Lei Dong, PhD, director and chief of medical physics at the Scripps Proton Therapy Center, San Diego, CA
Adaptive radiotherapy has been considered a necessary quality assurance practice for high-precision patient treatment, and we are at the very beginning of making this a clinical reality. In-room cone beam computed tomography (CBCT) is a powerful tool, providing the first step towards implementing adaptive proton therapy as a standard of practice. Our knowledge and confidence to perform these accurate treatments is only growing as we begin to take advantage of three-dimensional (3D) in-room imaging available to us.
Recent technological advances are improving the implementation of adaptive radiotherapy during intensity modulated proton therapy (IMPT). With the advent of IMPT, better dose distribution can be achieved than was previously possible with traditional radiotherapy. While the increased accuracy holds the promise for significant dosimetric advantages, such as decreasing dose to organs at risk, it is a double-edged sword as it is more sensitive to anatomical changes within or outside the target. This can be mitigated by advancements in high quality volumetric imaging using CBCT to implement adaptive treatment plans. 3D anatomical information about the patient during each treatment session and actual delivered doses can be tracked, allowing for adaption of the treatment plan throughout the course of therapy to ensure the most effective treatment.
Dynamic Patient Care with Volumetric Imaging
The conventional radiotherapy workflow assumes a static situation by basing the simulation, planning, and treatment on an initial computed tomography (CT) image. The majority of clinical situations, however, are dynamic, and the proton dose distribution is often impacted by changes in tumor shape and/or volume; organ or tumor movement; and anatomical changes, such as weight loss.
Volumetric imaging using CBCT not only improves visibility for better positioning (image guidance to set up patients more accurately), but it serves as a trigger, in many cases, to do an adaptive CT, allowing us to timely change a treatment plan to fit the patient situation. Scripps Proton Treatment Center (SPTC) is the first to have CBCT functionality in all of its gantry treatment rooms. Thus far, adaptive radiotherapy using CBCT is not only improving our accuracy but also the efficiency of our workflow.
Adaptive Radiotherapy at SPTC
Prior to having CBCT available, we were doing mainly two adaptive schedules for treatment evaluation. The first evaluation is at the end of the first week of treatment to evaluate potential systematic errors that may have been introduced during the simulation process. Simulation is the first event in the radiation therapy process. At this stage, many patients tend to be nervous and may tense their muscles, consequently affecting tumor or organ positioning. After the first week, patients often become more relaxed and positioning becomes more natural and reproducible.
The second adaptive radiotherapy evaluation is done between the third and fourth weeks of treatment, which is usually when the patient is responding to radiation treatment and the tumor is shrinking. This is a good point for adaptive radiotherapy not only because of tumor changes but also because it's usually the half way point, so there's still time to adapt the plan for the remaining treatments.
With the availability of CBCT on the ProBeam® System, we are able to monitor the patient's anatomy on a daily basis. Patients no longer need to be brought to the CT simulator to perform such evaluation. This saves time and resources at the simulation CT scanner. Using the deformable registration tool in Velocity™, Varian's tool for image and treatment registration, we can even calculate dose using the daily CBCT images. The software is capable of mapping the original calibrated Hounsfield numbers from the planning CT into the CBCT image while retaining the patient's current anatomical and positional information. The deformable image registration tool is a key technology for adaptive radiotherapy; contours of the target and normal structures can be propagated from the planning CT to the CBCT, instead of creating from scratch, which saves time and improves the workflow. Additionally, regions of over dose or under dose can be delineated when tracking the cumulative up-to-date dose delivered, allowing us to adapt and to achieve the desired dose more accurately.
Overcoming Current Barriers to Adaptive Radiotherapy
Technological and workflow challenges are the two main barriers to making adaptive radiotherapy standard practice. CBCT is technologically challenging because it has more scatter when using wide angle x-rays. Consequently, it can be more challenging to get quality CBCT images and accurate Hounsfield numbers for dose calculations.
There are also workflow challenges. For example, a physician needs to determine if adaptive radiotherapy is necessary, based on anatomical changes. When adaptive radiotherapy is deemed necessary, each adaptive plan is considered a new treatment plan and all of the quality assurance procedures need to be followed, greatly increasing the workflow demands.
With Varian's integrated systems, it has been helpful in managing the increased workflow when utilizing adaptive techniques. Varian is the only vender that offers an integrated end-to-end imaging system, from simulation to treatment planning to in-room imaging. Additionally, the integrated ARIA® oncology information system and database links together all of the information required to adapt plans such as recording treatment couch shifts. Finally, the automatic deformable image registration tool, anatomy checking, dose tracking, and online/remote reviewing of adapted plans allows for high precision adaptive radiotherapy with realistic clinical workflows.
The Beginning of a Bright Future
At SPTC, our goal is to give our patients the highest quality treatment possible, and adaptive radiotherapy enables us to get the best treatment outcomes for our proton therapy patients with lower toxicity without sacrificing tumor control. As we refine and develop adaptive radiotherapy techniques, we will be able to design treatment based on patient's changing geometry, allowing them to maximize the benefits of proton therapy.