Department of Radiation Oncology and Image-applied Therapy, Kyoto University, Kyoto, Japan.
Med Phys. 2013 Feb;40(2):021701. doi: 10.1118/1.4773033.
This study aims to evaluate the interfractional dose variations in the organs-at-risk (OARs) during pancreatic breathhold intensity-modulated radiotherapy (IMRT) and to assess the impacts of "planning organs-at-risk volume" (POV) structures generated by isotropically expanding the dose-limiting OARs, based on the comparison of the interfractional doses to the OARs between IMRT plans and conventional three-dimensional-conformal radiotherapy (3D-CRT) plans.
Thirty repeat CT scans were acquired from ten consecutive patients who were receiving chemoradiotherapy for pancreatic cancer. Six IMRT plans for each patient with two levels of prescription (45 and 51 Gy in 15 fractions) and 3 POV margin sizes (5, 7, and 10 mm) were generated based on the initial CT scan under predetermined constraints. Two 3D-CRT plans (39 and 42 Gy in 15 fractions) were simultaneously generated. The dose distribution of all of the treatment plans was recalculated with the repeat CT scans. The interfractional dose variations in the three OARs (stomach, duodenum, and small intestine) were evaluated, and the absolute volumes ≥39 Gy (V39Gy) of the OARs in the IMRT plans were compared to those in the 3D-CRT plans. Regression analyses were performed to assess the relative impact of the factors of interest on the interfractional dose variations of the OARs.
Substantial dose excesses to the three OARs were observed at all of the prescription dose levels and the POV margin sizes on the repeat CT scans. The safety threshold based on the mean stomach V39Gy on the recalculated 39 Gy-3D-CRT plans was 1.9 ml. Statistically significant and marginally insignificant mean V39Gy values above the safety thresholds were observed in the stomach in the 51 Gy-IMRT plans (2.6 and 2.1 ml with the 5- and 7-mm PRV margins, respectively (P = 0.015 and 0.085)). Only in the case of the 10-mm POV margin did the metric fall below the safety threshold to 1.5 ml (P = 0.634). The duodenum and the small intestine did not violate the safety thresholds (1.4 and 3.8 ml, respectively). From the multiple regression analyses, only the margin size (P < 0.001) and the POV V39Gy (P < 0.001) were significantly associated with the distribution of recalculated V39Gy for the stomach. Multiple factors, including the margin size (P = 0.020) and the POV V39Gy (P < 0.001) were associated with the recalculated V39Gy for the duodenum. However, none of the POV parameters for the small intestine were associated with the recalculated V39Gy.
Considerable interfractional dose variation was observed in three critical OARs. At the escalated prescription dose of breathhold IMRT, the dose variations could exceed the dose variations using 3D-CRT at the safe prescription dose level, indicating that a dose-escalation strategy based solely on the initial advantageous dose distribution in a breathhold IMRT can be problematic. Given the current limitations for predicting or coping with variation throughout the treatment course, the use of POV should be considered for safely delivering escalated doses to patients with pancreatic cancer.
本研究旨在评估在胰腺屏气调强放疗（IMRT）过程中危及器官（OAR）的分次剂量变化，并评估基于等比扩大剂量限制 OAR 的“计划危及器官体积”（POV）结构，根据 OAR 与 IMRT 计划和传统三维适形放疗（3D-CRT）计划之间的分次剂量比较，评估对 OAR 的影响。
连续十例接受胰腺癌放化疗的患者，共采集 30 次重复 CT 扫描。基于预定约束，为每位患者生成 6 个 IMRT 计划，每个计划有两个处方剂量（15 个分次 45 和 51 Gy）和 3 个 POV 边界大小（5、7 和 10 mm）。同时生成 2 个 3D-CRT 计划（15 个分次 39 和 42 Gy）。使用重复 CT 扫描重新计算所有治疗计划的剂量分布。评估三个 OAR（胃、十二指肠和小肠）的分次剂量变化，并比较 IMRT 计划和 3D-CRT 计划中 OAR 的绝对体积≥39 Gy（V39Gy）。进行回归分析，以评估感兴趣因素对 OAR 分次剂量变化的相对影响。
在重复 CT 扫描的所有处方剂量水平和 POV 边界大小上，三个 OAR 均观察到明显的剂量过剩。基于重新计算的 39 Gy-3D-CRT 计划中胃的平均 V39Gy 的安全阈值为 1.9 ml。在 51 Gy-IMRT 计划中，胃的 V39Gy 存在显著且边际不显著的均值超过安全阈值（分别为 2.6 和 2.1 ml，使用 5 和 7-mm PRV 边界（P = 0.015 和 0.085））。仅在 10-mm POV 边界的情况下，该指标降至 1.5 ml（P = 0.634）以下。十二指肠和小肠均未超过安全阈值（分别为 1.4 和 3.8 ml）。从多元回归分析来看，只有边界大小（P < 0.001）和 POV V39Gy（P < 0.001）与胃的重新计算 V39Gy 分布显著相关。多种因素，包括边界大小（P = 0.020）和 POV V39Gy（P < 0.001），与十二指肠的重新计算 V39Gy 相关。然而，小肠的 POV 参数均与重新计算的 V39Gy 无关。
在三个关键 OAR 中观察到相当大的分次剂量变化。在屏气 IMRT 的递增处方剂量下，剂量变化可能超过在安全处方剂量水平下使用 3D-CRT 的剂量变化，这表明仅基于屏气 IMRT 中初始有利剂量分布的剂量递增策略可能存在问题。鉴于目前对治疗过程中变化的预测或应对的局限性，对于接受胰腺癌治疗的患者，应考虑使用 POV 以安全地给予递增剂量。
