Phys Med Biol. 2013 Nov 21;58(22):8179-96. doi: 10.1088/0031-9155/58/22/8179.
Many tumor cells demonstrate hyperradiosensitivity at doses below 50 cGy. Together with the increased normal tissue repair under low dose rate, the pulsed low dose rate radiotherapy (PLDR), which separates a daily fractional dose of 200 cGy into 10 pulses with 3 min interval between pulses (20 cGy/pulse and effective dose rate 6.7 cGy min−1), potentially reduces late normal tissue toxicity while still providing significant tumor control for re-irradiation treatments. This work investigates the dosimetric and technical feasibilities of intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT)-based PLDR treatments using Varian Linacs. Twenty one cases (12 real re-irradiation cases) including treatment sites of pancreas, prostate, pelvis, lung, head-and-neck, and breast were recruited for this study. The lowest machine operation dose rate (100 MU min−1) was employed in the plan delivery. Ten-field step-and-shoot IMRT and dual-arc VMAT plans were generated using the Eclipse TPS with routine planning strategies. The dual-arc plans were delivered five times to achieve a 200 cGy daily dose (~20 cGy arc−1). The resulting plan quality was evaluated according to the heterogeneity and conformity indexes (HI and CI) of the planning target volume (PTV). The dosimetric feasibility of retaining the hyperradiosensitivity for PLDR was assessed based on the minimum and maximum dose in the target volume from each pulse. The delivery accuracy of VMAT and IMRT at the 100 MU min−1 machine operation dose rate was verified using a 2D diode array and ion chamber measurements. The delivery reproducibility was further investigated by analyzing the Dynalog files of repeated deliveries. A comparable plan quality was achieved by the IMRT (CI 1.10–1.38; HI 1.04–1.10) and the VMAT (CI 1.08–1.26; HI 1.05–1.10) techniques. The minimum/maximum PTV dose per pulse is 7.9 ± 5.1 cGy/33.7 ± 6.9 cGy for the IMRT and 12.3 ± 4.1 cGy/29.2 ± 4.7 cGy for the VMAT. Six out of the 186 IMRT pulses (fields) were found to exceed 50 cGy maximum PTV dose per pulse while the maximum PTV dose per pulse was within 40 cGy for all the VMAT pulses (arcs). However, for VMAT plans, the dosimetric quality of the entire treatment plan was less superior for the breast cases and large irregular targets. The gamma passing rates for both techniques at the 100 MU min−1 dose rate were at least 94.1% (3%/3 mm) and the point dose measurements agreed with the planned values to within 2.2%. The average root mean square error of the leaf position was 0.93 ± 0.83 mm for IMRT and 0.53 ± 0.48 mm for VMAT based on the Dynalog file analysis. The RMS error of the leaf position was nearly identical for the repeated deliveries of the same plans. In general, both techniques are feasible for PLDR treatments. VMAT was more advantageous for PLDR with more uniform target dose per pulse, especially for centrally located tumors. However, for large, irregular and/or peripheral tumors, IMRT could produce more favorable PLDR plans. By taking the biological benefit of PLDR delivery and the dosimetric benefit of IMRT and VMAT, the proposed methods have a great potential for those previously-irradiated recurrent patients.
许多肿瘤细胞在低于50 cGy 的剂量下表现出超敏感性。加上低剂量率下正常组织修复增加,脉冲低剂量率放疗(PLDR),即将每日 200 cGy 的分次剂量分为 10 个脉冲,每个脉冲之间间隔 3 分钟(20 cGy/脉冲,有效剂量率为 6.7 cGy min−1),可以降低晚期正常组织毒性,同时为再照射治疗提供显著的肿瘤控制。这项工作研究了使用瓦里安直线加速器进行强度调制放疗(IMRT)和容积调制弧形治疗(VMAT)的 PLDR 治疗的剂量学和技术可行性。招募了 21 例(12 例为真实再照射病例)包括胰腺、前列腺、骨盆、肺、头颈部和乳房的治疗部位。在计划交付中采用最低机器运行剂量率(100 MU min−1)。使用 Eclipse TPS 生成了十个场分步射击 IMRT 和双弧 VMAT 计划,并采用常规规划策略。双弧计划分五次进行,以实现每日 200 cGy 剂量(~20 cGy 弧−1)。根据计划靶区(PTV)的不均匀性和一致性指数(HI 和 CI)评估计划质量。根据每个脉冲靶区的最小和最大剂量评估保留 PLDR 超敏感性的剂量学可行性。使用二维二极管阵列和离子室测量验证了 VMAT 和 IMRT 在 100 MU min−1 机器运行剂量率下的输送精度。通过分析重复输送的 Dynalog 文件进一步研究了输送的可重复性。IMRT(CI 1.10–1.38;HI 1.04–1.10)和 VMAT(CI 1.08–1.26;HI 1.05–1.10)技术可实现可比的计划质量。IMRT 的最小/最大 PTV 剂量为每个脉冲 7.9 ± 5.1 cGy/33.7 ± 6.9 cGy,VMAT 为 12.3 ± 4.1 cGy/29.2 ± 4.7 cGy。在 186 个 IMRT 脉冲(场)中,发现有 6 个超过每个脉冲最大 PTV 剂量 50 cGy,而所有 VMAT 脉冲(弧)的最大 PTV 剂量都在每个脉冲 40 cGy 以内。然而,对于 VMAT 计划,对于乳房病例和大不规则靶区,整个治疗计划的剂量学质量较差。两种技术在 100 MU min−1 剂量率下的伽马通过率均至少为 94.1%(3%/3 mm),点剂量测量与计划值的差值在 2.2%以内。基于 Dynalog 文件分析,IMRT 的叶位置均方根误差为 0.93 ± 0.83 mm,VMAT 的叶位置均方根误差为 0.53 ± 0.48 mm。相同计划的重复输送的叶位置 RMS 误差几乎相同。一般来说,两种技术都适用于 PLDR 治疗。VMAT 对于每个脉冲更均匀的靶区剂量更有利,特别是对于位于中央的肿瘤。然而,对于大、不规则和/或外周肿瘤,IMRT 可以产生更有利的 PLDR 计划。通过利用 PLDR 输送的生物学益处和 IMRT 和 VMAT 的剂量学益处,所提出的方法对于那些先前接受过照射的复发患者具有很大的潜力。