Zhuang T, Djemil T, Qi P, Magnelli A, Stephans K, Videtic G, Xia P
The Cleveland Clinic Foundation, Cleveland, OH.
Med Phys. 2012 Jun;39(6Part17):3817. doi: 10.1118/1.4735575.
Stereotactic body radiotherapy has been an efficacious treatment modality for early stage non-small cell lung cancer. The accuracy of dose calculations is in question due to the presence of inhomogeneity. It was required in several clinical trials to calculate dose without heterogeneity correction. However, to better correlate the outcomes with the planned dose, accurate dose calculation with heterogeneity correction is highly desirable.
We compared the recalculated dose with Monte Carlo (MC) algorithm to the original Pencil Beam (PB) calculations for clinical lung SBRT plans. Thirty-one clinical plans that followed protocol guidelines were retrospectively investigated. Dosimetric parameters D1, D95 and D99 for the PTV and D1 for organs at risk were compared. Correlations of mean lung dose and V20 of lungs between two calculations were investigated.
Compared to the PB calculations without heterogeneity correction in clinical plans, we found that in terms of D95 of PTV, (1) the two calculations resulted in similar D95 for edge tumors with volumes greater than 25.1cc; (2) an average overestimation of 5% in PB calculations for edge tumors with volumes less than 25.1cc; and (3) an average overestimation of 9% or underestimation of 3% in PB calculations for island tumors with volumes smaller or greater than 22.6 cc, respectively. With heterogeneity correction, the PB calculation resulted in an average reduction of 23.8% and 15.3% in D95 for island and edge lesions respectively compared to the MC calculation. For organs at risks, no clinical meaningful differences were found among all the comparisons. Excellent correlations for mean dose and V20 of lungs were observed between the two calculations.
Using a single scaling factor to account for the differences in using heterogeneity correction may not be sufficient. To understand dose-response relation in Lung SBRT, accurate dose calculation such as the Monte Carlo algorithms is highly recommended.
立体定向体部放射治疗一直是早期非小细胞肺癌的一种有效治疗方式。由于存在不均匀性，剂量计算的准确性受到质疑。在一些临床试验中，要求在不进行不均匀性校正的情况下计算剂量。然而，为了使结果与计划剂量更好地相关联，非常需要进行准确的不均匀性校正剂量计算。
我们将采用蒙特卡罗（MC）算法重新计算的剂量与临床肺部立体定向体部放射治疗（SBRT）计划的原始笔形束（PB）计算结果进行了比较。回顾性研究了31个遵循方案指南的临床计划。比较了靶区的剂量学参数D1、D95和D99以及危及器官的D1。研究了两种计算之间肺平均剂量和肺V20的相关性。
与临床计划中未进行不均匀性校正的PB计算相比，我们发现，就靶区的D95而言，（1）对于体积大于25.1cc的边缘肿瘤，两种计算得出的D95相似；（2）对于体积小于25.1cc的边缘肿瘤，PB计算平均高估5%；（3）对于体积分别小于或大于22.6cc的岛状肿瘤，PB计算分别平均高估9%或低估3%。进行不均匀性校正后，与MC计算相比，PB计算得出的岛状和边缘病变的D95分别平均降低23.8%和15.3%。对于危及器官，所有比较均未发现具有临床意义的差异。两种计算之间观察到肺平均剂量和肺V20具有良好的相关性。
使用单一比例因子来解释使用不均匀性校正的差异可能是不够的。为了了解肺部SBRT中的剂量反应关系，强烈建议使用准确的剂量计算方法，如蒙特卡罗算法。
