Department of Oncology, Helsinki University Central Hospital, HUS, Finland.
Radiother Oncol. 2009 Oct;93(1):109-14. doi: 10.1016/j.radonc.2009.04.023. Epub 2009 May 10.
We estimated accuracy requirements for dose and position of the major salivary glands in head and neck intensity-modulated radiotherapy (IMRT) based on the dose response characteristics of the glands obtained by using the sigmoidal dose response model.
Dose response of the parotid and submandibular salivary glands was determined for 25 head and neck cancer patients treated by IMRT. Individual salivary gland functions were assessed by scintigraphy before and 6months after radiotherapy. Accuracy requirements were estimated by using the maximal slope of the fitted dose response model and average value of the dose gradients within the glands. In addition, systematic and random set-up errors were estimated for each patient by at least weekly portal imaging. We investigated the changes in the salivary gland mean doses (D(mean)) that would have occurred without correction of patient positioning. This was done by shifting the planned isocenter according to the obtained systematic set-up error and by recalculating the dose distribution in treatment planning system (TPS).
The maximal slope and D(50) values of the dose response model were -0.0411/Gy and 30.4Gy, respectively. The results suggested that spared fraction of individual salivary gland function can be estimated with an accuracy of +/-10%, if actual D(mean) of the gland is within +/-2.4Gy with the planned value. On the average, this was achieved with maximal systematic positional 3D shift of 3.0mm for the parotid glands and 2.7mm for the submandibular glands. The magnitude of systematic 1D set-up errors was 1.7+/-1.3mm (mean+/-SD) while that of systematic 3D errors was 3.4+/-1.6mm. The SD of random set-up errors was 1.5mm. The magnitude of D(mean) shifts due to set-up errors was 1.5+/-1.4Gy. The steepness of dose gradients within the glands was 0.8+/-0.5Gy/mm in the most critical direction (toward the glands).
When substantial part of salivary gland function is intended to be spared in head and neck IMRT, narrow dosimetric and positional tolerances should be adopted for the major salivary glands due to steep dose response curve obtained for the glands.
我们基于使用 sigmoidal 剂量反应模型获得的腺体剂量反应特征，估计了头颈部调强放疗（IMRT）中主要唾液腺剂量和位置的精度要求。
对 25 例接受 IMRT 治疗的头颈部癌症患者的腮腺和颌下腺的剂量反应进行了研究。在放疗前和放疗后 6 个月，通过闪烁扫描评估个体唾液腺的功能。通过拟合剂量反应模型的最大斜率和腺体内部剂量梯度的平均值来估计精度要求。此外，通过每周至少进行一次门控成像来估计每位患者的系统和随机摆位误差。我们研究了在不纠正患者定位的情况下，唾液腺平均剂量（Dmean）会发生的变化。这是通过根据获得的系统摆位误差移动计划等中心点，并在治疗计划系统（TPS）中重新计算剂量分布来实现的。
剂量反应模型的最大斜率和 D50 值分别为-0.0411/Gy 和 30.4Gy。结果表明，如果腺体的实际 Dmean 在计划值的正负 2.4Gy 以内，则可以以正负 10%的精度估计个体唾液腺功能的保留分数。平均而言，这可以通过腮腺最大系统 3D 移位 3.0mm 和颌下腺 2.7mm 来实现。系统 1D 摆位误差的幅度为 1.7+/-1.3mm（平均值+/-标准差），而系统 3D 误差的幅度为 3.4+/-1.6mm。随机摆位误差的标准差为 1.5mm。由于摆位误差导致的 Dmean 移位幅度为 1.5+/-1.4Gy。腺体内部最关键方向（朝向腺体）的剂量梯度陡峭度为 0.8+/-0.5Gy/mm。
当头颈部 IMRT 中意图保留大部分唾液腺功能时，由于获得的腺体剂量反应曲线较陡，主要唾液腺应采用较窄的剂量和位置容限。
