Department of Biomedical Engineering and Environmental Sciences, National Tsing Hua University, Hsinchu, Taiwan, Republic of China.
Department of Medical Imaging and Radiological Sciences, Chung Shan Medical University, Taichung, Taiwan, Republic of China.
PLoS One. 2021 Apr 23;16(4):e0250243. doi: 10.1371/journal.pone.0250243. eCollection 2021.
We measured the overshoot effect in a linac and reduced it using block correction, reverse-sequence correction, and index correction. A StarTrack detector was used on a Varian iX. Five segments, 1 × 10 cm2 in area, were designed; the centers were at -4, -2, 0, 2, and 4 cm on the x axis for measuring the overshoot effect on a 10 × 10 cm2 collimator setting. Block correction was applied to two segments. The first was on the new first segment at -6 cm, and the other was on the new last segment at 6 cm. Both two new segments were obtained from the 10 × 10 cm2 collimator setting. The order of segments was reversed in reverse-sequence correction. Reverse-sequence correction averages the dose at every segment after two irradiations. When we used MLC Shaper, index correction reduced the first segment's index (cumulative radiation occupation) by 60% and increased the last segment's radiation occupation by 60% in a new MLC.log file. As for relative dose, the first segment had an overdose of 52.4% and the last segment had an underdose of 48.6%, when irradiated at 1 MU at 600 MU/min. The relative doses at the first segment, irradiated at 1 MU, after block correction, reverse-sequence correction, and index correction were applied decreased from 152.5% to 95.1%, 104.8%, and 100.1%, respectively. The relative doses at the last segment, irradiated at 1 MU, after block correction, reverse-sequence correction, and index correction were applied increased from 48.6% to 97.3%, 91.1%, and 95.9%, respectively. The overshoot effect depended on the speed of irradiation. High irradiation speeds resulted in notable overdosing and underdosing at the first and last segments, respectively. The three corrections mitigated the overshoot effect on dose. To save time and effort, the MLC.log file should be edited with a program in the future.
我们在直线加速器中测量了超调效应，并使用块校正、逆序校正和指数校正来降低超调效应。在瓦里安 iX 上使用了 StarTrack 探测器。设计了五个区域为 1×10cm2 的段，中心点位于 x 轴上的-4、-2、0、2 和 4cm，用于测量 10×10cm2 准直器设置下的超调效应。对两个段进行了块校正。第一个在新的第一段上，位于-6cm，另一个在新的最后一段上，位于 6cm。这两个新段都来自 10×10cm2 准直器设置。在逆序校正中，段的顺序颠倒。逆序校正在两次照射后平均每个段的剂量。当我们使用 MLC Shaper 时，指数校正会在新的 MLC.log 文件中使第一段的指数（累积辐射占用）减少 60%，使最后一段的辐射占用增加 60%。对于相对剂量，当以 600MU/min 的速度以 1MU 照射时，第一段的剂量过高，为 52.4%，最后一段的剂量过低，为 48.6%。在应用块校正、逆序校正和指数校正后，以 1MU 照射的第一段的相对剂量分别从 152.5%降至 95.1%、104.8%和 100.1%。以 1MU 照射的最后一段的相对剂量分别从 48.6%增加到 97.3%、91.1%和 95.9%。超调效应取决于照射速度。高照射速度会导致第一段和最后一段的剂量明显过高和过低。三种校正方法都减轻了剂量的超调效应。为了节省时间和精力，将来应该使用程序编辑 MLC.log 文件。
