Delivery accuracy of image guided radiation therapy using Elekta Infinity\u27s on-board imaging

Using the technique developed by Vinci et al. (2007), this project quantified the accuracy and precision of the Elekta Infinity using an end-to-end test for the entire image guided radiation therapy (IGRT) treatment process. The IGRT capabilities of the on-board imaging systems for megavoltage (MV) planar imaging and kilovoltage cone beam computed tomography (kVCBCT) were both evaluated. The accuracy of the on-board imaging systems will impact the size of planning treatment margins. A CIRS Radiosurgery head phantom with a Gafchromic EBT2 film dosimetry block insert was used to measure the dose distributions in the three orthogonal planes. A coplanar, isocentric, seven-field treatment plan was created for a cylindrical target volume located at the center of the film cube. The phantom was manually positioned on the treatment couch, either correctly aligned to the room lasers or with intentional misalignments of &177 5 mm in three directions of couch motion. Prior to treatment, one of the IGRT modalities was utilized to identify misalignment; the calculated realignment values were used to shift the phantom prior to treatment delivery. After treatment, the delivered dose distributions were analyzed and compared to the calculated planar dose distributions. Two metrics were analyzed from the film&58 positional alignment error of the 70&37 isodose line and comparison of spatial shifts of the 80&37 dose points. Positional alignment error quantified displacement between the midpoints of the measured and calculated dose profiles; shifts at the 80&37 dose points indicated distortions in the profiles. Investigation of the accuracy of the couch motion revealed a systematic 1 mm error at fixed locations along the longitudinal axis. Using a non-invasive immobilization device and onboard image guidance, setup margins on the Elekta Infinity should be set to 2 mm using kVCBCT and 3 mm using MV planar imaging. To use the Elekta Infinity for high precision deliveries with minimal planning target volumes would require tighter tolerances in couch motion; a couch with rotational as well as translational motion seems advisable. For high precision deliveries, image guidance using kVCBCT is recommended over MV planar image guidance