Comparing dose distributions: DTA and dose-difference

Radiation therapy plan quality assurance often hinges on comparing calculated dose distributions with measured dose distributions. One of the most common techniques to compare dose distributions is the combined use of distance-to-agreement (DTA) and dose difference. In this post I will give an overview of these concepts.

A planar dose distribution from IMRT.

Dose difference is a very straight forward comparison of dose at corresponding points in two distributions. Given a point a_p in the planned distribution and the corresponding point a_m in the measured distribution, the dose difference is simply D(a_m) - D(a_p). A passing criterion is used, e.g. 3% of planned dose, such that if the measured dose difference is <= 3% the measured distribution "passes" at that point. The drawback of the dose difference test is that it is not robust in high gradient regions, as small misalignments can cause large dose differences.

Distance-to-agreement (DTA) is also very straight forward conceptually. Given a point a_p in the planned distribution and the corresponding point a_m in the measured distribution, the distance-to-agreement is the nearest point in the measured distribution from a_m, such that D(a_m + r) = D(a_p). As with dose difference, a passing criterion is chosen, e.g. 3 mm. If the matching dose level is found within a radius of <= 3 mm, the measured distribution "passes" at that point, . This technique is quite robust against misalignments in high gradient regions, as the matching dose level will still be nearby. However, this technique is prone to failure in low gradient regions, where even small misalignments can require a large radius to find the matching dose level. To avoid this to some extent, a dose threshold value can be used, such that dose below of the x% isodose line is not considered, where x% is usually a low dose, penumbra region.

In order to overcome the lack of  robustness in high and low gradient regions for dose difference and DTA respectively, the two tests are often used in conjunction.  This is done by evaluating the tests independently and then defining a point as passing if it passes either test. Thus a distribution might pass 70% for DTA and 70% for dose difference, but 90% for the combined test.

What we've seen is that both of these tests are conceptually quite straight forward, though each has its limitations. The subtleties and challenges arise in the efficient implementation of the algorithms (DTA in particular).

I discuss the gamma test in this post.


Further reading:

• I. J. Yeo and J. O. Kim, A Procedural Guide to Film Dosimetry, Medical Physics Publishing, 2004, ISBN: 9781930524194
• W. B. Harms, Sr., D. A. Low, J. W. Wong, and J. A. Purdy, A software tool for the quantitative evaluation of 3D dose calculation algorithms, Med. Phys. 25, p.c1830 (1998); http://dx.doi.org/10.1118/1.598363
• J. Van Dyk, R. B. Barnett, J. E. Cygler, and P. C. Shragge, "Commissioning and quality assurance of treatment planning computers," Med. Phys. 26, 261–273 (1993). http://dx.doi.org/10.1118/1.598801

Image by MBq and licensed under CC license terms.