Dose-effect analysis of facial bone morphology following radiation therapy for head and neck rhabdomyosarcoma

Introduction: With the long-term survival of paediatric rhabdomyosarcoma patients improving, the impact of late adverse effects becomes more important. These late effects in head-neck-rhabdomyosarcoma survivors include poor cosmesis associated with facial deformation. This study aims to correlate facial deformation with radiation dose and to propose new dose–effect models. Materials and Methods: We evaluated 3D facial images of children treated with radiotherapy enrolled in our multi-center cross-sectional study with a primary head-neck-rhabdomyosarcoma. Facial deformation was calculated from 3D image analysis at follow-up and compared to a healthy control population (n = 537). The radiation dose delivered to all individual facial bones and sutures was extracted from original radiotherapy plans. Dose-effect probability curves were constructed by converting the dose to its equivalent 2 Gy/fraction EQD2 using binary logistic regression. Results: Thirty-four survivors with a median follow-up of 9.2 years were included. Survivors with facial deformation all received significantly higher mean doses to the corresponding bony structures (p < 0.001). The ethmo-maxillary complex shows increased susceptibility for facial deformation at 28GyEQD2 and a 50% probability of growth deformation at 51GyEQD2. The mandibular complex shows increased growth deformation probability at 26GyEQD2 and a 50% probability at 41GyEQD2. The threshold for increased risk of bone deformation for individual facial bones varied from 26 GyEQD2 to 43 Gy EQD2 and a 50% probability for deformation varied from 44 Gy EQD2 to 55 Gy EQD2. Conclusion: Dose thresholds for specific facial bones impacting cosmesis in survivors of paediatric sarcoma are shown. These data are essential to improve treatment planning thereby limiting the debilitating late adverse effect of facial deformation.