较长靶区放疗计划的两种剂量验证方法研究

Preliminary Study on Two Dose Verification Methods for Long Target Radiotherapy Plan

目的利用剂量拼接(Dose Composing,DC)和改变源到探测器距离(Alter Distance,AD)验证较长靶区放疗计划的剂量分布。方法选取18例靶区长度>26 cm的放疗计划,并借助50 cm×30 cm×30 cm虚拟水模体分别计算源到探测器的距离(Source to Detector Distance,SDD)为70 cm和100 cm时机架角度归零的验证计划。利用PTW OCTAVIUS1500二维矩阵在Elekta Axesse加速器实测两种方式下的剂量分布。DC方式需要按特定程序测量两次并进行DC,即先向靶端平移5 mm的倍数距离测量1次,然后180°旋转矩阵并向枪端平移相同距离再测量1次;AD方式仅需通过升床将探测器高度置于计划层面测量1次。比较两种验证方法不同标准下的γ通过率、测量点数量、验证覆盖度和测量耗时。结果两种剂量验证方法的γ通过率均随参考标准的降低而提高,差异有统计学意义(P<0.05),其中2 mm/2%和2 mm/3%时,DC组和AD组γ通过率差异无统计学意义(P>0.05);3 mm/3%时,DC组γ通过率为低于AD组,差异有统计学意义(P<0.05)。AD组参与γ分析的测量点低于DC组[(469.8±111.2)vs.(1819.4±428.1)]。两组的验证覆盖率比较差异无统计学意义(P>0.05)。DC组测量时间约为AD组的3倍。结论两种方法均可实现较长靶区放疗计划的剂量学验证,其中DC方法测量点更多,验证覆盖范围更长,但测量耗时较长;AD方法随SDD的变化验证覆盖范围有一定的限值,但操作简单、验证效率更高。研究结果为放疗计划剂量学质量保证提供了一定的参考。

Objective To verify dose distribution of radiotherapy plans in longer target areas by using dose composing(DC)and alter distance(AD)from source to detector.Methods Eighteen radiotherapy plans with target area length>26 cm were selected,and the verification plans of source to detector distance(SDD)being 70 cm and 100 cm were calculated by means of 50 cm×30 cm×30 cm virtual water model,respectively.The dose distribution was measured by the Elekta Axesse accelerator using PTW OCTAVIUS1500 two-dimensional matrix.DC method required two measurements and dose splicing according to specific procedures.Firstly,multiple distance measurement of 5 mm was carried out to the target end for one time,and then the matrix was rotated 180°and translated to the gun end for the same distance for another measurement.In AD mode,the detector height only needed to be measured at the planned level once by lifting the bed.Theγpass rate,number of measurement points,verification coverage and measurement time of the two verification methods under different standards were compared.Results Theγpassing rate of two DQA methods increased with the decrease of the criterion,and the difference was statistically significant(P<0.05).At 2 mm/2%and 2 mm/3%,theγpassing rates of the DC group and AD group were not statistically significant(P>0.05).At 3 mm/3%,γpassing rate in DC group was lower than that in AD group,and the difference was statistically significant(P<0.05).The measurement points ofγanalysis in AD group were lower than those in DC group[(469.8±111.2)vs.(1819.4±428.1)].There was no significant difference in verification coverage between the two groups(P>0.05).The measurement time of the DC group was about three times that of the AD group.Conclusion Both methods can achieve dose verification of long-target radiotherapy plan.The DC method has more measurement points and longer coverage,but it takes more time.The AD method has a certain limit on the measurement coverage,but it is simpler to operate and more efficient.The results provide a reference for the dosimetric quality assurance of radiotherapy plans.