Dosimetric evaluation of treatment plans made with images gained from cone beam CT in patients with head and neck cancer undergoing adaptive radiotherapy

Background and Aim: In head and neck cancer radiotherapy, intensity-modulated radiotherapy producing high dose gradients and image-guided radiotherapy enhancing treatment accuracy are employed due to the proximity of target and normal tissues. Anatomical changes and position errors during treatment hinder the precise delivery of dose to the target volume. When rigid corrections are insufficient to address anatomical changes in Cone Beam CT images, adaptive radiotherapy is considered. Adaptive radiotherapy systematically monitors treatment changes and re-optimizes the treatment plan when necessary. Cone Beam CT images, a vital component of adaptive radiotherapy, are of lower quality than planning CT images due to acquisition differences. Attention is crucial for both adaptive radiotherapy and dosimetric calculations due to artifacts in Cone Beam CT images, as they may reduce geometric and dosimetric accuracy in plans. Material and Methods: In our study, treatment plans were generated using planning CT images for 20 patients undergoing radiotherapy for head and neck cancer. Various CT/ Cone Beam CT calibration techniques were applied to Cone Beam CT and synthetic CT (synthetic CT: deformed images obtained from Cone Beam CT) images, and differences among calculated plans were investigated. Dose tracking was also performed on weekly Cone Beam CT images throughout the treatment. Target volume and organ-at-risk doses were evaluated based on patients' dose-tracking plans. Differences between the calculated reference plan and Cone Beam CT / synthetic CT plans were assessed using gamma index criteria. Results: When compared with the reference plans and gamma index criteria, synthetic CT plans showed a higher pass rate in gamma analysis. For the patient's treatment plan and accumulated dose plan, dose differences in the PTV54 target volume were found to be -3.88% and -1.57% for D%98 and D%95, respectively. Throughout the entire treatment, an increase of 6.53% and 7.71% in dose was observed for the left and right parotids, respectively. The average change in neck circumference was found to be 1.18±0.65 cm. Conclusion: The usability of synthetic CT has been confirmed in adaptive radiotherapy applications. In adaptive radiotherapy applications for head and neck cancer, it was found that the optimal time for monitoring and replanning the dose received by the target volume and critical organs is during the 4th and 5th weeks. Additionally, it was concluded that adaptive radiotherapy should be considered on a patient-specific basis.