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December 6, 2004

Lung Cancer - A Pittsburgh Rad Therapist’s Experience With the Latest Stereotactic Radiosurgery
By William Vogel, RT(R)
Radiology Today
Vol. 5 No. 25 Page 22

Editor’s note: William Vogel, RT(T), is lead radiation therapist in the stereotactic radiosurgery program at the University of Pittsburgh Medical Center Shadyside. This article is a firsthand account of his experience on the stereotactic surgery team there.

What originated as a technique to treat tumors in the head and neck with submillimeter accuracy is becoming an extracranial radiosurgery and radiotherapy tool.

Stereotactic radiosurgery is a complicated procedure involving many hours of preparation for both the radiosurgery team and the patient. As oncology teams improve the ability to account for the body’s natural breathing motion, the technology is steadily becoming an effective tool in the war on lung cancer.

Several different techniques are currently being incorporated into treatments to help compensate for respiratory motion. Gating has become more common, but even the most advanced gating system can compensate for only the anterior/posterior movement of the chest.

The new Synchrony respiratory tracking system for the Accuray CyberKnife allows for the compensation of diaphragmatic motion as well. At the University of Pittsburgh Medical Center (UPMC) Shadyside, we use one of the original prototype CyberKnife Stereotactic Radiosurgery systems. The CyberKnife is a 6-megavolt linear accelerator attached to a robotic arm guided by a state-of-the-art computer and near–real-time imaging systems. The CyberKnife was originally developed and used for brain lesions but several years ago was upgraded with a fiducial tracking system. With the ability to track fiducials—tiny gold landmarks imbedded in tissue surrounding tumors—we have been able to treat many extracranial sites. There are presently 15 facilities in the United States that use the CyberKnife, as well as two in Japan, and so far the treatment outcomes have been dramatic.

Our CyberKnife has been here for more than 10 years and has served us well. We have treated more than 600 patients to various anatomical sites with submillimeter accuracy. Our success rate with spinal lesions led us to develop a lung program, but unlike a lesion involving a vertebral body, a lung lesion moves significantly. We have found that depending on the lesion’s location, it can move up to 3 centimeters in any direction with each breath. Using a breath-holding method originally conceived by our physicist, we have successfully treated more than 50 cases. But for every case that has arrived at our door, I’m sure several others were not so fortunate. Although this breath-holding technique provides precision, it can be more than a patient with lung cancer can endure.

Screening Candidates
Many factors are considered during the initial consultation between the radiation oncologist and thoracic surgeon. If the tumor is small enough and/or not located near critical anatomical structures, the patient may be a good candidate for conventional surgery. But if the conditions aren’t advantageous for surgery, radiofrequency ablation becomes an option. Generally speaking, only lesions considered inoperable are considered for CyberKnife radiosurgery—and then only if conventional radiation or chemotherapy has not succeeded in the past.

The most crucial screening factor may be whether the patient has the ability to withstand breath holds. Holding your breath for 15 seconds seems easy enough; now imagine doing it 150 times in one-minute intervals to complete a typical CyberKnife lung treatment using the breath-hold technique. Don’t forget you’d be lying on your back with your arms extended above your head for four hours straight. Also consider the issues common among lung cancer patients: diminished lung capacity, emphysema, and a constant supply of portable oxygen. Most of our patients have difficulty breathing after being lifted from their wheelchairs.

In our facility, we treat only what many consider worst-case scenarios, such as a patient with a tumor lodged against a critical structure such as the heart, or a patient in which spinal cord radiation tolerances have been reached. The radiation oncologist and thoracic surgeon weigh all such factors, which doesn’t necessarily make the patient a candidate for CyberKnife treatment, but allows him or her to continue further into the process.

Placing Fiducials
For the CyberKnife to be able to track a patient’s movement, gold fiducials must be placed within 4 centimeters of the lesion. Fiducials are tiny gold seeds approximately 0.8 X 4 millimeters. These fiducials serve as radiographic landmarks for the CyberKnife computer’s image-guidance system. Generally, the thoracic surgeon will place four fiducials in a diamond pattern around the lesion using CT guidance. With the four fiducials, the CyberKnife has the ability to track patient motion to 1/10 millimeter in six directions.

Placing these fiducials requires a skilled hand. If not handled delicately, a patient often develops a pneumothorax requiring a chest tube to be placed and another day of monitoring in the hospital. Also, if a pneumothorax develops while placing the first fiducial, the patient will have to wait for the lung to heal before repeating the process.

Approximately one week after the fiducial placement, the patient returns to have an immobilization device made and to undergo a CT simulation for treatment planning. Allowing one week between placing the fiducials and planning treatment is crucial because the CyberKnife tracks not only the individual fiducial but also the spatial relationships between the fiducials. At Shadyside, we have found that since the accuracy is to 1/10 millimeter, even a small amount of inflammation can affect the spatial relationship of the fiducials. Each set of fiducials has its own unique pattern after placement. If you do not wait for the swelling to subside, the pattern will not match and the system won’t be able to track them.

Unlike conventional immobilization devices used in radiation therapy, an immobilization device for CyberKnife treatments must also consider the patient’s comfort since treatments generally range between two and six hours. The CT exam must include at least 200 slices at 1.25 millimeters to image the treatment area.

The CT images are transferred to the CyberKnife computer system where the radiation oncologist outlines the treatment volumes and critical structures and prescribes a dose accordingly. The physicist then locates and marks the fiducials in the computer, allowing the image-guidance system to know exactly where the fiducials are located in respect to the tumor volume and nearby critical structures. The CyberKnife computer then develops a treatment plan according to the information it has received.

Unlike conventional radiation therapy treatment planning, the CyberKnife uses an inverse planning system, which allows the computer to pick the direction, distance, total number, and energy of the beams for treatment. These beams are referred to as nodes. A complex lung tumor can require between 150 and 250 nodes. To ensure accuracy, a breath hold is required for each of these nodes. And since the CyberKnife is not constrained by the usual isocentricity of conventional linear accelerators, the distance from the tumor and direction control the uniformity of the dose distribution.

Planning Treatment
Once the plan is completed, the patient returns for a simulation of the actual treatment. The patient is placed on the CyberKnife treatment table and maneuvered into position over the orthogonal CyberKnife imaging cameras. Several real-time images are taken and the patient is realigned into the exact same position as the initial CT. The CyberKnife computer system uses 6-D technology—right/left, anterior/posterior, superior/inferior, roll, pitch, and clockwise/counterclockwise—and all of these adjustments are needed within 0.5 millimeters. These images can be viewed on a computer at the therapist’s console. Once the patient is properly positioned, the simulation begins. A series of 30 images are taken and compared with the originals on the treatment console.

We have successfully treated more than 50 patients using this technique, but during these simulations we have found three patients who were unable to go through with the treatment process for various reasons.

One woman had traveled from New York and had been staying in a hotel during the entire treatment process. To look at her, you would never imagine she had stage IV lung cancer. In her case, a large mass had lodged directly against the arch of her aorta. CyberKnife was her only hope, but the tumor was so close to the heart that its beating caused the fiducials to move so erratically that treatment was impossible.

Imagine telling this woman, after all she had been through, “I’m sorry, but nothing can be done.” Thinking of her always makes me wonder how many people were rejected before they ever made it to this step. For every patient we have successfully treated, there had to be at least one who was unable to pass the initial consult because the person was unable to hold his or her breath for at least 15 seconds. This 15-second breath hold has become a mountain too difficult for most lung patients to climb, but with recent technological advancements, this mountain has been tunneled through and the opportunity of having a stereotactic radiosurgery treatment is available for all who need it.

This has all become possible with the development of Synchrony. Synchrony is a new dynamic image-guided targeting tool incorporated into the CyberKnife system. It allows for the robotic arm to move and compensate for motion during breathing. With Synchrony, treatments that once took more than four hours and involved a multitude of breath holds are now completed in approximately 45 minutes. A series of infrared markers are placed on the patient’s chest and abdomen. Each of these infrared markers directs a beam of visible light toward an array of Synchrony cameras mounted on the treatment room ceiling.

Reference Images
The treatment begins with the respiratory pattern being tracked by the Synchrony system. Then, as is done with the standard breath-holding technique, a series of orthogonal images are captured to locate the fiducials. The information from these two systems is immediately correlated to precisely locate the tumor and compensate for the patient’s breathing pattern. Throughout the treatment, the system automatically acquires new images and correlates them to the infrared motion, allowing for the adjustment of the patient’s breathing pattern.

Infrared detection has been used in the past for respiratory gating systems, but unlike conventional respiratory gating, Synchrony takes into account all lung motion. We have found through our breath-holding studies that a majority of the motion of breathing comes from the diaphragm. Four-dimensional gating accounts only for the anterior/posterior motion of the chest, but since CyberKnife tracks not only the infrared markers but also the gold implanted fiducials, the system compensates in real time for all motion. With this type of compensation, margin around tumor volumes can be greatly reduced, resulting in the treatment of less healthy surrounding tissue. That also allows for tumors adjacent to critical structures, such as the heart, to be given a much higher dose of radiation.

Accounting for respiratory motions expands the possibilities of using stereotactic radiosurgery to treat tumors in patients’ lungs. Our treatment consists of one fraction, in which we give 2,000 centigrays to the 80% isodose line. We have found this to be an effective tool, yet we have discovered with time several patients returning after various scans and tests for another treatment. The new protocol established for stereotactic surgery to the lung with Synchrony involves giving 6,000 centigrays in three fractions. That is three times the current dose and delivered effectively in almost half the time.

Imagine that you are given an option between being treated with wide marginal field, which would probably do some damage to normal healthy tissue, for 15 minutes per day over a six-week period. Or, you can be treated in three sessions and treating less margin and have the same treatment, sparing healthy tissues in three sessions. Which would you choose?

Granted, for most newly diagnosed lesions, a radiation oncologist would probably advise on beginning with a regimen of conventional radiation to ensure the irradication of lymph node involvement. But what if you could condense the treatment to three weeks of traditional radiotherapy followed by three fractions of stereotactic treatment? And what about those patients who have been treated conventionally and have had a recurrence? These examples illustrate the impact of stereotactic treatment advances.

There are currently several hospitals in the United States using the Synchrony system for the treatment of various lung and abdominal lesions. At Boulder (Colo.) Community Hospital, more than 80 patients have been treated with the new system. Most deliver a significantly higher dose in half the time of our standard breath-holding technique. We at UPMC Shadyside are slated to be completely operational with our new system by this coming April and hope to treat 200 lung cases in our first year with the upgraded system.

— William Vogel, RT(R), is lead radiation therapist in the stereotactic radiosurgery program at the University of Pittsburgh Medical Center Shadyside.

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