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Volume 20, Number 2, 2015

How to compare treatment plans? Personalized perspective

Tomasz Piotrowski, Agata Jodda


Recently Kumar et al.1 presented a comparative study of two different volumetric modulated arc therapy delivery techniques. The growing numbers of new planning algorithms, delivery methods, and technologies in the recent years has caused the number of published planning comparisons to increase almost exponentially. Therefore, discussion on how to find the best way of comparing treatment plans is quite urgent.

We fully agree with the authors of the commented study that simple planning comparisons are susceptible to bias. However, even a simple comparison should be made according to clearly defined criteria, which allows to compare treatment plans prepared for two equivalent techniques implemented on two different systems (e.g. VMAT on Varian and Elekta). Clearly defined criteria enable standardization of the obtained results. Unfortunately, the authors of the commented study do not provide elementary information about normalization of doses for the two equivalent VMAT plans created on the Eclipse and Monaco treatment planning systems nor any information about the constraints of the doses to organs at risk and to the tumor that were used during the optimization process. We agree with the authors that technological differences between two different solutions, such as Varian and Elekta, could pose a problem in data interpretation, but these differences can also be minimized. For example, according to the paper published by Knoos et al.,2 the algorithm-specific differences of dose distribution could be ignored if the compared algorithms include similar methods of accounting for changes in electron transport. In the commented paper, algorithms are equivalent and, therefore, do not bias the comparisons. Unfortunately, another important aspect concentrated on the arcs used for plan preparation was not deeply discussed in the commented study. Generally, if the comparisons are performed, operator/planner dependant degrees of freedom should be minimized. The comparison between two-arc Varian VMAT and one-arc Elekta VMAT is controversial. Even if the two-arc technique (Varian VMAT) is an accepted standard of care, the results of one-arc VMAT Varian technique should be added. The reason is that the two-arc VMAT technique doubles the number of potential beamlets used during the optimization process as compared to the one-arc VMAT technique. Finally, the comparisons of parameters that describe dose distribution and parameters that describe dose delivery (e.g. estimated times or monitor units) should be carefully checked by appropriate statistical tests, which should be clearly reported in the study. Referring to the commented study, we would like to note that selecting the t-Student test is not always a good idea. Sometimes, a wrong selection of a statistical test leads to misinterpretation of results. An appropriate selection of statistical tests depends on many independent factors, among which the most important are: (i) dependence or independence of the compared groups; (ii) number of compared groups and observations made for each group; (iii) normality or non-normality of the distribution and (iv) information of the statistical dispersion of the observations made for the compared groups. 3

Signature: Rep Pract Oncol Radiother, 2015; 20(2) : 77-78

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