Evaluation of contour propagation and dose deformation errors using the hybrid and biomechanical algorithms for head and neck cancer
Authors: Lee Tsair-Fwu , Lee Shen-Hao , Chiu Chi-Min , Yang Pei-Ying , Chiu Yu-Hao , Yang Jack , Huang Yu-Jie , Fang Fu-Min , Chao Pei-Ju , Tseng Chin-Dar 
Published: April 2023

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 (Planx and Planp). Planx included photon beam therapy (5250cGy/25 sessions) and Planp involved proton beam therapy (1680cGy/8 sessions). We used two adjuvant treatment immobilization devices (immobilization) to scan computed tomography (CT) images: Planx included CTx and immobilizationx, and Planp included CTp and immobilizationp. 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 Rim+b, and the range of only the body contour is defined as Rb. We generated four settings as follows: Hy_Rim+b, Bio_Rim+b, Hy_Rb, and Bio_Rb. We mapped organs at risk (OARs) to Planp 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 Planp. 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 Planp. We mapped the received dose of OARs in Planx to Planp with dose deformation, and evaluated the percent dose difference [dose diff.(%)] between the four settings and Planx. In terms of contour propagation, the overlapping image of the horizontal section (transversal) showed that because the range set by Hy_Rim+b and Bio_Rim+b includes immobilization, Hy_Rim+b deforms in the oral cavity and esophagus area, and for Bio_Rim+b significant deformations around the body contour lead to misregistration. The Hy_Rb and Bio_Rb settings are not obviously deformed in the overlapping images. We assessed the consistency of dissemination of OARs contours by using the DSC. The average DSC of Hy_Rim+b and Bio_Rim+b is 0.63 and 0.32, respectively; the average DSC of Hy_Rb and Bio_Rb 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_Rim+b and Bio_Rim+b in Planx are 13.15% and 17.82%, respectively, while those of Hy_Rb and Bio_Rb 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 OARs 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.