Amer A M, Mackay R I, Roberts S A, Hendry J H, Williams P C
North Western Medical Physics, Christie Hospital NHS Trust, Manchester, UK.
Radiother Oncol. 2001 Nov;61(2):143-50. doi: 10.1016/s0167-8140(01)00440-6.
To use radiobiological modelling to estimate the number of initial days of treatment imaging required to gain most of the benefit from off-line correction of systematic errors in the conformal radiation therapy of prostate cancer.
Treatment plans based on the anatomical information of a representative patient were generated assuming that the patient is treated with a multi leaf collimator (MLC) four-field technique and a total isocentre dose of 72 Gy delivered in 36 daily fractions. Target position variations between fractions were simulated from standard deviations of measured data found in the literature. Off-line correction of systematic errors was assumed to be performed only once based on the measured errors during the initial days of treatment. The tumour control probability (TCP) was calculated using the Webb and Nahum model.
Simulation of daily variations in the target position predicted a marked reduction in TCP if the planning target volume (PTV) margin was smaller than 4 mm (TCP decreased by 3.4% for 2 mm margin). The systematic components of target position variations had greater effect on the TCP than the random components. Off-line correction of estimated systematic errors reduced the decrease in TCP due to target daily displacements, nevertheless, the resulting TCP levels for small margins were still less than the TCP level obtained with the use of an adequate PTV margin of approximately 10 mm. The magnitude of gain in TCP expected from the correction depended on the number of treatment imaging days used for the correction and the PTV margin applied. Gains of 2.5% in TCP were estimated from correction of systematic errors performed after 6 initial days of treatment imaging for a 2 mm PTV margin. The effect of various possible magnitudes of systematic and random components on the gain in TCP expected from correction and on the number of imaging days required was also investigated.
Daily variations of target position markedly reduced the TCP if small margins were used. Off-line correction of systematic errors can only partly compensate for these TCP reductions. The adequate number of treatment imaging days required for systematic error correction depends on the magnitude of the random component compared with the systematic component, and on the size of PTV margin used. For random components equal to or smaller than the systematic component, 3 consecutive treatment imaging days are estimated to be sufficient to gain most of the benefit from correction for current clinically used margins (6-10 mm); otherwise more days are required.
运用放射生物学模型估算在前列腺癌适形放射治疗中，为从系统误差的离线校正中获取最大益处所需的初始治疗成像天数。
基于一名代表性患者的解剖学信息生成治疗计划，假设该患者采用多叶准直器（MLC）四野技术治疗，总等中心剂量为72 Gy，分36次每日剂量给予。根据文献中测得数据的标准差模拟分次治疗间靶区位置的变化。假设仅在治疗初始阶段基于测得的误差对系统误差进行一次离线校正。使用Webb和Nahum模型计算肿瘤控制概率（TCP）。
如果计划靶区体积（PTV）边界小于4 mm，靶区位置每日变化的模拟预测TCP会显著降低（2 mm边界时TCP降低3.4%）。靶区位置变化的系统成分对TCP的影响大于随机成分。对估计的系统误差进行离线校正减少了因靶区每日位移导致的TCP降低，然而，小边界时最终的TCP水平仍低于使用约10 mm的合适PTV边界所获得的TCP水平。校正预期的TCP增益幅度取决于用于校正的治疗成像天数以及所应用的PTV边界。对于2 mm的PTV边界，在初始6天治疗成像后进行系统误差校正估计可使TCP增益2.5%。还研究了各种可能大小的系统和随机成分对校正预期的TCP增益以及所需成像天数的影响。
如果使用小边界，靶区位置的每日变化会显著降低TCP。系统误差的离线校正只能部分补偿这些TCP降低。系统误差校正所需的足够治疗成像天数取决于随机成分与系统成分相比的大小，以及所使用的PTV边界大小。对于随机成分等于或小于系统成分的情况，估计连续3天治疗成像足以从当前临床使用的边界（6 - 10 mm）校正中获得最大益处；否则需要更多天数。
