Room: AAPM ePoster Library
Purpose: To develop methodologies for offline adaptive radiotherapy of prostate and head-and-neck treatments using Velocity.
Methods: A cohort of 30 prostate and 10 head-and-neck radiotherapy patients with daily CBCTs and noticeable anatomy changes were retrospectively analyzed using Velocity. For the head-and-neck patients, Velocity’s adaptive assessment tools were used to create adapted CTs for each fraction of treatment on which the initial plan was recalculated and the resulting dose analyzed. For the prostate patients, Velocity’s adaptive assessment tools proved inadequate due to the algorithms’ inability to handle contrast on the planning CT or very large differences in bladder volume. Instead, structure-based deformation was used to create the adapted CTs in Velocity prior to dose analysis.
Results: The head-and-neck patients demonstrated increasing larynx mean dose and decreasing PTV coverage throughout treatment. Spinal cord and parotid doses demonstrated trends for individual patients but not across all patients. Increases of as much as 20%, 10%, and 15% were seen for the larynx mean, cord maximum, and parotid mean dose, respectively. PTV coverage dropped by as much as 20%. The prostate patients demonstrated increasing bladder V65 and rectum V70 with decreasing bladder fill, with high fraction-to-fraction variation. Calculation time alone for one patient with 35 fractions can take more than 5 hours, with added time when using structure-based deformation.
Conclusion: Velocity is an effective tool for offline assessment and adaptation of head-and-neck radiotherapy. We recommend weekly assessment for head-and-neck patients using Velocity, with adaptive replanning midway through treatment or when dose constraints are violated. Velocity is an inefficient tool for offline assessment and adaptation of prostate radiotherapy for patients with large variations in bladder fill and contrast-enhanced planning CTs. However, we have developed a methodology that allows it to be used to determine what interventions might be necessary for prostate patients with variable bladder fill.