Liu D, Chen D, Chetvertkov M, Altman M, Li H, Wen N, Glide-Hurst C, Ajlouni M, Levin K, Movsas B, Chetty I
Henry Ford Health System, Detroit, Michigan.
Wayne State University, Detroit, Michigan.
Med Phys. 2012 Jun;39(6Part17):3817. doi: 10.1118/1.4735576.
We hypothesize that PTV margin dose is an important factor for local tumor control. We evaluated dose distributions for patients originally treated with pencil-beam (PB)-based plans and retrospectively calculated with Monte Carlo (MC) method, with emphasis on the spatial region between the ITV and PTV (PTV-margin), where the largest dose differences were expected.
Forty-six stage I-II lung cancer patients with 51 lesions treated with SABR were retrospectively analyzed (23 central and 28 peripheral tumors). All patients received 4DCT imaging, and an ITV was generated from the maximum intensity projection and subsequent review of four 4DCT phases. An isotropic 3mm ITV-to-PTV margin was used. The iPlan TPS was used to generate the original treatment plans using PB-based heterogeneity correction. MC doses were recalculated using the same MUs as in the PB plan. Dose distributions for the ITV, PTV-margin, and PTV were analyzed using generalized equivalent uniform dose (gEUD) with a = - 20. Student's paired t-test elucidated differences between PB and MC-based gEUD and the two different tumor locations.
Mean ITV and PTV volumes were 24.2 cc (range: 2.2 to 99.3 cc) and 50.4 cc (range: 6.4 to 229.7 cc), respectively. The mean gEUDs of ITV, PTV-margin and PTV, normalized to PB-based 100% isodose were 1.02+/-0.04, 1.01+/-0.04 and 1.01+/-0.04 for PB-based plans, compared to 0.94+/-0.06, 0.88+/-0.08 and 0.90+/-0.08 (all p<0.05) for MC-based plans. The maximum overestimations with the PB algorithm in the PTV-margin average dose were 10.4% and 19.6% (p < 0.05) for peripheral tumor cases and central tumor cases, respectively.
PB-based dose distributions showed the highest dose overestimation (relative to MC) in the PTV-margin spatial region. Analysis of spatial dose differences is an important precursor toward assessment of patterns-of-local failure, to be investigated in future work to explore possible association between dose and regions of failure. Acknowledgement: supported in part by grants from NIH R01 CA106770 and from Varian Medical Systems.
我们假设计划靶体积（PTV）边缘剂量是局部肿瘤控制的一个重要因素。我们评估了最初采用基于笔形束（PB）计划治疗的患者的剂量分布，并采用蒙特卡罗（MC）方法进行回顾性计算，重点关注内部靶体积（ITV）和PTV之间的空间区域（PTV边缘），预计该区域存在最大剂量差异。
对46例接受立体定向消融放疗（SABR）治疗的I-II期肺癌患者的51个病灶进行回顾性分析（23个中央肿瘤和28个周围肿瘤）。所有患者均接受4DCT成像，并通过最大强度投影以及随后对四个4DCT时相的复查生成ITV。采用各向同性3mm的ITV到PTV边缘。使用iPlan治疗计划系统（TPS），利用基于PB的不均匀性校正生成原始治疗计划。使用与PB计划相同的机器跳数（MU）重新计算MC剂量。使用α=-20的广义等效均匀剂量（gEUD）分析ITV、PTV边缘和PTV的剂量分布。采用学生配对t检验阐明基于PB和基于MC的gEUD之间以及两种不同肿瘤位置之间的差异。
ITV和PTV的平均体积分别为24.2立方厘米（范围：2.2至99.3立方厘米）和50.4立方厘米（范围：6.4至229.7立方厘米）。基于PB计划，ITV、PTV边缘和PTV的平均gEUD（归一化为基于PB的100%等剂量线）分别为1.02±0.04、1.01±0.04和1.01±0.04，而基于MC计划的分别为0.94±0.06、0.88±0.08和0.90±0.08（所有p<0.05）。对于周围肿瘤病例和中央肿瘤病例，PB算法在PTV边缘平均剂量方面的最大高估分别为10.4%和19.6%（p<0.05）。
基于PB的剂量分布在PTV边缘空间区域显示出最高的剂量高估（相对于MC）。空间剂量差异分析是评估局部失败模式的重要前提，有待在未来工作中进行研究，以探索剂量与失败区域之间可能的关联。致谢：部分得到美国国立卫生研究院（NIH）R01 CA106770资助以及瓦里安医疗系统公司的资助。
