5 Things You Need to Know About IGRT

More than half of patients with cancer require radiation in the course of managing their disease, according to the National Cancer Institute. Treatment plans have increased in complexity to meet the challenge of irradiating malignant tissues while sparing adjacent normal anatomy. And because the human body is dynamic and in constant motion, a radiation beam may or may not precisely match the target tumor.

That’s where image guided radiation treatment (IGRT) comes in — to offer sophisticated dose distribution plans carefully mapped before treatment begins and are verified on a daily basis. Equipment manufacturers and clinical thought-leaders have developed innovative approaches to the challenges presented by treating a moving target and propelled radiation therapy into new capabilities.

So when it comes to IGRT, the essential ingredients are daily targeting for the most accurate treatment, cone-beam CT to improve workflow, precise respiratory gating to track tumor movement during therapy, multimodality imaging that provides image guidance and precision in guiding radiosurgery.

1. Daily targeting with low-dose, hi-res images

“We are now doing real-time image guidance, or online image guidance,” explains Tim Fox, PhD, director of medical physics in the Emory University department of radiation oncology. In the past, once the initial planning images were completed and a treatment plan devised, technologists would obtain weekly megavoltage (mV) portal images to verify where treatment was delivered. Megavoltage energy, used for treatment purposes, produces cloudy images, where kilovoltage (kV) images are clear and show contrast between soft tissue and bone. Additionally, kV images generate a lower radiation dose to the patient.

Linear accelerator manufacturers have integrated kV x-ray units onto their basic treatment machines so that clinicians are able to determine exactly where the tumor is located each day when the patient is positioned for treatment.

For example, both of the Varian Medical Systems linacs — Clinac and Trilogy —feature the On-Board Imager (OBI) automated system to obtain high-resolution x-ray images to identify precise tumor location while automatically adjusting patient position when necessary. By combining low-dose, high-resolution kV x-ray imaging with integrated software control of treatment parameters, this system in intended to increase accuracy while facilitating efficient patient throughput. The OBI dynamic targeting IGRT is available as an upgrade for most Clinac systems currently installed.

“I think IGRT is most useful when daily targeting is critical,” asserts Hoke Han, MD, radiation oncologist at Hollywood Radiation Oncology in Florida. He explains that studies have shown that when the dose is escalated, the cure rate increases in prostate cancer treatment; however, complication rates can increase as well without careful targeting. Clinicians must target daily to insure that the prostate is exactly in the center of the field and therefore minimize radiation dose to the bladder and rectum.

Using the BrainLAB ExacTrac X-ray 6D automated IGRT system coupled with im-planted fiducial markers, high-resolution x-rays pinpoint internal tumor position immediately prior to treatment. The system will robotically correct patient set-up discrepancies, and track patient movement throughout the treatment. Han reports that BrainLAB is working on a respiratory gating capability for this system that can be installed on all existing linacs.

James Rubenstein, MD, medical director of Radiation Therapy Services headquartered in Ft. Myers, Fla., uses several different radiation treatment systems throughout their 66 centers in 14 states. Clinicians in their centers use an array of imaging modalities for initial targeting activities, and they have used implanted radio-opaque fiducials to guide treatment.

Rubenstein expects development of radiofrequency emitters to be implanted as fiducials so that a linac could be set to track the radiofrequency beam. “You could have those fiducials with dosimeters in them, so that if a fiducial lies outside the beam field, you would know that.” He believes there are prototypes for RF emitters and diodes in development, some awaiting FDA approval.

2. Cone-beam CT

In addition to planar x-rays, many treatment centers perform cone-beam CT images. Fox describes the process as taking the OBI system, rotating it around the patient and using the generated images to reconstruct a tomographic data set. With this approach, images can produce a three-dimensional anatomic section that is 14 to 17 centimeters wide. While radiographic imaging requires about three minutes to perform, the process with cone-beam CT almost doubles the time required because they need to rotate the entire gantry around the patient and then wait for image reconstruction.

Clinicians frequently are concerned that including IGRT in the treatment process will negatively impact throughput.Fox and his colleagues developed home-grown data mining software to examine clinical efficiency issues of IGRT performed on 2,700 patients using the OBI system installed on their Varian Clinac 23EX treatment system, and published their results in the June 2005 issue of the Journal of the American College of Radiology. While they ascertained there was a learning curve of approximately one month for this new approach, they found workflow efficiency improved. 

Bichan Micaily, MD, FACR, professor and associate chairperson for radiation oncology at Temple University Hospital describes the imaging process for IGRT, using their Elekta Synergy S linac with the XVI kilovolt radiographic imager. After obtaining initial treatment planning images using a variety of imaging modalities that may include CT with or without PET and/or MRI to define tumor dimensions accurately and establish bony landmarks for future location activities, they transmit those image data sets to a Pinnacle planning system, and once a plan is devised, to the Synergy S. In addition, they use a laser system to line up the patient for treatment. Bony landmarks provide definition of the target area.

This approach has proven valuable to performing IMRT because the Synergy S offers a multileaf collimator with 4mm leaf thickness at the isocenter.

“You want the target in the isocenter of the beam,” Micaily says. When they have made necessary changes, they take another CT image (with the XVI imager attached to the linac) and co-register data sets again to make sure they have targeted accurately. “Generally after only one shift, you’ll be fine.”

Roxana Taveira, BS, CMD, RT(T)(R), who serves as manager of radiation oncology at Temple University Hospital, says that they treat between 75 and 85 patients a day on two Elekta platforms and the Elekta Synergy S with multileaf collimator installed in September 2005.

On the IT side of the equation, Taveira recommends establishing a robust network and storage capabilities for managing the image data sets. They have 2 to 3 terabytes (TB) of available storage space since patients may receive daily treatment for up to eight weeks.

At William Beaumont Hospitals in Royal Oak, Mich., corporate chairman of radiation oncology, Alvaro Martinez, MD, FACR, describes their use of IGRT for treatment of tumors in the head, neck, prostate, breast and lung. Their Elekta Synergy linac employs kV imaging which they use to perform cone-beam CT. After the initial planning phase, they are able to track motion of tumors and then adapt the plan and re-plan to develop the smallest margin possible around the tumor.

When asked about whether or not daily kV imaging is required, Martinez says it depends on individual patient condition. Those who are easy to align in the linac and who exhibit little internal organ motion or organ deformation would not require as frequent imaging as those who have a less stable clinical situation. 

While this process works quite well for tumors in rigid areas where there is not much motion, such as the head and neck or retroperitoneal areas, tumors in soft tissue may require another approach.

3. Respiratory gating

Shalom Kalnicki, MD, FACRO, professor and chairman of the department of radiation oncology at the Montefiore Medical Center and Albert Einstein College of Medicine in the Bronx, explains that they use IGRT for treating lung cancer. First, they obtain a CT scan with a 4D scanner that creates a cine-CT loop to analyze movement of the tumor as correlated with the respiratory cycle using an external beacon taped to the patient’s sternum. A system of infrared beams and cameras produces the respiratory cycle information that is integrated with PET/CT image data sets. 

“We fuse the image data set and look at movement and deformation of the target. Then we select the phase of breathing where the target is most stable and has the least deformity,” says Kalnicki. Their Varian Trilogy linac is equipped with RPM respiratory gating functionality. “We establish when the gate will be open and the beam is on, and when the gate is closed and the beam is off.” They use the PET/CT image data sets to provide separation between tumor and collapsed lung, better visualization of the involvement of lymph nodes and assessment of areas within the tumor and lymph nodes that have a different metabolic rate that could require a higher radiation dose.

Besides the obvious advantage of respiratory gating for lung tumors, Kalnicki explains this approach is beneficial in treating tumors of the liver, pancreas and stomach because they are located directly under the lung fields and the diaphragm, so they move as well.

John Buatti, MD, professor and head of the department of radiation oncology in the University of Iowa Center of Excellence in IGRT uses Siemens Medical Solutions technology throughout the imaging and treatment chain. They use the Somatom Sensation Open large-bore CT which includes support for respiratory-gated CT, a Magnetom Trio 3T MRI system, four Oncor linear accelerators, one of which is in a special configuration that is well-suited for stereotactic radiosurgery, and are expecting delivery of a biograph PET/CT scanner in the coming months.

Buatti notes that while respiratory gating may not play a large role in diagnostic activities, it is quite beneficial for targeting. The downside is an increase in the number of images from a couple hundred to a couple thousand. In their radiation oncology department, they have a several- terabyte server because they must keep images for several months after treatment has been completed. Images are kept offsite in the central IT department of the hospital.

He expects that in the future, these activities will develop into image response-guided therapy. Through capturing multiple sets of images throughout the course of treatment, they would be able to detect changes in the tumor. For example, if a portion of the tumor has been destroyed in the early stages of treatment, they could possibly reduce the treatment area.  While Buatti stresses the importance of having additional information, he also recognizes that it will obviously add to the number of images that must be managed.

Rubenstein concurs that adaptive radiation therapy is on the horizon. This dynamic planning scenario would take changes in the tumor bed into account, such as a patient who develops ascites during treatment who requires plan modification.


4. Multimodality IGRT

Joseph Motta, MD, FACS, director of urology, laparoscopic and minimally invasive urologic surgery at St. Vincent’s Hospital in Staten Island, N.Y., employs brachytherapy or radioactive seed implantation for patients who have failed radiation therapy for prostate cancer. They have used a GE LightSpeed VCT 64-slice CT, fluoroscopy, real-time x-ray imaging and transrectal ultrasound to fine-tune seed placement in a surgical implantation procedure. They use the Bard Palladium 103 and Iodine 125 seeds placed through a hollow needle, in conjunction with computer-generated image guidance via VariSeed software that renders ultrasound images in real time.


5. Critical accuracy in IGRT

Perhaps nowhere is image guidance more essential than when employed in radiosurgery. Using a CyberKnife Robotic Radiosurgery system that is used to treat tumors anywhere in the body with sub-millimeter accuracy, Gregory Gagnon, MD, CyberKnife program director and acting chairman of the department of radiation oncology at Georgetown University explains the system is a different approach to radiation therapy. Considered “non co-planar” the robotic arm has six joints, just like a human arm, so that it can be pointed at a target from any angle. This enables the system to generate up to 1,000 radiation beams delivered from any angle.

“The robot is very precise, and we’ve measured the pointing precision at 92 microns, which is the thickness of a human hair, or thereabouts,” says Gagnon. “It takes a 500-pound accelerator, and places the beam within the thickness of a human hair in precision over and over again, without a lot of scatter, so they aren’t ‘fuzzy’ on the edges.”

Not restricted to head and neck tumors anymore, and no longer requiring a stereotactic frame, they have used this system to treat tumors throughout the body, including lung tumors. About half of the tumors they treat are outside of the head.

While regular treatment might require six weeks of multiple fractions, with this system, they are capable of completing treatment within three days. “Since there are 1,000 beams, each one will carry 1000th of a dose, so you can give a high dose very quickly.” The amount of radiation to the skin would be comparable to that produced during a CT scan. “We can treat three or four different tumors in the same patient in the same day, and we rarely get any failures in the area we treat.”


Conclusion

Image guidance has become integral to the process of delivering radiation treatment. With the advent of IMRT and radiosurgery, its importance gained urgency. The use of daily high-resolution imaging at treatment time has gained significance in managing the demanding aspects of dose distribution.