Sci—Fri PM: Delivery — 12: Performance of Two Automated Image Guidance Techniques Employing Low Dose CBCT

Image‐guided radiotherapy (IGRT) is becoming the standard treatment for prostate cancer. One approach employs a linear accelerator‐mounted, cone‐beam computed tomography (CBCT) system to acquire 3D images at treatment. Typically, radiation therapists manually register such images to the planning CT to determine couch shifts, thereby ignoring anatomical rotations and deformations. Furthermore, CBCT systems deliver significant imaging doses over long fractionation schedules. Therefore, we continue developing image guidance (IG) techniques relying on automatic registration and low dose CBCT images. Our “global” method computes couch corrections while the “local” variant provides a deformable transformation that can be used to adapt the original plan. Previous validation with Varian's On‐board Imager (OBI v1.3) showed that IG error is maintained despite reducing the mAs to 15% of the standard 1300. Recent improvements in OBI v1.4 result in similar image quality at 680 mAs (“pelvis” mode). Additionally, “pelvis spotlight” mode was introduced with additional lateral collimation and 720 mAs employed over 200 degrees. Retesting showed that global IG error is 3.5 ±1.0 mm irrespective of the OBI version down to 10% of the standard dose. Local IG required 20% of the standard dose to achieve 1.8 ± 0.6 mm accuracy with OBI v1.4 pelvis, while the 2.0 ± 0.5 mm error was maintained down to 15% with OBI v1.3 and v1.4 spotlight. In absolute terms, the dose savings achieved by our IG methods are cumulative with those offered by the upgrade. Our local IG technique has great potential to significantly improve the precision of radiation therapy.