Low D A, Chao K S, Mutic S, Gerber R L, Perez C A, Purdy J A
Mallinckrodt Institute of Radiology, Division of Radiation Oncology, St. Louis, MO 63110, USA.
Int J Radiat Oncol Biol Phys. 1998 Oct 1;42(3):681-92. doi: 10.1016/s0360-3016(98)00273-9.
A commercial serial tomotherapy intensity-modulated radiation therapy (IMRT) treatment planning (Peacock, NOMOS Corp., Sewickley, PA) and delivery system is in clinical use. The dose distributions are highly conformal, with large dose gradients often surrounding critical structures, and require accurate localization and dose delivery. Accelerator and patient-specific quality assurance (QA) procedures have been developed that address the localization, normalization, and delivery of the IMRT dose distributions.
The dose distribution delivered by serial tomotherapy is highly sensitive to the accuracy of the longitudinal couch motion. There is also an unknown sensitivity of the dose distribution on the dynamic mutlileaf collimator alignment. QA procedures were implemented that assess these geometric parameters. Evaluations of patient positioning accuracy and stability were conducted by exposing portal films before (single exposure) and after (single or double exposure) treatments. The films were acquired with sequential exposures using the largest available fixed multileaf portal (3.36 x 20 cm2). Comparison was made against digitally reconstructed radiographs generated using independent software and appropriate beam geometries. The delivered dose was verified using homogeneous cubic phantoms. Radiographic film was used to determine the localization accuracy of the delivered isodose distributions, and ionization chambers and thermoluminescent dosimetry (TLD) chips were used to verify absolute dose at selected points. Ionization chamber measurements were confined to the target dose regions and TLD measurements were obtained throughout the irradiated volumes. Because many more TLD measurements were made, a statistical evaluation of the measured-to-calculated dose ratio was possible.
The accelerator QA techniques provided adequate monitoring of the geometric patient movement and dynamic multileaf collimator alignment and positional stability. The absolute delivered dose as measured with the ionization chamber varied from 0.94 to 0.98. Based on these measurements, the delivered monitor units for both subsequent QA measurements and patient treatments were adjusted by the ratio of measured to calculated dose. TLD measurements showed agreement, on average, with the ionization chamber measurements. The distribution of TLD measurements in the high-dose regions indicated that measured doses agreed within 4.2% standard deviation of the calculated doses. In the low-dose regions, the measured doses were on average 5% greater than the calculated doses, due to a lack of leakage dose in the dose calculation algorithm.
The QA system provided adequate determination of the geometric and dosimetric quantities involved in the use of IMRT for the head and neck. Ionization chamber and TLD measurements provided accurate determination of the absolute delivered dose throughout target volumes and critical structures, and radiographic film yielded precise dose distribution localization verification. Portal film acquisition and subsequent portal film analysis using 3.36 x 20 cm2 portals proved useful in the evaluation of patient immobilization quality. Adequate bony landmarks were imaged when carefully selected portals were used.
一种商业串行断层放射治疗调强放射治疗（IMRT）治疗计划系统（Peacock，NOMOS公司，宾夕法尼亚州塞威克利）及输送系统正在临床中使用。剂量分布具有高度适形性，在关键结构周围通常存在大剂量梯度，并且需要精确的定位和剂量输送。已经开发出加速器和针对患者的质量保证（QA）程序，以解决IMRT剂量分布的定位、归一化和输送问题。
串行断层放射治疗所输送的剂量分布对纵向治疗床运动的准确性高度敏感。剂量分布对动态多叶准直器对准也存在未知的敏感性。实施了评估这些几何参数的QA程序。通过在治疗前（单次曝光）和治疗后（单次或双次曝光）拍摄射野片来评估患者定位的准确性和稳定性。使用最大可用的固定多叶射野（3.36×20平方厘米）进行连续曝光来获取射野片。将其与使用独立软件和适当射束几何形状生成的数字重建射线照片进行比较。使用均匀立方模体验证所输送的剂量。用射线照相胶片确定所输送等剂量分布的定位准确性，并用电离室和热释光剂量测定（TLD）芯片验证选定部位的绝对剂量。电离室测量限于靶区剂量区域，TLD测量在整个照射体积内进行。由于进行了更多的TLD测量，因此可以对测量剂量与计算剂量之比进行统计学评估。
加速器QA技术对患者几何运动、动态多叶准直器对准和位置稳定性提供了充分的监测。用电离室测量的绝对输送剂量在0.94至0.98之间变化。基于这些测量结果，后续QA测量和患者治疗的输送监测单位根据测量剂量与计算剂量之比进行调整。TLD测量结果平均与电离室测量结果一致。高剂量区域TLD测量值的分布表明，测量剂量与计算剂量在计算剂量的4.2%标准差范围内一致。在低剂量区域，由于剂量计算算法中缺乏漏射线剂量，测量剂量平均比计算剂量大5%。
QA系统对头部和颈部IMRT使用中涉及的几何和剂量学量提供了充分的测定。电离室和TLD测量在整个靶区体积和关键结构中准确测定了绝对输送剂量，射线照相胶片实现了精确的剂量分布定位验证。使用3.36×20平方厘米射野获取射野片并随后进行射野片分析，在评估患者固定质量方面证明是有用的。当使用精心选择的射野时，可以对足够的骨性标志进行成像。
