Introduction: The application of 3D volumetric imaging modalities in treatment planning of radiation therapy can provide more precisely define tumor localization, and computed tomography (CT) is the most common accepted method for treatment planning. Given the lack of a CT scanner stationed in all radiotherapy departments and equipping most of the medical linear accelerators with only electronics portal imaging device (EPID) device, the use of this technology for megavoltage cone-beam computed tomography (MV-CBCT) imaging in such cases as urgent palliative radiotherapy treatments, leads to precise localization of the tumor and increases the accuracy of the beam sent to the target area. Since MV-CBCT imaging is possible using EPID, this study is aimed at conducting MV-CBCT and investigating the possibility of using these images in the treatment planning of the radiation therapy.
Materials and Methods: Initially, the projection data of an anthropomorphic head phantom was acquired at 3o increments for a 360° of gantry rotation around the patient, and then the 2D projections were reconstructed with Feldkamp cone-beam algorithm. The intensity-to- electron-density calibration was performed for MV-CBCT images to converts the reconstructed gray-level intensity of each image voxel into an electron density value. The reconstructed MV-CBCT images were sent to the treatment planning software, and after the registration with KVCT images, two independent plans were generated on the both CT datasets. Finally, the dose volume histogram (DVH) of the anatomical targets contoured with MV-CBCT was compared to DVH of the same targets contoured with KVCT images.
Results: There was a high level of agreement between the MV-CBCT and KVCT plans with the anatomical structures of the skull as the target. The shift between the co-ordinate positions of the anatomical points identified on the MV-CBCT from the reference planning CT points, and the calculated dose based on the both CT datasets have an acceptable level of agreement.
Conclusion: The results of this study show that MV-CBCT images could be effective for treatment planning modality include single fraction palliative treatment. However, low contrast resolution and extended scan acquisition time are the two main limiting factor that eliminate this imaging modality only for palliative treatments at easily distinguishable area, such as brain.