Department of Radiation Oncology, Wayne State University, Detroit, Michigan 48201, USA.
Med Phys. 2010 Jun;37(6):2457-65. doi: 10.1118/1.3425792.
To experimentally simulate IMRT delivery using two human cell models in vitro and test the hypothesis that a loss in effective dose resulting from the prolongation of megavoltage x-ray treatment delivery time would be greatly reduced in corresponding IMRT simulations using higher-LET radiation.
The effect of prolonging the delivery time of a treatment fraction was investigated in vitro using human PC-3 prostate and HGL21 glioblastoma tumor cell lines. Cells were irradiated with x rays from a conventional linear accelerator or with neutrons from a clinical d(48.5)+Be radiotherapy beam and maintained at 37 degrees C throughout. The delivery time for six closely spaced doses, simulating six multiple-port segments, was varied from acute to 60 min for x-ray irradiation, and acute to 120 min for neutron irradiation. Cell survival was measured following summed doses for the six segments of 0.5-6 Gy for x rays and 0.16-2 Gy for neutrons, covering the most likely range of dose per fraction used in clinical radiotherapy.
Prolonging x-ray delivery time (from initiation of segment 1 to initiation of segment 6) from 5 to 45 min resulted in a loss in effective total dose (in equivalent 2 Gy multifraction treatments) of 5.6% in the PC-3 cell line and 11.7% in the HGL21 cell line. More clinically common prolongations of 5-30 and 5-15 min resulted in effective dose reductions of 3.8% and 1.7% for PC-3, and 7.3% and 2.9% for HGL21. A loss of less than 0.5% in effective dose was observed for prolongations up to 45 min of similarly effective neutron irradiation of PC-3 and HGL21 cells.
Prolonged delivery times of photon fractions could have a significant impact on treatment outcome especially for tumors with a low alpha/beta ratio and short repair halftime. These effects are significant at delivery times commonly associated with IMRT and are variable with cell type. X-ray IMRT should therefore always be planned to minimize dose-fraction delivery time. However, if IMRT treatments are delivered with high-LET radiation, this considerably reduces the dependence of the biological effect on fraction delivery time even out to 2 h.
通过体外培养的两种人类细胞模型来模拟调强放射治疗（IMRT）的实施，并验证假设，即由于延长兆伏级 X 射线治疗的实施时间而导致的有效剂量损失，在使用更高传能线密度（LET）的放射治疗模拟中会大大减少。
使用人前列腺癌细胞系 PC-3 和胶质母细胞瘤细胞系 HGL21，在体外研究延长治疗剂量分割的实施时间的影响。细胞用常规线性加速器的 X 射线或临床 d（48.5）+Be 放射治疗束的中子照射，并在整个过程中保持在 37℃。6 个紧密间隔的剂量的实施时间从急性（照射开始至第 1 个分割结束）延长至 60 分钟，用于 X 射线照射，从急性延长至 120 分钟，用于中子照射。对于 X 射线的 0.5-6 Gy 和中子的 0.16-2 Gy 六个分段的总和剂量，测量细胞存活情况，涵盖临床放射治疗中最常见的剂量分割。
从第 1 个分割开始至第 6 个分割开始，将 X 射线的传输时间从 5 分钟延长至 45 分钟，导致 PC-3 细胞系的有效总剂量（相当于 2 Gy 多分割治疗）损失 5.6%，HGL21 细胞系损失 11.7%。在 PC-3 中，更常见的 5-30 分钟和 5-15 分钟的延长导致有效剂量减少 3.8%和 1.7%，在 HGL21 中，有效剂量减少 7.3%和 2.9%。对于 PC-3 和 HGL21 细胞的同样有效的中子照射，延长至 45 分钟，观察到有效剂量的损失小于 0.5%。
光子剂量分割的延长传输时间可能会对治疗结果产生重大影响，特别是对于α/β比值低和修复半衰期短的肿瘤。这些影响在与 IMRT 相关的常见传输时间中很显著，并且随细胞类型而异。因此，X 射线 IMRT 应始终计划将剂量分割的传输时间最小化。然而，如果使用高 LET 射线进行 IMRT 治疗，即使在 2 小时的时间内，这也会大大降低生物学效应对剂量分割传输时间的依赖性。
