Volume 52, No. 30
'Beam shaper' sharpens brain surgeons' aim
By Lillian Kim
Neurosurgeons at Emory Clinic can attack brain tumors more effectively and safely than ever, due to a new device that allows them to precisely contour high-energy radiation beams.
By doing so, this "beam shaper," called a "micro-multileaf collimator," can attack irregularly shaped brain lesions, sending the maximum possible amount of radiation into the lesion while minimizing the radiation that affects surrounding normal tissue.
This is especially important in brain surgery, when lesions may be near or wrapped around critical nerves such as the optic or auditory/facial nerves. Indiscriminate radiation can damage these nerves, reducing their function.
"This system allows us not only to treat tumors that are spherical in shape but also tumors of irregular shapes, which are by far the majority," said neurosurgeon Nelson Oyesiku, assistant professor of neurosurgery.
The clinic currently is the only medical center in the Southeast that possesses the beam shaper technology, which received federal approval in 1997.
The use of radiation as a surgical tool, termed stereotactic radiosurgery, has been in practice since the 1940s, and today more than 30,000 people worldwide undergo radiosurgery each year for cranial tumors. Neurosurgeons favor stereotactic radiosurgery for selected conditions because it offers an effective, noninvasive alternative to conventional brain surgery, which requires opening up the skull. The procedure is applied to a variety of brain tumors such as meningiomas, acoustic neuromas, metastatic tumors and pituitary tumors. It also is used for certain vascular malformations of the brain.
Traditional radiosurgery does involve risks, since it employs a circular beam of radiation that can at best approximate a lesion's shape. By comparison, the micro-multileaf collimator consists of 52 individually motorized tungsten "leaves" that can be positioned to create a radiation beam of any shape, aimed from any angle.
Emory neurosurgeons use the micro-multileaf collimator in concert with another technological innovation, the ExacTrac, to offer fractionated stereotactic radiotherapy--a technique that divides a single radiation treatment into several treatment sessions, allowing the neurosurgeon to limit the risk to surrounding normal tissue. The ExacTrac also allows precise repositioning of a patient undergoing a series of radiation treatments, ensuring that radiation is consistently aimed at exactly the same place.
"Say a patient comes in for treatment on Monday," Oyesiku explained, "The ExacTrac will remember the exact position the patient was in when he comes in for treatment on Wednesday, so that the lesion can be found again and the radiotherapy machine will direct the beams specifically to the same position."
In conventional radiotherapy, permanent skin markers such as tattooed dots are used to realign a patient. A surgeon using the ExacTrac system instead attaches several "body markers," detectable by CT scan, to multiple points on the patient's body. Just before treatment, the patient lies supine on a comfortable vacuum cushion "couch" that automatically shifts him or her into the precise position used in the last treatment session. Two infrared cameras continuously track the body markers to maintain correct orientation.
With the ExacTrac, patients can be consistently repositioned to within 1.5 millimeters; without this technology it is impossible for patients and their physicians to precisely recreate the patient's position on the treatment table, according to Oyesiku.
This is particularly vital in stereotactic radiosurgery, which depends on a high degree of precision. The ExacTrac also can be used to treat cancers outside the brain.
The micro-multileaf collimator and the ExacTrac are both manufactured by BrainLAB, a company headquartered in Munich, Germany, that specializes in neurosurgery and radiation therapy systems.