Room: Exhibit Hall | Forum 5
Purpose: Adaptive-Radiotherapy (ART) with frequent imaging has been used to improve dosimetric accuracy by accounting for anatomical variations; including tumor shrinkage, weight loss and inter-fraction organ motion. The process to evaluate which patients will benefit from ART methods can be time consuming, error prone and require additional diagnostic quality fan-beam CT scans. Introduced here is a methodology that simplifies the evaluation of ART need while reducing uncertainty in the process.
Methods: Daily iterative-CBCT (iCBCT) scans were collected on a Linac for Prostate and H&N patients. These iCBCTs along with the planning-CT, structures, treatment plan and dose-files were transferred to the Velocity system for ART analysis. Weekly ART evaluation was performed every 5th fraction by deforming the planning-CT to the iCBCT and migrating the HU-values into the iCBCT frame-of-reference to generate a synthetic-CT matching the daily anatomy but with accurate e-density information. The RT-plan was then transferred to this synthetic-CT for dose calculation in the treatment planning system. These weekly synthetic-CT dose calculations were accumulated back to the planning-CT to evaluate the need for ART.
Results: Both H&N and Prostate patients demonstrated differences in their accumulated dose and dose volume histograms as compared to the initial treatment plan. The H&N example exhibited under-dosing of the PTV targets as well as increased dose to several OARs including; parotid glands and larynx. The Prostate example showed similar results with under-dosing to the PTV and increased dose to the penile bulb and anorectum.
Conclusion: This work demonstrated an efficient method to evaluate delivered dose partway through the treatment course and the impacts on targets and OARs for determining the need for ART. Using daily iCBCTs and the planning-CT to produce synthetic CT-scans for accurate daily dose calculation and ART evaluation may be repurposed for on-line ART by incorporating daily RT-plan re-optimization using Knowledge-Based planning.
Funding Support, Disclosures, and Conflict of Interest: The work presented in this abstract was supported by a grant from Varian Medical Systems.
Cone-beam CT, Treatment Planning, Image-guided Therapy