Schopfer Mathieu, Bochud François O, Bourhis Jean, Moeckli Raphaël
Institute of Radiation Physics, Lausanne University Hospital and University of Lausanne, Lausanne, Switzerland.
Radiation-Oncology Department, Lausanne University Hospital and University of Lausanne, Lausanne, Switzerland.
Med Phys. 2020 Sep;47(9):3845-3851. doi: 10.1002/mp.14348. Epub 2020 Jul 18.
To validate a delivery quality assurance (DQA) protocol for tomotherapy based on the measurement of the leaf open times (LOTs). In addition, to show the correlation between the mean relative LOT discrepancy and the dose deviation in the planning target volume (PTV).
We used a LOT measurement algorithm presented in a previous work on our two tomotherapy treatment units (TOMO1 and TOMO2). We generated TomoPhant plans with intentional random LOT discrepancies following Gaussian distributions of -6%, -4%, -2%, 2%, 4%, and 6%. We irradiated them on the Cheese Phantom with two ion chambers and collected the raw data on both our treatment units. Using the raw data, we measured the actual LOTs and verified that the induced discrepancies were highlightable. Then, we calculated the actual dose using Accuray's standalone dose calculator and verified that the calculated dose agreed with the ion chamber measurement. We randomly chose 60 clinical treatment plans, delivered them in air, and collected the raw detector data. We measured the actual LOTs from the raw data and calculated the corresponding dose distributions using Accuray's standalone dose calculator. We assessed the Pearson coefficient correlation of the deviation between expected and actual dose in the PTV (a) with the mean relative LOT discrepancy and (b) with the γ-index pass rate for different tolerances.
The mean relative discrepancy between actual (measured by the algorithm) and expected LOTs on the modified TomoPhant plans was 1.10 ± 0.05% on TOMO1 and 0.02 ± 0.03% on TOMO2, respectively. The agreement between measured and calculated dose was 0.2 ± 0.3% on TOMO1 and 0.1 ± 0.3% on TOMO2, respectively. On clinical plans, the means of the relative LOT discrepancies ranged from -3.0 % to 1.4%. The dose deviation in the PTVs ranged from -1.6% to 2.4%. The Pearson coefficient correlation between the mean relative LOT discrepancy and the dose deviation in the PTV was 0.76 (P ≈ 10 ) on TOMO1 and 0.65 (P ≈ 10 ) on TOMO2, respectively. There was no correlation between the γ-index pass rate and the dose deviation in the PTV.
The method made it possible to measure and to correctly highlight the LOT discrepancies on the TomoPhant plans. The dose subsequently calculated was accurate. On clinical plans, the mean LOT discrepancy correlated with the dose deviation in the PTV. This makes the mean LOT discrepancy a handy indicator of the plan quality.
基于叶片开启时间（LOTs）的测量来验证螺旋断层放射治疗的输出质量保证（DQA）方案。此外，展示计划靶区（PTV）中平均相对LOT差异与剂量偏差之间的相关性。
我们使用了先前在我们的两台螺旋断层放射治疗设备（TOMO1和TOMO2）上的一项工作中提出的LOT测量算法。我们生成了具有遵循-6%、-4%、-2%、2%、4%和6%高斯分布的故意随机LOT差异的螺旋断层模体计划。我们使用两个电离室在奶酪模体上对它们进行照射，并在我们的两台治疗设备上收集原始数据。使用原始数据，我们测量了实际的LOTs，并验证了诱导的差异是可突出显示的。然后，我们使用医科达的独立剂量计算器计算实际剂量，并验证计算出的剂量与电离室测量结果一致。我们随机选择60个临床治疗计划，在空气中进行照射，并收集原始探测器数据。我们从原始数据中测量实际的LOTs，并使用医科达的独立剂量计算器计算相应的剂量分布。我们评估了PTV中预期剂量与实际剂量之间偏差的皮尔逊系数相关性：（a）与平均相对LOT差异的相关性，以及（b）与不同容差下的γ指数通过率的相关性。
在修改后的螺旋断层模体计划上，实际（通过算法测量）与预期LOTs之间的平均相对差异在TOMO1上为1.10±0.05%，在TOMO2上为0.02±0.03%。测量剂量与计算剂量之间的一致性在TOMO1上为0.2±0.3%，在TOMO2上为0.1±0.3%。在临床计划中，相对LOT差异的平均值范围为-3.0%至1.4%。PTV中的剂量偏差范围为-1.6%至2.4%。PTV中平均相对LOT差异与剂量偏差之间的皮尔逊系数相关性在TOMO1上为0.76（P≈10），在TOMO2上为0.65（P≈10）。γ指数通过率与PTV中的剂量偏差之间没有相关性。
该方法能够测量并正确突出螺旋断层模体计划上的LOT差异。随后计算出的剂量是准确的。在临床计划中，平均LOT差异与PTV中的剂量偏差相关。这使得平均LOT差异成为计划质量的一个便捷指标。
