Targeting Tumors with Motion Control

Focused radiation continues to be an effective means for treating cancerous tumors, particularly when there is a clear shot at the target. Getting a clear shot, however, is no simple task, especially since the target is often surrounded by healthy tissue, can grow, shrink or shift over time, and is affected by changes in the patient’s anatomy.

10/01/2008


Focused radiation continues to be an effective means for treating cancerous tumors, particularly when there is a clear shot at the target. Getting a clear shot, however, is no simple task, especially since the target is often surrounded by healthy tissue, can grow, shrink or shift over time, and is affected by changes in the patient’s anatomy.

To address this challenge for clinicians, TomoTherapy Incorporated of Madison, WI, developed its Hi•Art treatment system, a radiation therapy device that effectively utilizes 3D CT imaging to ensure accurate treatments for patients on a daily basis. The Hi•Art treatment system was designed to leverage ring-gantry geometry similar to that used in CT scanning to deliver intensity-modulated radiation therapy (IMRT) from all angles around the patient, while minimizing side effects for patients.

Graham Reitz, research engineer at TomoTherapy, says, “Most cancer centers and solution providers now know the need for image guidance. Our co-founders, Rock Mackie and Paul Reckwerdt got together 17 years ago at the University of Wisconsin to mount a linear accelerator for the Hi•Art treatment system on a ring gantry. This allowed helical fan-beam delivery of IMRT, with integrated megavoltage CT imaging. This advance lets clinicians better focus radiation beams, and treat the most complex cases.”

“We were able to give spinal cord compression patients an additional 20 to 30 Gy using the advanced IMRT capabilities of TomoTherapy,” says Chester Ramsey, PhD, Thompson Cancer Survival Center, Knoxville, TN.

Ring-gantry positioning

Essentially, the patient lays on a table or couch that slides into the rotating gantry for continuous 360° delivery of helical IMRT using tens of thousands of narrow beamlets, all targeting the tumor and individually optimized to contribute to the total tumor dose. Handling the couch’s x-y-z motion is Galil Motion Control’s integrated DMC-2153 5-axis controller. Currently, the system places the table to within

“We considered several motion controllers for the Hi•Art system and Galil’s controller met our requirements for its Ethernet-based control of stepper and servo motors, ability to multitask, SSI feedback capability and robust programming language,” added Reitz. “It also had to be small enough to fit in our very small, allowed space.”

Galil’s PID Compensation feature controls the z -axis for handling the “up/down” motion of the couch. An integrated motor and stepper driver work in concert with the DMC-2153 to control the y -axis for moving the couch in and out of the bore, and the x -axis for right/left lateral movements. The DMC-2153 controller also accepts two forms of feedback—incremental and SSI absolute—to double check and maintain couch position should there be any disruption due to power loss, and to ensure precise synchronization of the x-y axis.

Reitz adds, “Along with integrated CT guidance, the two key differentiators of our systems over conventional systems are the ring gantry design for helical delivery, and our binary multi-leaf collimator for beam shaping and modulation.” He explains that since the width of the helical fan-beam IMRT projected to the axis is 40 cm, and that maximum couch length is 160 cm, very large volumes can be treated in a single, simple set-up. In fact, it is possible to treat anywhere within a cylindrical volume 40 cm in diameter by 160 cm long. Even larger diameter volumes can be treated with a reduced number of beams.

Typically, tens of thousands of beamlets are included in a precisely-optimized treatment. A single beamlet corresponds to the radiation emitted through an open leaf of the system’s patented multi-leaf collimator (MLC) with the gantry at any given angle, during any given rotation. The need for a large number of beam angles to achieve a highly-conformal dose distribution is much like the need for a large number of image projection angles in CT imaging.

Lane Rosen, MD, director, radiationocology at Willis-Knighton Cancer Center, Shreveport, LA, explains the benefits of this approach: “The system’s TomoImage capability gives you even better visualization and accuracy of patient positioning than the step-and-shoot systems, while the helical treatment delivery provides better coverage and conformality to the target than previous rotational IMRT systems.”

To accommodate TomoTherapy’s needs, Reitz says Galil customized the controller so it could easily accept all of the SSI devices for feedback. “We used lots of the I/O provided for various tasks, like machine shutdown, clutch status, and emergency stops. It also sends signals to its embedded computer to perform motion calculations while our system’s Linux computer calculates, coordinates and talks directly to the Galil controller to provide redundancy and increased safety.

Today, TomoTherapy has more than 150 Hi•Art treatment systems installed at clinics around the world, treating tumors that result in prostate cancer, lung metastasis, scalp carcinoma, multiple lesions and more.


Author Information

John Hayes is a Senior Application Engineer at Galil Motion Control. Contact him at johnh@galilmc.com .




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