Cone beam calibration curves in dose calculations for a VMAT head and neck radiotherapy treatment

Abstract

Introduction Cone beam computer tomography (CBCT) dose calculation can provide many advantages for adaptive radiotherapy despite the loss of accuracy associated, mainly due to beam scatter that causes Hounfield number (HU) inconsistency. Purpose Analyze the differences between CT and CBCT dose calculation for a volumetric modulated arc therapy (VMAT) treatment, using different calibration curves. Materials and methods It has been used an Alderson RANDO phantom with tissue equivalent material. CBCT images were obtained in a Varian’s On-Board Imager (v1.4), using a standard dose head protocol. Planning CT images were obtained in a Toshiba Aquilion LB. A VMAT plan has been calculated in Eclipse (v10) using 4 calibration curves for CBCT images: Standard, Varian CBCT calibration, measured with a CATPHAN 504 phantom and measured with a CIRS 062M phantom, placed between RANDO slices. For each image was defined 54 point in different areas (air, bone, soft tissue) and clinical structures (PTV, spinal cord, parotids, mandible, oropharynx). HU and dose were obtained for CT and CBCT points and dose–volume–histogram for clinical structures. Results For the points analyzed the average difference was 45 HU (standard deviation 147), the average dose difference was less than 2% for all calibration curves. CBCT-DVHs were in excellent agreement with CT-DVHs. Minimum, maximum, mean and median doses agreed very well. In general, differences were less than 2%. The smallest differences were obtained for CIRS-calibration curve. Conclusion CBCT images for a head and neck treatment can be used to calculate dose in adaptive radiotherapy in order to evaluate changes with the original treatment using the calibration curves analyzed.