Evaluation of Interfractional Setup Uncertainties and Calculation of Adequate CTV-PTV Margin for Head and Neck Radiotherapy using Electronic Portal Imaging Device

Background: The evaluation of treatment-associated errors is important in the radiotherapy process, particularly those resulting related to patient setup.Objective: This research aimed to assess the interfractional setup errors and determine the Clinical Target Volume to Planning Target Volume (CTV to PTV) margin in patients undergoing ۳-Dimensional Conformal Radiation Therapy (۳DCRT) for head and neck cancer by means of electronic portal imaging device.Material and Methods: In this analytical study, ۳۰۰ portal images were acquired from ۵۰ patients undergoing ۳DCRT for head and neck cancer. Using the portal images of Lateral (LAT) and Antero-Posterior (AP) fields, population systematic (∑) and random (σ) errors were obtained in the lateral, longitudinal, and vertical directions. Finally, based on the International Commission on Radiation Units and Measurements (ICRU) Report ۶۲’s, Stroom’s and Van Herk’s methods, Planning target volume margins were determined.Results: The translational shift ranges were ۰-۸.۱ mm in the ML, ۰-۹ mm in the SI (AP), ۰-۸.۸ mm in the SI (LAT), and ۰-۱۰ mm in the AP directions. The population systematic and random errors were respectively ۳.۲۳۰, ۲.۷۵۳, and ۲.۹۹۷ mm, and ۱.۴۷۶, ۱.۸۵۳, and ۱.۷۱۵ mm in X, Y, and Z directions. The calculated PTV margins using the ICRU-۶۲, Stroom’s, and Van Herk’s formulae were ranging from ۳.۲۳۶-۳.۵۵۱, ۶.۶۰۵-۷.۴۹۳, and ۷.۹۳۲-۹.۱۰۸ mm, respectively. Conclusion: A PTV margin of ۷.۵-۹.۵ mm seems safe for ensuring adequate treatment volume coverage. In addition, the EPID is an effective equipment for verifying patient positioning and reducing treatment setup errors.

Keywords:

Head and Neck Cancer , Radiotherapy , Patient Positioning , Setup Errors , Systematic Error