov 11.
To start total body irradiation (TBI) treatments, physical parameters are measured for a magna field irradiation.
6 MV photon beam from Clinac 600 CD linear accelerator (Varian, USA) with fully opened collimator at 45° and gantry at 270° provided a diamond shaped magna field with diagonal dimension 224 cm at 4.0 m source skin distance (SSD). The flatness of the radiation field was measured in the presence of locally designed acrylic beam spoiler and beam flatness filter. Central Axis Depth dose data (CADD), tissue maximum ratios and entrance dose pattern are measured using large phantoms. Methods for clinical dose estimation using semi-conductor diodes and TLD were standardized.
PVC beam flattener at the shielding tray position and the presence of acrylic beam spoiler in the radiation field provided a flatness of 100.15% ± 0.44% compared to open beam flatness 101.6 ± 1.5%. A reduction of 2% in percentage depth dose was observed at 10 cm depth in the presence of 15 mm acrylic beam spoiler. However, no changes are observed in the TMRs with presence of beam spoiler. The measured ionization ratios clearly showed change of beam quality with the introduction of beam spoiler. The presence of 15 mm beam spoiler ensured entrance dose 100% at skin and remaining unchanged within 1% upto a depth of 10 mm. Phantom measurements show good agreement between calculated and measured doses.
The paper recommends use of modified CADD parameters for treatment planning, if calibration of output is carried out in the presence of beam spoiler.
为了开始全身照射（TBI）治疗，需要测量大磁场照射的物理参数。
Clinac 600 CD 直线加速器（美国瓦里安）的 6 MV 光子束，在 45°完全打开准直器，在 270°旋转机架，提供了一个对角线尺寸为 224cm 的菱形大磁场，在 4.0m 源皮距（SSD）处。在场设计的丙烯酸束散器和束平滤波器存在的情况下，测量辐射场的平坦度。使用大型体模测量中心轴深度剂量数据（CADD）、组织最大比和入射剂量模式。使用半导体二极管和 TLD 进行临床剂量估算的方法已经标准化。
在屏蔽托盘位置的 PVC 束平坦器和辐射场中的丙烯酸束散器的存在，与开放束平坦度 101.6±1.5%相比，提供了 100.15%±0.44%的平坦度。在存在 15mm 丙烯酸束散器的情况下，在 10cm 深度处观察到深度剂量的百分比降低了 2%。然而，在存在束散器的情况下，TMR 没有变化。测量的离子化比清楚地显示了随着束散器的引入，束质发生了变化。存在 15mm 束散器可确保在皮肤处的入射剂量为 100%，并在 10mm 深度内保持不变，变化不超过 1%。体模测量显示计算剂量与测量剂量之间具有良好的一致性。
如果在存在束散器的情况下进行输出校准，则建议在治疗计划中使用修改后的 CADD 参数。
