  The accuracy of deformable image registration (DIR) has a significant
dosimetric impact in radiation treatment planning. We evaluated accuracy of
various DIR algorithms using variations of the deformation point and volume.
The reference image (Iref) and volume (Vref) was first generated with virtual
deformation QA software (ImSimQA, Oncology System Limited, UK). We deformed
Iref with axial movement of deformation point and Vref depending on the types
of deformation that are the deformation1 is to increase the Vref (relaxation)
and the deformation 2 is to decrease . The deformed image (Idef) and volume
(Vdef) acquired by ImSimQA software were inversely deformed to Iref and Vref
using DIR algorithms. As a result, we acquired deformed image (Iid) from Idef
and volume (Vid) from Vdef. The DIR algorithms were the Horn Schunk optical
flow (HS), Iterative Optical Flow (IOF), Modified Demons (MD) and Fast Demons
(FD) with the Deformable Image Registration and Adaptive Radiotherapy Toolkit
(DIRART) of MATLAB. The image similarity between Iref and Iid was calculated
using the metrics that were Normalized Mutual Information (NMI) and Normalized
Cross Correlation (NCC). When moving distance of deformation point was 4 mm,
the value of NMI was above 1.81 and NCC was above 0.99 in all DIR
algorithms.When the Vref increased or decreased about 12%, the difference
between Vref and Vid was within 5% regardless of the type of deformation.The
value of Dice Similarity Coefficient (DSC) was above 0.95 in deformation1
except for the MD algorithm. In case of deformation 2, that of DSC was above
0.95 in all DIR algorithms. The Idef and Vdef have not been completely restored
to Iref and Vref and the accuracy of DIR algorithms was different depending on
the degree of deformation. Hence, the performance of DIR algorithms should be
verified for the desired applications.
