Physical Aspects of Stereotactic Radiosurgery
Jeffrey A. Fiedler, MSc
Gamma Knife Center, Barrow Neurological Institute, Mercy Healthcare Arizona, Phoenix, Arizona
Stereotactic radiosurgery is a demanding treatment modality that depends upon the accurate and precise geometric and dosimetric delivery of potentially damaging levels of radiation. To ensure the availability of safe, reproducible, and clinically efficacious treatment, all of the equipment utilized must be thoroughly examined and continually tested to characterize and monitor its performance, as well as to detect problems as far in advance as possible before patient care might be affected. Medical Physics is the specialty and field of practice responsible for technical matters related to stereotactic radiosurgery. This article briefly describes the education, clinical training, and experience of a typical Medical Physicist. How this knowledge is applied to transform physical formulae, mathematical models, and technical specifications into a safe, accurate, reliable, and relatively simple noninvasive procedure for the treatment of deep or inoperable intracranial lesions is emphasized. A description of a typical Gamma Knife treatment is outlined to provide the reader with a glimpse of Gamma Knife procedures.
Key Words: Gamma Knife, medical physics, stereotactic radiosurgery
Initially conceived as a noninvasive alternative to open neurosurgery,8 stereotactic radiosurgery has evolved into a therapeutic modality now used as both postoperative adjuvant treatment and to boost radiation doses in conjunction with conventional external beam radiation therapy.6 Rapid advances in technology, especially those in computing, have profoundly affected procedures followed in the delivery of stereotactic radiosurgery.2 Thorough acceptance-testing and commissioning of the equipment, its continuous monitoring, and evaluation of the device’s performance and beam characteristics are critical to accurate delivery of the high-dose levels used in radiosurgical techniques.
|Figure 1. Flowchart representation of the process established for physicians applying to be listed as Authorized Users under the medical teletherapy license obtained for the Gamma Knife operated at the Barrow Neurological Institute of St. Joseph’s Hospital and Medical Center. GK=Gamma Knife, STX=stereotactic.
This article briefly describes each of the physics-related aspects of licensing, commissioning, and operating a Gamma Knife. Selection and use of equipment as well as analysis of data are discussed to provide the reader an overview of some of the medical-physics issues dealt with in establishing a Gamma Knife-based stereotactic radiosurgery program.
Role of Medical Physicists
Physicians who practice stereotactic radiosurgery usually lack the necessary training in mathematics and physics, the technical expertise, experience, dedicated specific technical interest, and time required to comprehensively perform labor-intensive equipment and beam testing. The person assigned these duties must be thoroughly familiar with medical applications of ionizing radiation in addition to having an educational background that provides a high level of facility in the subject areas outlined above. Medical Physicists specializing in therapeutic radiological physics, who usually have graduate degrees in biomedical engineering, nuclear engineering, nuclear physics, medically applied physics or biophysics and experience with the procedures and operations of radiation oncology clinics, easily satisfy these requirements. Many Medical Physicists take examinations analogous to those completed by physicians to attain certification in their subspecialty area of practice from the American Board of Medical Physics (ABMP), the American Board of Radiology (ABR), or both. These credentials are important in states that license the practice of Medical Physics since licensure and hence the ability to work in the profession may be denied without them. Certification by the ABR, a medical specialty board that also certifies physicians, confers eligibility for full membership in the American College of Radiology and results in a listing in the American Directory of Medical Specialists.
The role of a physicist involved with any stereotactic radiosurgery program may be defined as comprising four major areas: (1) ensuring the safe, proper operation of radiation-emitting devices and all ancillary equipment used in the design and delivery of radiation treatment, (2) collecting and analyzing all relevant radiation-dose data used in calculating radiation-dose distributions and corresponding exposure times, (3) designing treatment plans according to directions provided by the attending neurosurgeon and radiation oncologist, and (4) monitoring the implementation of machine settings during the delivery of treatment to assure compliance with the geometric/timing parameters specified in the treatment plan.
Federal or state agencies require detailed procedures describing how each of these tasks will be achieved. Before any institution is issued a radioactive materials license for medical use of the Cobalt 60 (60Co) source supply associated with any Gamma Knife, documentation of specialized training in the application of ionizing radiation using stereotactic techniques is also required for each medical practitioner who wants to prescribe radiation doses or to be directly involved in the delivery of treatment (Fig. 1). The task of collecting, collating, writing up, and submitting all of this information in the appropriate format to the government agency in charge thus also becomes an important part of a physicist’s duties. This administrative/regulatory aspect of the physicist’s job never ends, since actual licensure marks just the beginning of a whole series of processes directed at maintaining compliance with the terms of the license, for both the site and all authorized users.