Fernandez M Castrillon, Venencia C, Garrigó E, Caussa L
Instituto de Radioterapia - Fundacion Marie Curie, Cordoba, Argentina.
Med Phys. 2012 Jun;39(6Part17):3809. doi: 10.1118/1.4735543.
To compare measured and calculated doses using Pencil Beam (PB) and Monte Carlo (MC) algorithm on a CIRS thorax phantom for SBRT lung treatments.
A 6MV photon beam generated by a Primus linac with an Optifocus MLC (Siemens) was used. Dose calculation was done using iPlan v4.1.2 TPS (BrainLAB) by PB and MC (dose to water and dose to medium) algorithms. The commissioning of both algorithms was done reproducing experimental measurements in water. A CIRS thorax phantom was used to compare doses using a Farmer type ion chamber (PTW) and EDR2 radiographic films (KODAK). The ionization chamber, into a tissue equivalent insert, was placed in two position of lung tissue and was irradiated using three treatments plans. Axial dose distributions were measured for four treatments plans using conformal and IMRT technique. Dose distribution comparisons were done by dose profiles and gamma index (3%/3mm).
For the studied beam configurations, ion chamber measurements shows that PB overestimate the dose up to 8.5%, whereas MC has a maximum variation of 1.6%. Dosimetric analysis using dose profiles shows that PB overestimates the dose in the region corresponding to the lung up to 16%. For axial dose distribution comparison the percentage of pixels with gamma index bigger than one for MC and PB was, plan 1: 95.6% versus 87.4%, plan 2: 91.2% versus 77.6%, plan 3: 99.7% versus 93.1% and for plan 4: 98.8% versus 91.7%. It was confirmed that the lower dosimetric errors calculated applying MC algorithm appears when the spatial resolution and variance decrease at the expense of increased computation time.
The agreement between measured and calculated doses, in a phantom with lung heterogeneities, is better with MC algorithm. PB algorithm overestimates the doses in lung tissue, which could have a clinical impact in SBRT lung treatments.
在用于立体定向体部放疗（SBRT）肺部治疗的CIRS胸部体模上，使用笔形束（PB）和蒙特卡罗（MC）算法比较测量剂量和计算剂量。
使用由配备Optifocus多叶准直器（西门子）的Primus直线加速器产生的6MV光子束。剂量计算通过iPlan v4.1.2治疗计划系统（BrainLAB）使用PB和MC（水剂量和介质剂量）算法完成。两种算法的调试均通过在水中重现实验测量来进行。使用CIRS胸部体模，通过Farmer型电离室（PTW）和EDR2射线照相胶片（柯达）比较剂量。置于组织等效插入物中的电离室放置在肺组织的两个位置，并使用三个治疗计划进行照射。使用适形和调强放疗（IMRT）技术对四个治疗计划测量轴向剂量分布。通过剂量剖面和伽马指数（3%/3mm）进行剂量分布比较。
对于所研究的射束配置，电离室测量表明PB高估剂量高达8.5%，而MC的最大变化为1.6%。使用剂量剖面的剂量学分析表明，PB在对应于肺部的区域高估剂量高达16%。对于轴向剂量分布比较，MC和PB的伽马指数大于1的像素百分比，计划1：95.6%对87.4%，计划2：91.2%对77.6%，计划3：99.7%对93.1%，计划4：98.8%对9l.7%。已证实，当以增加计算时间为代价降低空间分辨率和方差时，应用MC算法计算的剂量学误差更低。
在具有肺部异质性的体模中，MC算法在测量剂量和计算剂量之间的一致性更好。PB算法高估了肺组织中的剂量，这可能对SBRT肺部治疗产生临床影响。
