An Aperture-Preserved Approach to Adaptive Radiotherapy of Prostate Cancer Using IMRT and VMAT

Abstract

Over the course of a patient’s radiotherapy treatment, there are often changes of internal anatomy relative to the planning CT scan. Because patients are typically treated with a treatment plan created based on one planning CT, dose is likely deposited in healthy tissue locations not targeted by the initial treatment plan, thus leading to less dose given in the target region. To combat this problem, adaptive radiation therapy (ART) was introduced where a new treatment plan is created based on the anatomy of the patient imaged on the treatment day. However, ART often cannot be feasibly routinely applied due to the involved time-intensive workflow required for implementation. For patients treated with VMAT and IMRT, part of this workflow is delivery quality assurance (QA) to verify the accuracy of radiation fields. This study introduces a new approach to remove the need for delivery QA for adapted plans in an attempt to shorten the ART clinical workflow. The implemented approach is a partial re-optimization of the initial plan maintaining the segment shapes from the original plan and minimum monitor unit (MU) constraints. Dose will be delivered through aperture shapes that have already passed QA protocol before the patient’s initial treatment, eliminating the DQA step from the ART workflow. CT images from 20 prostate cancer patients were retrospectively chosen for this study. Each of these patients had 5 CT-on-rails (CTOR) images taken, one from each week of their treatment, in addition to their original planning CT. Partially re-optimized VMAT and IMRT plans were created for all 100 images and compared to dosimetry directly calculated onto the treatment CT from the original plan. Goals for treatment adaption included regaining coverage lost on the treatment day CT as well as sparing organs at risk. Dosimetric benefits of this adaption method could be seen in many treatment plans. Successful ART plans, with significant change of anatomy between initial plan and ART plans, were delivered and measured on treatment machine. They show similar DQA quality as original plans, demonstrating that partial re-optimization can be performed and delivered to a patient without the need for delivery QA. With the shortened workflow of this technique, adaptive therapy may become a viable option for patients that typically may not qualify for a full adapted plan.