CBCT image registration for adaptive radio and proton therapy of prostate cancer

The application of cone-beam CT (CBCT) image guidance in external beam therapy for prostate cancer treatment is questioned by sub-optimal image quality and procedural constraints. Such factors may frustrate the effort of applying volumetric imaging to quantify anatomical changes and evaluate replanning. This study investigates the impact of anatomical changes and the potential of using CBCT for IMRT and IMPT in prostate patients. Deformable image registration (DIR) is applied in 9 prostate cancer patients imaged with CBCT, to transform the planning CT and produce a virtual CT with updated patient anatomy. Daily image guidance was simulated for repeated CBCT scans relying on either soft tissue targeting or bone anatomy matching. Automated landmark detection was used to evaluate the residual distance for landmarks distributed over the region of interest (target and organs-at-risk) after DIR. DIR allowed the mean distance for these landmarks to be reduced from 3.79 to 2.00 mm. IMRT and IMPT plans were created and recalculated over virtual CTs. Rectum dose increase was observed in 8 of 9 patients, and for 3 cases there was constraints violation, accordingly to QUANTEC guidelines, of up to 75 Gy on 19% of rectum volume. The most important CTV coverage loss was around 5% of the prescribed dose for a proton therapy case. For both types of treatment, PTV inter-fraction movement and organs-at-risk structural variations, may result in loss of target coverage and/or early excessive damage of acute-responding tissue components. Therefore, DIR and dose recalculation are important tools for dose distribution verification on PTV and organs at risk, in order to assure prescribed dose delivery and avoid future complications. (author)