New index for quantitative comparison of dose distributions in radiotherapy

Delivery of intensity modulated radiotherapy (IMRT) requires patient specific quality assurance (QA) tests. These tests normally include a two-fold comparison of dose distributions: (a) comparison of planned dose created in the employed treatment planning system (TPS) and dose computed by using third-party software; (b) comparison of planned (predicted) and delivered doses.

We describe a new approach to compare dose distributions in radiotherapy.

The suggested approach employs a novel dose congruency index (DCI) to evaluate differences between compared dose distributions. The new index is dependent on dose and dose increment ( $\delta D$ ). The "pass" and "fail" criteria for dose congruency assessment are given by the conditions $DCI\le 1$ and $DCI>1$ , respectively. The employment of DCI was demonstrated by comparing 3D dose distirbutions in stereotactic body radiation therapy (SBRT) and stereotactic intracranial radiosurgery (SRS). The DCI was also computed for predicted and measured portal doses.

For the compared SBRT dose distributions, the DCI computed with $\delta D$  = 1%, 1.5%, and 2% of the prescribed dose resulted in passing rates (i.e., relative number of dose levels with $DCI\le 1$ ) of 34.1%, 61.5%, and 100%, respectively. For the compared SRS dose distributions, the DCI with $\delta D$  = 1%, 1.5%, and 2% of the prescribed dose had passing rates of 25.3%, 33%, and 41.8%, respectively. For the compared 2D distributions, the DCI passing rates averaged over the two arcs employed in the studied case, were as follows: 14.5% for $\delta D$  = 1%, 48.8% for $\delta D$  = 1.5%, and 85.5% for $\delta D$  = 2%.

It is demonstrated that the suggested index can be used to compare 2D and 3D dose distributions in radiotherapy.