Sponsored by an educational grant from Varian Medical Systems.
A range of imaging technology is necessary for optimum management of cancers. Simulation is generally considered the first step of many in preparing a patient for treatment. Today, simulation can be accomplished using a myriad of modalities, including fluoroscopy, CT, PET, and MRI. After simulation, treatment-planning computers are used to map out the radiation dose distribution in and around the tumor, using information acquired by the simulator.
The Acuity? system from Varian Medical Systems (Palo Alto, Calif) combines planning, simulation, and motion data for verification of patient plans. High-resolution, interactive 2-D digital images enable more precise, efficient planning. Auto-setup of 3-D planning information ensures data integrity and accurate machine setup. Respiration motion data-captured using Acuity and Varian’s real-time position management (RPM?) Respiratory Gating System-allows physicians to account more accurately for patient motion. Combined, these factors allow a clinic to optimally and efficiently complete a crucial step in the care of radiation oncology patient.
Five hospitals were interviewed for this article and all use the system for the many procedures it was designed to support. Overlake Hospital Medical Center (Bellevue, Wash) uses Acuity exclusively for brachytherapy. As a department that is not capacity limited at this time, the dedicated CT/Sim supports all simulation from simple to complex and has allowed the department to immediately focus on an efficient MammoSite program.
Four of the hospitals use Acuity for basic simulation of bone metastases and whole-brain treatment. All site representatives agreed that Acuity is the most efficient way to simulate these types of patients when they are immediately scheduled to start treatment. According to Arthur L. Boyer, PhD, director of the radiation physics division of the department of radiation therapy at Stanford University School of Medicine (Palo Alto, Calif), features of Acuity make it quite efficient for all general simulation processes, once one has learned to use them. He comments that software-driven controls for centering treatment fields using video fluoroscopy images acquired from the flat-panel amorphous silicon X-ray imaging device are excellent.
Wilrijk Hospital (Antwerp, Belgium) uniquely uses Acuity to simulate the isocenter of the plan before sending it for a CT/Sim. Another hospital’s representative said that plan isocenters are defined dosimetrically; and yet another has a dedicated CT scanner in the department, so simulating the isocenter prior to the CT/Sim would not be necessary because it would be a duplication.
 |
| Plan evaluation for IMRT often is completed with orthogonal pairs of setup fields. From left to right are AP and Lat planning DRRs (above) and AP and Lat Acuity verification images (below). |
Other ways that Acuity is used for basic simulation are in the case of palliative lung (at Norfolk & Norwich University Hospital in Norwich, UK); in difficult clinical electron setups, where the physicians want to take a picture to localize (Emory Crawford Long Hospital in Atlanta); and for MammoSite and high-dose rate (HDR) patients and large-size patients, who are more easily simulated, or “simmed,” on Acuity. Most breast treatments at Stanford Cancer Center are simulated with Acuity. The process involves the use of the digital video fluoroscopy along with the field light and laser back-pointer. Couch “kicks” and the use of half-blocked fields are easily planned using Acuity’s features.
Plan Verification
Norfolk & Norwich University Hospital (NNUH) completes plan verification on Acuity, and finds it useful to be able to verify multi-leaf collimator settings (MLCs) before going to the accelerator. New starts on the accelerator take 15-20 minutes, compared to 20-30 minutes prior to acquiring an Acuity system, and the physicians find that setups are consistently reproducible after plan verification on Acuity. NNUH analyzes motion in the plan using fluoroscopy for some body sites, mainly the larynx (swallowing) and lung (respiration). Using the exact treatment couch and immobilization for all radical patients, and having actual couch parameters adds another element of quality assurance to the process: If, at set up, the parameters are out of tolerance, it is an indication that something is wrong, according to Jenny Tomes, head of radiotherapy in the department of radiotherapy and oncology at NNUH.
Overall process and workflow efficiencies can be improved by completing plan verification on Acuity with cone-beam CT (CBCT). Reducing the amount of time spent on the accelerator taking pretreatment verification images and analyzing them increases patient throughput on the machines, thus reducing waiting lists.
Tomes expects her department to use Acuity for 3-D plan verification for some sites where avoidance of organs at risk (OARs) is critical or where organ motion could result in a geographic miss. The ability to assess conformal avoidance of organs at risk and confirming accuracy in targeting the planning volume requires the ability to see soft tissue, which normally cannot be visualized on conventional fluoroscopic or portal imaging.
Boyer and his colleagues at Stanford continue to find it useful to verify the isocenter of conformal and intensity-modulated plans after the plans are computed and approved. Often, changes occur in the isocenter position between the acquisition of the initial planning CT and the approval of a final plan. A recheck of the isocenter skin marks and the acquisition of digital images in the final treatment position provides more security for the delivery of the technically demanding 3-D conformal radiotherapy (CRT) and IMRT plans. The digital images acquired on the Acuity are better references than DRRs for the isocenter check images acquired with the Portal Vision electronic portal imaging devices available on Stanford’s linear accelerators.
According to Boyer, verifying the projection of apertures formed by MLCs is less important than precise verification of the isocenter. Once the isocenter is set accurately, the field shapes fall into place. The majority of the multiple treatment fields are delivered at oblique angles that are difficult to interpret unless compared with digital indications of the target volumes superposed on the digitally reconstructed radiographs derived from CT scan acquisitions.
From the acquisition of a planning CT or simulation, Boyer finds that he routinely needs to schedule patients for the first treatment 3-5 days later. Their plan verification step is a small but significant perturbation of the overall process. The process time is driven by the time gaps in the overall process that allow relatively small dosimetry and therapist staffs, along with a small effective medical staff, to plan advanced radiation medicine for a large number of patients.
 |
| From left to right are images representing planning DRR, plan verification on Acuity, and treatment verification on Varian’s On-Board Imager system. This supports complete image verification in an efficient manner for both patient and clinician. |
Stanford has ongoing research protocols to analyze tumor motion using the Varian digital video-fluoroscopy available on Acuity. These protocols involve the use of Varian’s RPM Respiratory Gating System. Staff members at Stanford are implementing applications of the RPM to treatments of the breast, lung, pancreas, and liver. The use of the system in hypofractionated techniques appears especially useful, Boyer says, and having actual couch parameters available at the accelerator instead of having to acquire them on the first day of treatment is a small but helpful advantage.
Boyer states that the direct entry of planning data and imaging files into a common server on the network is an enabling necessity for cancer management at this level of technology utilization. Acuity fits into a seamlessly networked system of computer workstations that form the technical foundation for delivering advanced techniques, including 3-D conformal radiation therapy, IMRT, cranial and extracranial stereotactic radiotherapy, and high-dose radiotherapy. All are guided by imaging acquired by CT, positron emission tomography enhanced by computerized tomography (PET/CT), MRI, CR, electronic portal imaging devices (EPID), and all use 3-D computerized treatment planning. It is difficult to quantify the impact of a single part of the process on the efficiency of the entire interdependent system.
Along with her colleagues at the Emory Crawford Long Hospital, Anna Iwinski Sutter, MS, DABR, completes plan verification of 3-D plans on Acuity and finds it useful to be able to verify MLCs before going to the accelerator. It is easier to compare a digital image with a DRR versus a portal image, especially after shifting their patients to isocenter, she explains. New starts on the accelerator at Emory are 15-30 minutes, compared to 30-45 minutes previously when not using Acuity.
Sutter also sees value in having actual couch parameters available at the accelerator instead of having to acquire them on the first day of treatment, “if you are using good immobilization, which keeps the patient in the same position on the couch day after day,” she says.
Depending on the patient, Sutter says some definite advantages exist in overall process/workflow efficiencies by completing plan verification on Acuity-“especially since we have to shift to find the isocenter on every patient. If we had to do this on the machine, finding the isocenter would take much longer.”
3-D Simulation and CBCT
CBCT imaging involves acquiring multiple kV projections as the gantry rotates 360?. A filtered back-projection algorithm is employed to reconstruct the volumetric images. Users cite various advantages in using CBCT for planning as opposed to a conventional CT device. CBCT also is viewed as a valuable tool for 3-D plan verification.
Mark Daniels, chief dosimetrist at Overlake, explained that the compatibility of CT with a fluoro-based simulator would allow for improved imaging of target areas, while not requiring the patient to be moved to a dedicated CT unit to achieve the same level of imaging. Further, with the combination of a digital fluoro simulator and CT, the field verifications, source-to-skin distances (SSDs), and multi-leaf collimator (MLC) projections all could be correlated with a CT scan and edited accordingly-versus using fusion techniques that can introduce error. Finally, the patient positions (or patient size) commonly seen in radiation therapy that could prohibit the ability to use a CT for treatment planning would be able to be performed using the CBCT, allowing for more conformal techniques to be employed for those patients.
 |
| Overlake Hospital’s brachytherapy suite features Varian’s Acuity, which helps to maintain a smooth workflow. |
CBCT for treatment planning is important for certain applications, according to Boyer. In particular, hypofractionated treatments on a linear accelerator equipped with CBCT allow the position of the target volume to be verified before each treatment. Since there are a small number of treatments, and since each treatment can tolerate very little error, CBCT is essential in this setting. Acuity can then be used to acquire a reference CBCT as part of the planning process prior to treatment, which allows the patient to be positioned and scanned with the same digital system that will be used to position and scan during the therapy procedure. These CBCT data sets become the reference to which the CBCT acquired on the treatment machine is compared. Having identical geometry and file formats facilitates this process. In more conventional settings, the CBCT acquired by Acuity can be used to provide reference digitally reconstructed radiographs (DRRs) of exceptional quality as reference images for conformal and intensity-modulated radiation therapy (IMRT).
Acquiring CBCT with Acuity has the advantage that the clearance of a woman’s arms and elbows is greater than is available in the bore of a CT scanner. The greater clearance on Acuity will allow for more comfortable patient positioning in this case. Whether Stanford will continue to use conventional CT or exclusively adopt CBCT on Acuity requires further study.
Simulation and Brachytherapy
Brachytherapy is a complex treatment that is used to treat many types of cancer, including prostate, breast, lung, and esophageal. Radioactive seeds are placed in or near the tumor, giving a high radiation dose to the tumor while reducing the radiation exposure to the surrounding healthy tissues.
Acuity is uniquely adapted to brachytherapy positioning and imaging. Distortion-free digital fluoroscopy images are useful aids in catheter and seed placement as well as verification. Images are accessible through Varian’s BrachyVision brachytherapy planning system and can be used immediately for treatment planning. These images also can be exported via DICOM RT to other brachytherapy planning systems.
Many hospitals do not specifically purchase Acuity for its practical use of brachytherapy. Rose Guerrero, director of cancer services at Overlake Hospital, says that without Acuity, the facility would have to move the patient from CT/Sim to HDR or purchase a C-arm, which would be very disruptive to operational flow and would not provide the additional flexibility or features currently available on Acuity.
Overlake Hospital initially considered purchasing a refurbished simulator; however, it would not have been able to integrate it with other departmental machines and functions. After seeing Acuity in action at Emory Crawford Long, it became clear to Guerrero and her team that Acuity was a better long-term and comprehensive solution. Overlake’s facility was designed for a dedicated HDR unit, and the room design accommodates both Acuity and the HDR. The design also enabled three vaults to be built-two linear accelerator vaults and a third vault to house the HDR and Acuity. The convenience and efficiency of having a dedicated simulator has enabled Overlake to launch its MammoSite program smoothly. With this dedicated vault, the department was able to practice simulations prior to working with the first patient, which enabled the staff to learn processes, timing, and setups on the machine. With a dedicated simulator, staff members are able to freely schedule and treat HDR patients. The process is seamless and time effective for the department, physicians, and patients.
Stanford also has a dedicated vault. A slanted conduit through the shielding provides access to the adjacent control area, which also includes a treatment-planning suite for HDR. The digital planar-view images can be directly transferred to an HDR planning system that is equipped to use them. There also is considerable potential for using cone-beam CT for planning and delivering HDR treatments.
Having the patient on the Acuity treatment table throughout the imaging, applicator placement, planning, and treatment delivery was an essential feature when developing the treatment suite at Stanford. Because even the smallest shift in applicator position can have considerable dosimetric consequences if the patient is moved between planning and treatment delivery, carrying out the procedure in a single room simplifies quality assurance.
Before the HDR suite was built, Stanford’s HDR planning was carried out in one room shielded for diagnostic X-rays, and delivery was performed in a linear accelerator vault, which meant that external beam treatment time for several patients was lost in order to treat each HDR patient. The scheduling problems were considerable.
The Role in Image-Guided Radiotherapy (IGRT)
Stanford Cancer Center intends to integrate CBCT with PET/CT and MRI target volume localization. The availability of digital images free of geometric distortions lends itself to a number of combinations of both planar and CBCT imaging acquired on Acuity with new research into molecular targeting therapeutics as well as diagnostic agents, according to Boyer.
 |
| 3-D verification of planning CT and CBCT is a significant part of the IGRT process. |
At Stanford, hypofractionated prostate cases are now treated with gold markers implanted in the prostate, as soft-tissue visualization of the prostate margins is impossible with planar imaging. With CBCT on Acuity and on the treatment machine, the implantation of the markers may be avoided. Similarly, gold markers are used in hypofractionated lung and pancreas protocols. The combination of CBCT on Acuity and the treatment accelerator can obviate the additional surgical procedures associated with inserting these markers in the patient, Boyer notes.
Stanford Cancer Center is in the process of installing RPM systems on all accelerators, scanners, and simulators. Motion gating is already in use for breast and some lung treatments. It will be particularly important for hypofractionated lung treatments, in which the total dose delivered during each session is too large to allow extra lung tissue to receive a high dose or for any of the target volume to be underdosed. This area is also one of active research.
NNUH believes that Acuity will be a significant component of its IGRT initiative. In the UK routine, IGRT on the treatment machines would be untenable with current waiting lists, according to Tomes, but 3-D verification might be the next best thing. Tomes and her colleagues have been assessing the effects of motion due to swallowing and respiration for some time. They currently assess this motion using fluoroscopy and saving a movie loop of the motion before the CT acquisition. Analyzing the movie loop gives the clinician an idea of what margins are required when defining the volumes. They have not yet implemented gating procedures.
Acuity works with Varian’s RPM Respiratory Gating System to help physicians plan and verify treatment techniques based on the patient’s respiration motion data. With gated fluoroscopy simulation, Acuity obtains digital images of the movement of internal anatomy, while the Respiratory Gating System records the patient’s external breathing pattern and range of motion. Acuity features Varian’s Exact Couch for accurate patient positioning and immobilization. Using the same patient-positioning system for planning, simulation, and treatment significantly reduces setup times and chances of error in treatment. Acuity automatically captures table parameters in simulation, and eliminates additional data entry on the Clinac. All QA procedures can be done on Acuity with the Exact Couch, saving time on the treatment unit.
In conclusion, Acuity is used in a number of different ways to optimize the clinical process. All sites agree that in meeting the specific needs of their departments, Acuity offers a high degree of flexibility. Whether simulating for palliative treatment, planning for brachytherapy, verifying IMRT setup reproducibility, or setting up reference images for IGRT, Acuity is the solution.
Laura Gater is a contributing writer for Medical Imaging.
Using PET/CT for simulation
The addition of PET to the CT study allows the physician to better define the gross treatment volume (GTV) to better treat a patient’s disease. “PET/CT has definitely impacted our ability to accurately perform treatment planning for tumors in the head, neck, and lung regions,” says Ashish Chawla, MD, of the Anne Arundel Medical Center (Annapolis, Md). “By determining both anatomic and metabolic information about these lesions, and fusing this information using specialized software for treatment-planning purposes, we can be more confident that all of the active tumor is well within our radiation portals and will receive the full dose required for cure.” The clinical intent of PET/CT has changed such that the initial treatment volume will better define the regional extent of risk. According to Paul Schilling, MD, of the Community Cancer Center of North Florida (Gainesville, Fla), the use of hybrid imaging has changed clinical intent in some cases because: A) some patients are found to have metastatic disease, so they might not need radiation therapy; or B) the intended schedule of radiation therapy would be different. PET/CT treatment planning changes patient management in two important ways. It gives physicians important information about the clinical stage of the tumor. For example, PET/CT is a highly accurate method for staging the extent of lymph node disease in lung cancer, which can dictate whether or not the patient is a good surgical candidate or would be better served by treatment, such as radiation and chemotherapy. Second, PET/CT can change management of the local tumor when radiation fields are altered in response to metabolic information about the primary tumor site. PET/CT also provides information crucial to dose escalation. The fact that the PET and CT data sets have the same DICOM origin accomplishes the data set registration. These sets are sent to the treatment planning system-in this case, the Varian Eclipse, which recognizes their common DICOM origin and gives them the tools to display the “fused” data sets for GTV contouring. “We have seen with this dual-modality approach to simulation that our treatment plan has modified from what we would have done without the PET information,” says Michael McCullough, PhD, clinical medical physicist at Anne Arundel Medical Center. -LG |
Respiratory gated CT imaging in simulation
Using GE Healthcare’s Advantage 4D CT with Varian Medical Systems’ RPM respiratory gating system for simulation and planning has impacted radiation therapy planning by giving oncologists a glimpse into future applications and technology. Physicians now clearly understand how internal organ motion takes place in the chest and upper abdomen, and they can plan accordingly. The technology also has helped physicians change their clinical planning intent by providing improved fidelity in anatomic visualization free from helical motion artifacts. With 4-D CT simulation and planning, physicians are able to obtain a gated CT scan and then retrospectively query the movement of a tumor and/or normal tissue inside the body.
The thorax and upper abdomen are obvious candidates for 4-D. Lung cancer in lower lobes shows tremendous movement with respiration. In short, any organs with motion resulting from the patient’s respiration are candidates for 4-D CT. Physicians have learned that a tumor in the abdomen and thorax moves with respiration motion in all three directions. The ranges could be as large as 2 cm-3 cm, which is very significant in terms of planning, notes Andrew Wu, PhD, of the University of Pittsburgh Medical Center (UPMC of Shadyside, Pa). According to Carl Nuesch, MD, of the Texas Oncology Center-Balcones (Austin, Tex), “One can no longer look at an axial CT image and believe that the tumor is totally static. Breath-holding techniques have major pitfalls as well-some patients cannot hold their breath for an appropriate duration.” Wu’s greatest challenge is trying to obtain a patient’s regular breathing pattern. Currently, he uses the voice coaching method. “We teach patients to follow the voice instruction to breathe regularly; however, it is difficult to have a patient breathing regularly without the patient getting hyperventilated,” he says. “To overcome this problem, we have designed a system so that patients will be able to self-regulate their breathing by watching their own breathing pattern.” -LG |
