Evaluation of contour propagation and dose deformation errors using the hybrid and biomechanical algorithms for head and neck cancer

Abstract This study aimed to evaluate whether the hybrid and biomechanically deformable image registration (DIR) algorithm of the RayStation treatment planning system would produce contour propagation and dose deformation errors in the head and neck due to the inclusion of adjuvant therapeutic fixtures. We analyzed the treatment plans of two groups of patients with head and neck cancer (Plan x and Plan p ). Plan x included photon beam therapy (5250cGy/25 sessions) and Plan p involved proton beam therapy (1680cGy/8 sessions). We used two adjuvant treatment immobilization devices (immobilization) to scan computed tomography (CT) images: Plan x included CT x and immobilization x , and Plan p included CT p and immobilization p . Using the hybrid (Hy) and biomechanical (Bio) algorithms of the RayStation treatment planning system, we controlled the registration to analyze the contour propagation and dose deformation. The range of immobilization including the body contour is defined as R im+b , and the range of only the body contour is defined as R b . We generated four settings as follows: Hy_R im+b , Bio_R im+b , Hy_R b , and Bio_R b . We mapped organs at risk (OAR s ) to Plan p by using contour propagation through the aforementioned four settings. Contour propagation uses the results of overlapping image display, the Dice similarity coefficient (DSC), and the contour drawn by the physician on Plan p . We used the results shown in the overlapping images in the contour propagation and evaluated them with the DSC and the contour drawn by the physician in Plan p . We mapped the received dose of OAR s in Plan x to Plan p with dose deformation, and evaluated the percent dose difference [dose diff.(%)] between the four settings and Plan x . In terms of contour propagation, the overlapping image of the horizontal section (transversal) showed that because the range set by Hy_R im+b and Bio_R im+b includes immobilization, Hy_R im+b deforms in the oral cavity and esophagus area, and for Bio_R im+b significant deformations around the body contour lead to misregistration. The Hy_R b and Bio_R b settings are not obviously deformed in the overlapping images. We assessed the consistency of dissemination of OAR s contours by using the DSC. The average DSC of Hy_R im+b and Bio_R im+b is 0.63 and 0.32, respectively; the average DSC of Hy_R b and Bio_R b is 0.94 and 0.83, respectively. The results of the overlapping image and DSC evaluation showed that the two algorithms can reduce the error by excluding immobilization in the registration range of contour propagation. We found that the hybrid algorithm is superior to the biomechanical algorithm. In terms of dose deformation, the average dose differences of Hy_R im+b and Bio_R im+b in Plan x are 13.15% and 17.82%, respectively, while those of Hy_R b and Bio_R b are 3.32% and 5.13%, respectively. We found that the average dose error of the hybrid algorithm is smaller than that of the biomechanical algorithm. Considering the setting where the registration range does or does not include immobilization, the average dose of OAR s differs by 9.83% for the hybrid algorithm and 12.69% for the biomechanical algorithm. In conclusion, we found that the hybrid and biomechanical algorithms of the RayStation treatment planning system increase the error of contour propagation and dose deformation because the registration range includes head and neck immobilization. The results show that the hybrid algorithm is more suitable for the head and neck than the biomechanical algorithm. Therefore, we suggest using the hybrid algorithm for clinical planning of DIR, and excluding immobilization from taking the patient's body contour as the registration range.