British Columbia Cancer Agency, Vancouver Cancer Centre, Vancouver, British Columbia, Canada.
Med Phys. 2012 Apr;39(4):1917-24. doi: 10.1118/1.3692177.
The purpose of this work is threefold: (1) to explore biological consequences of the multileaf collimator (MLC) calibration errors in intensity modulated radiotherapy (IMRT) of prostate and head and neck cancers, (2) to determine levels of planning target volume (PTV) and normal tissue under- or overdose flagged with clinically used QA action limits, and (3) to provide biologically based input for MLC QA and IMRT QA action limits.
Ten consecutive prostate IMRT cases and ten consecutive head and neck IMRT cases were used. Systematic MLC offsets (i.e., calibration error) were introduced for each control point of the plan separately for X1 and X2 leaf banks. Offsets were from - 2 to 2 mm with a 0.5 mm increment. The modified files were imported into the planning system for forward dose recalculation. The original plan served as the reference. The generalized equivalent uniform dose (gEUD) was used as the biological index for the targets, rectum, parotid glands, brainstem, and spinal cord. Each plan was recalculated on a CT scan of a 27 cm diameter cylindrical phantom with a contoured 0.6 cc ion chamber. Dose to ion chamber and 3D gamma analysis were compared to the reference plan. QA pass criteria: (1) at least 95% of voxels with a dose cutoff of 50% of maximum dose have to pass at 3 mm/3% and (2) dose to chamber within 2% of the reference dose.
For prostate cases, differences in PTV and rectum gEUD greater than 2% were identified. However, a larger proportion of plans leading to greater than 2% difference in prostate PTV gEUD passed the ion chamber QA but not 3D gamma QA. A similar trend was found for the rectum gEUD. For head and neck IMRT, the QA pass criteria flagged plans leading to greater than 4% differences in PTV gEUD and greater than 5% differences in the maximum dose to brainstem. If pass criteria were relaxed to 90% for gamma and 3% for ion chamber QA, plans leading to a 5% difference in PTV gEUD and a 5%-8% difference in brainstem maximum dose would likely pass IMRT QA. A larger proportion of head and neck plans with greater than 2% PTV gEUD difference passed 3D gamma QA compared to ion chamber QA.
For low modulation plans, there is a better chance to catch MLC calibration errors with 3D gamma QA rather than ion chamber QA. Conversely, for high modulation plans, there is a better chance to catch MLC calibration errors with ion chamber QA rather than with 3D gamma QA. Ion chamber and 3D gamma analysis IMRT QA can detect greater than 2% change in gEUD for PTVs and critical structures for low modulation treatment plans. For high modulation treatment plans, ion chamber and 3D gamma analysis can detect greater than 2% change in gEUD for PTVs and a 5% change in critical structure gEUD since either QA methods passes the QA criteria. For gEUD changes less than those listed above, either QA method has the same proportion of passing rate.
本研究旨在：（1）探讨多叶准直器（MLC）校准误差对前列腺癌和头颈部癌症调强放疗（IMRT）的生物学后果，（2）确定计划靶区（PTV）和正常组织在临床使用的 QA 动作限制下的剂量低于或高于规定剂量的水平，（3）为 MLC QA 和 IMRT QA 动作限制提供基于生物学的输入。
使用了 10 例连续前列腺 IMRT 病例和 10 例连续头颈部 IMRT 病例。分别针对计划的每个控制点的 X1 和 X2 叶片组引入系统的 MLC 偏移（即校准误差）。偏移量从-2 到 2 毫米，增量为 0.5 毫米。修改后的文件被导入规划系统进行正向剂量重新计算。原始计划作为参考。广义等效均匀剂量（gEUD）被用作目标、直肠、腮腺、脑干和脊髓的生物学指标。每个计划都在一个直径为 27 厘米的圆柱形模体的 CT 扫描上进行计算，该模体上有一个轮廓为 0.6 cc 的离子室。对离子室和 3D 伽马分析与参考计划进行了比较。QA 通过标准：（1）剂量截止值为最大剂量的 50%的至少 95%的体素必须通过 3 毫米/3%，（2）腔室内的剂量在参考剂量的 2%以内。
对于前列腺病例，发现 PTV 和直肠 gEUD 的差异大于 2%。然而，导致前列腺 PTV gEUD 差异大于 2%的较大比例计划通过了离子室 QA，但未通过 3D 伽马 QA。直肠 gEUD 也出现了类似的趋势。对于头颈部 IMRT，QA 通过标准标记了导致 PTV gEUD 差异大于 4%和脑干最大剂量差异大于 5%的计划。如果将伽马的通过标准放宽到 90%，将离子室 QA 的通过标准放宽到 3%，那么导致 PTV gEUD 差异大于 5%和脑干最大剂量差异大于 5%-8%的计划可能会通过 IMRT QA。与离子室 QA 相比，更多的头颈部计划 PTV gEUD 差异大于 2%通过了 3D 伽马 QA。
对于低调制计划，3D 伽马 QA 比离子室 QA 更有可能捕捉到 MLC 校准误差。相反，对于高调制计划，离子室 QA 比 3D 伽马 QA 更有可能捕捉到 MLC 校准误差。离子室和 3D 伽马分析 IMRT QA 可以检测到 PTV 和关键结构的 gEUD 变化大于低调制治疗计划的 2%。对于高调制治疗计划，离子室和 3D 伽马分析可以检测到 PTV 和关键结构 gEUD 的变化大于 2%，并且由于任何 QA 方法都通过了 QA 标准，因此关键结构 gEUD 的变化大于 5%。对于小于上述 gEUD 变化的情况，任何 QA 方法的通过率都相同。
