A symmetric nonrigid registration method to handle large organ deformations in cervical cancer patients.

Purpose: Modern radiotherapy requires assessment of patient anatomical changes. By using unidirectional registration methods, the quantified anatomical changes are asymmetric, i.e., depend on the direction of the registration. Moreover, the registration is challenged by the large and complex organ deformations that can occur in, e.g., cervical cancer patients. The aim of this work was to develop, test, and validate a symmetric feature-based nonrigid registration method that can handle organs with large-scale deformations.

Methods: A symmetric version of the unidirectional thin plate spline robust point matching (TPS-RPM) algorithm was developed, implemented, tested, and validated. Tests were performed by using the delineated cervix and uterus and bladder in CT scans of five cervical cancer patients. For each patient, five CT scans with a large variability in organ shape, volume, and deformations were acquired. Both the symmetric and the unidirectional algorithm were employed to calculate the registration geometric accuracy (surface distance and surface coverage errors), the inverse consistency, the residual distances after transforming anatomical landmarks, and the registration time. Additionally, to facilitate the further use of our symmetric method, a large set of input parameters was tested.

Results: The developed symmetric algorithm handled successfully the registration of bladders with extreme volume change for which TPS-RPM failed. Compared to the unidirectional algorithm the symmetric algorithm improved, for the registration of organs with large volume change, the inverse consistency by 78% and the surface coverage by 46%. Similarly, for organs with small volume change, the symmetric algorithm improved the inverse consistency by 69% and the surface coverage by 13%. The method allowed for anatomically coherent registration in only 35 s for cervix-uterus and 151 s for bladder, while keeping the inverse consistency errors around 1 mm and the surface matching errors below 1 mm. Compared to rigid alignment the symmetric method reduced the residual distances between anatomical landmarks from a range of 5.8 +/- 2-70.1 +/- 20.1 mm to a range of 1.9 +/- 0.2-8.5 +/- 5.2 mm.

Conclusions: The developed symmetric method could be employed to perform fast, accurate, consistent, and anatomically coherent registration of organs with large and complex deformations. Therefore, the method is a useful tool that could support further developments in high precision image guided radiotherapy.