Barrow_Quarterly_1997_Physical_Aspects_of_Stx_Radiosurgery

Barrow Neurological Institute
Barrow Quarterly - Volume 13, No. 1, 1997
Physical Aspects of Stereotactic Radiosurgery
Jeffrey A. Fiedler, MSc
Gamma Knife Center, Barrow Neurological Institute, Mercy Healthcare Arizona, Phoenix, Arizona
Abstract
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.
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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
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.

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.
Licensure
In the United States, the federal government has jurisdiction over all non-naturally occurring (reactor- and/or cyclotron-produced)
radioactive material. However, under the terms of Nuclear Regulatory Commission (NRC) regulation 10 CFR 150, state
governments (which have jurisdiction over all naturally occurring radioisotopes and all radiation-producing devices such as x-ray
tubes and accelerators located within their geographic boundaries) may enter into an agreement with the NRC to subcontract this
function and to administer their own programs for licensing radioactive materials. Arizona has undertaken to follow this route,
making it an Agreement State, with the Arizona Radiation Regulatory Agency (ARRA) assigned responsibility for relevant
licensing and radiation safety issues.
All ARRA license applications require documentation supporting the qualifications of each person who wishes to prescribe or
directly supervise the medical use of the isotope in question. Information concerning licensee self-regulation procedures, facilities
and equipment available on site dedicated to or available for use in conjunction with the proposed medical application, and/or its
associated quality assurance/radiation safety testing program(s) is also necessary. A complete set of diagrams and calculations
upon which the thicknesses of primary and secondary shielding barriers are based also must accompany the license application
for those spaces housing the applied-for amount of radioactive material. Data and specifications providing information such as
the total average and maximum source activity (emissions per unit time), construction, and housing of the radioactive source(s)
to be used also are mandatory.
For the Barrow Neurological Institute’s Gamma Knife, ARRA Medical Teletherapy License 7-424 allows medical use of up to
6600 Ci of 60Co activity (2.442 x 1014 decay events per second, each 1.25 MeV in average energy), with up to 9900 Ci of 60Co
permitted on site during reloading. This represents the highest amount of activity licensed for medical use at a single installation
in the state.

Figure 2. Examples of beam profiles measured for the Model B Gamma Knife at the University of Pittsburgh with
corresponding profiles taken from beam models constructed by this system’s treatment planning computer: (A) 4-mm
collimator, x-axis; (B) 4-mm collimator, y-axis; (C) 4-mm collimator, z-axis; (D) 18-mm collimator, x-axis; (E) 18-mm collimator,
y-axis; (F) 18-mm collimator, z-axis. The coordinate (100, 100, 100) represents the Gamma Knife isocenter. LGP = Leksell
GammaPlan. [Courtesy of Ann H. Maitz, MSc]

Commissioning
Prior to delivery and placement inside the Gamma Knife, each radiation source is wiped with a cotton swab, which is then
surveyed to confirm the integrity of source encapsulation. At the time of loading, a subset of sources is wipe tested again. When
all sources have been loaded, the transport casks and handling equipment used for storage or manipulation of radioactive
materials also must be examined carefully to ensure that the treatment site itself has not been contaminated inadvertently during
the loading procedure. The new, electrically operated Model B Gamma Knife arrives with its own portable, self-shielded loading
machine. The strict isolation provided by this loader significantly decreases the probability of any site contamination when
compared with the temporary hot-cell labs constructed on-site for loading of the hydraulically operated Model U Gamma Knife. It
also decreases the total time required for loading and reloading operations.
After a Gamma Knife has been fully
assembled and loaded, its manufacturer
(Elekta Radiosurgery, Inc., Atlanta, GA)
completes a variety of device inspections
and machine-function tests to ensure that it
is operational and working within the set of
stringent mechanical and geometric
tolerances specified for it at time of
purchase, and that a minimum dose-rate of
300 cGy/min† is available at the intersection
point of all 201 radiation beams originating
from the sources housed within.
Once the machine has been certified as
properly installed and operational by the
manufacturer, the Gamma Knife physicist
conducts a detailed and comprehensive
series of tests.9 (1) The dose-rate at the
beam-intersection focus is measured and
defined exactly, a procedure known as
calibration.12 (2) The dose distributions
delivered by each of the four different beam-
defining collimators provided with the unit
are fully characterized or mapped (each
produces a “sphere” of radiation dose near
4, 8, 14, or 18 mm in diameter, centered at
the beam-intersection focus). This beam
mapping is accomplished by measuring and
graphing radiation dose as a function of
position along each of the x (right/left), y
(anterior/posterior), and z (superior/inferior)
axes (Fig. 2). Graphs showing a select set
of equal dose points in each of the axial (x-
y), sagittal (y-z), and coronal (x-z) planes,
known as isodose plots, are also produced
(Fig. 3). (3) The relative output factors for
each of the four collimator helmets are
determined. The output factor is the ratio of
absolute dose delivered per unit time by a
specific collimator relative to the dose
delivered over the same period of time by
the largest available collimator (Table 1). (4)
Attenuation of the radiation beam by the
interchangeable collimator plugs (which may
be used for blocking of selected beam
channels to shield normal radiation-sensitive
anatomic structures during Gamma Knife
treatment) is measured. (5) The geometric
accuracy, precision, and reproducibility of
the mechanical attachments and
accessories used during stereotactic
irradiation—which define the treatment
coordinates—are confirmed. (6) The three-

dimensional spatial correspondence
between the beam isocenter, the maximum
dose point within the dose-distribution
pattern produced by each collimator, and
the geometric center of the Gamma Knife’s
source-holding hemisphere is evaluated. (7)
All of the data measured above are
correlated with those stored in the Gamma
Knife treatment-planning computer and used
to construct the beam models applied during
calculation of radiation-dose distributions for
individual treatment plans. (8) The accuracy
and linearity of the unit’s treatment timing
controls are verified. (9) All of the unit’s
safety features, including its entry door
interlock circuits, emergency treatment
interrupt and stop switches, patient audio
and visual communication links between the
treatment room and control area, and all of
the facility radiation monitors, are tested
thoroughly.
Each
of
the
tests
above
has
specific
problems
in
obtaining
correct
data
values
associated
with
it.
For
example,
most
ionization
chambers
used
for
conventional
external
beam
radiation
therapy
dose
measurements
are
so
Figure 3. Sample isodose (8-mm collimator) plots through the isocenter of the
University of Pittsburgh’s Model B Gamma Knife presented in the (A) axial, (B)
coronal, and (C) sagittal planes. Dose levels of 10%, 20%, 30%, 50%, 70%, and

large
that
they
perturb
any small cross-sectional area fields to
which they are exposed. This changes the
amount of internally scattered radiation the
measuring point would otherwise receive in
any tissue-mimicking medium used for
measurement purposes.3,4
Thermoluminescent dosimeters (TLDs),
devices that release absorbed radiation
energy in the form of light when heated, are
sufficiently small (i.e., 1 mm x 1 mm x 3 mm)
for the above effect not to be manifested.
Since they are approximately three times as
dense as human soft tissue, however, the
location of the effective point of
measurement, in terms of human soft-tissue
depths, is difficult to define accurately.
Diode dosimeters also are sufficiently small
not to change the effects of any field of
radiation in which they are immersed.
Diodes tend to decay with increased
accumulated radiation exposure and are
sometimes temperature-sensitive, which
makes them, at best, reliable only for
relative (ratio mode) dosimetry
measurements. X-ray sensitive film is widely
used and well suited to obtaining beam
profiles and isodose distributions. The
specific dose-responsive curve of each
batch of film used must be known before
any meaningful dose data can be obtained
from the optical density patterns of films
exposed for beam characterization
purposes.5
The 201 60 Co sources of the Gamma Knife
are almost evenly distributed in five rings
constituting part of a hemispherical shell
inside the unit (Fig. 4).
The radiation emitted from each source is directed toward the center of what would be the corresponding whole sphere. This
point, where the central axes of all beams intersect (Fig.5), is called the isocenter. The isocenter establishes the principal spatial
point of reference for all dose measurements and treatment geometry subsequently used in medical applications of the Gamma
Knife. The importance of this spatial coordinate is further emphasized by the fact that the manufacturer’s specifications state that
the location of the unit isocenter will always be defined to within ±0.3 mm.
Daily Operations During Patient Treatment
To prevent problems with the unit that might create significant unexpected delays in
treatment, the Gamma Knife physicist completes a comprehensive operational and
safety check of the unit and verifies the accuracy of its control timer. After the unit
and treatment suite have passed this inspection, the attending neurosurgeon
affixes the Leksell stereotactic headframe (Fig. 6) to the patient’s skull so that the
intended treatment target is located as near as possible to its center. The frame
defines the location and orientation of the Leksell Cartesian coordinate system
used in target localization and treatment.13 It holds the patient rigidly in position
during both imaging and irradiation and permits the attachment of a variety of
three-dimensional fiducial marking systems used in conjunction with different
imaging modalities such as magnetic resonance (MR) imaging, computed
tomography (CT), and angiography.
90% (relative to the point of maximum dose) are plotted. Dose level increases as
an observer moves from the outside to the center of each isodose distribution
shown. [Courtesy of Ann H. Maitz,MSc]
Figure 4. (A) Cross-sectional diagram showing beams from 60Co sources within
the Model U Gamma Knife directed to focus radiation at the target within a patient.
Sources within the Model B form five “rings” that are either parallel to or slightly
offset from an axial anatomic plane through the patient’s head (as opposed to the
arrangement shown here, which is more symmetric about the top of the patient’s
head). (B) Cross-sectional view through long axis of the Model B Gamma Knife.
[Courtesy of Elekta Instruments, Inc.]

A series of thin-slice axial images, extending both superiorly and inferiorly to the
target(s) of interest, is obtained so that the target can be observed clearly. A
second set of the same images, but exhibiting the signal intensity effects of an
injected contrast agent, also may be acquired, depending on the patient’s
diagnosis. If MR imaging is used, a set of images in the coronal orientation through
the anatomy of interest is usually recorded. Typically, the capability to superimpose
graphics on these images at the MR/CT control console is taken advantage of to
perform crude measurements of the target’s geometry and to obtain the
coordinates of the target center in the Leksell frame of reference before imaging
has been completed.
During treatment planning, the fiducial markers visible on each of the acquired
cross-section/projected images are entered as data points. With this set of
information, the treatment-planning computer is able to correlate the position of all
image pixels with three-dimensional geometric coordinates in Leksell space. This
process of image registration is critical to ensure accurate definition of the
treatment geometry.10,11 Consequently, standards for image quality are high.
Physicists specializing in diagnostic imaging usually maintain and ensure the
optimal performance of MR imaging devices, CT scanners, and angiographic
imaging arrays. However, a quick, easily completed series of tests performed
during imaging or on the Gamma Knife treatment-planning computer can verify that
no image distortion has occurred.
Before treatment planning, the patient will
have had his or her skull radii measured with
the Gamma Knife skull scaling instrument—a
large acrylic sphere with 25 evenly
distributed holes for insertion of a measuring
rod (Fig. 7). The patient also will have been
set in the treatment position on the Gamma
Knife unit to determine a comfortable degree
of chin and neck flexion, known as the
treatment gamma angle . After the measured
skull radii have been entered into the
treatment planning computer, outlines of the
target and critical anatomic structures may
Figure 5. Diagram showing headframe
fixation and intersection of all 201 beam
paths inside of a collimator helmet for the
Model U Gamma Knife. [Courtesy of Elekta
Instruments, Inc.]
Figure 6. Photograph of the Leksell stereotactic headframe with fixation
pins shown passing through headframe support bars. The attachments at
the sides of the headframe are brackets used to set the patient at the
appropriate y and z coordinates. [Courtesy of Elekta Instruments, Inc.]

be superimposed graphically onto the
previously entered cross-sectional/projected
image data. The treatment-planning
computer is capable of detecting the outlines
of internal anatomy using semi-automatic
image segmentation, but the user can
always manually enter or edit graphically
overlayed internal structure boundaries as
well. It is good practice to take the time
required to outline target and critical
structure volumes because these data can
later be used to judge exactly how well any
designed dose distribution covers the target
or misses critical anatomical structures
through a process known as dose-volume
histogram analysis.
A dose-calculation matrix encompassing the
anatomic region of interest is defined. Since
radiation doses are only computed and
mapped within this volume, the matrix must
be large enough to include critical
neighboring radiation-sensitive structures
such as the brain stem, optic nerve(s),
retina, or skin surface (depending on the
location of the target to be treated). If the
matrix is too large, however, spatial
resolution in the dose calculation may be
lost. Therefore, care must be exercised in
locating and sizing the dose-calculation
matrix to achieve the appropriate balance
between inclusion of nearby dose-limiting
structures and geometric sharpness in
definition of the dose values obtained.
Radiation dose is deposited at the target
location by applying one of the “spheres” of dose at a specific desired location. Radiation dose summates according to the
Principle of Superposition, and multiple such spheres or shots as referred to by Gamma Knife users may be entered into the
treatment plan if sized and spaced appropriately to follow the contours of nonspherically shaped target volumes (all collimator
sizes may be combined or used more than once each if desired, Fig. 8). Further refinements in dose shaping are possible by
exposing the patient to each shot for different lengths of time (shot weighting) or by using as many as 100 available collimator
plugs to block the beams delivered to the target or to immediately adjacent critical structures from any of the unit’s 201 beam
channels.
As shots are added, the treatment-planning computer recalculates the radiation-dose distribution in real time. The point receiving
the maximum level of radiation dose is always set as the 100% dose point, with dose at all other points defined relative to it. This
mathematical redefinition of dose levels, known as normalization, is useful since all doses within the calculation matrix are
presented as ratio values. The dose distribution can thus be shaped according to accepted values for the ratio of dose(s) to
specific critical surrounding tissue(s) relative to the dose level required for therapeutic effects to be observed within the target
tissue.
Figure 7. Photograph of the Leksell stereotactic headframe with skull scaling
instrument (used to measure the patient’s skull radii) attached. Note the
measuring rod (upper right), which has been inserted through one of the 25 hole
positions at which radii are recorded. [Courtesy of Elekta Instruments, Inc.]

After the planned dose-distribution has been reviewed and approved by the attending neurosurgeon and radiation oncologist, the
prescription dose to be administered is selected. It usually depends on the volume of tissue being dosed between maximum and
50% dose levels (the 50% isodose volume), the patient’s diagnosis and staging, any previous radiation that may have been
administered, and the intent of treatment.
Sometimes the Leksell frame support bars prevent placement of the patient at desired shot coordinates, but neither the physicist
nor the treatment-planning computer can always reliably predict if and when this situation will occur while placing shots during
design and calculation of the treatment plan. To address this problem, the extreme x, y, and z values listed among the shot
coordinates are noted when the final approved plan is printed. Taking the extra step of checking that these plan coordinates can
actually be reached by setting the patient in the treatment position (without delivering any radiation) allows the plan to be
adjusted if certain shot locations cannot be achieved physically.
During treatment delivery, the physicist continuously verifies that the collimator helmet(s), Leksell frame coordinates, patient’s
position, and plugging patterns specified in the treatment plan are set correctly.1 If plugs are added or deleted from the patterns
provided in the plan, the physicist adjusts the duration of the shot accordingly. Changes in the plug pattern may be necessary
since the attending physician usually checks beam channels that could potentially direct radiation to the lenses of the eyes.
Because a patient’s eyes may move during treatment and their lenses thus are not fixed at the locations defined for them (as
taken from the cross-sectional images entered into the treatment planning system earlier), extra precautions for shielding the
eyes may be necessary.
Figure 8. (A) Individual inserts that define beam diameter for the 18-, 14-, 8-, and 4-mm collimators. Each collimator helmet
has 201 such identical inserts, which may be interchanged with solid plugs to block the beam from any channel. (B) The
plugging pattern menu option of the GammaPlan is shown on the computer screen. Plugging patterns improve the
conformational dose. The flattened line around the inferior surface of the lesion corresponds to the 50% isodose curve that
results from this particular blocking pattern. [Courtesy of Elekta Instruments, Inc.]

When the treatment is complete, documentation noting the actual treatment administered is signed by the attending
neurosurgeon, radiation oncologist, and physicist. The Gamma Knife unit is shut down to prevent its unauthorized use, and the
patient’s computer-treatment planning information is backed up and archived for future reference. A flow chart condensing all of
the treatment planning and delivery steps described above is presented in Figure 9.
Conclusion
Although not a widely known profession, Medical Physics plays a crucial role in many technologically intensive medical
disciplines, including (but not necessarily limited to) cardiology, diagnostic and interventional radiology, neurology, neurosurgery,
nuclear medicine, ophthalmology, pathology, and radiation oncology. In particular, the contributions and responsibilities of
medical physicists in supporting the safe, accurate, and reliable delivery of cyclotron, Gamma Knife, or linear accelerator-based
stereotactic radiosurgery make them an integral part of any interdisciplinary team involved in this practice.7
Acknowledgment
The author thanks Ann H. Maitz, MSc, Medical Physicist at the University of Pittsburgh Center for Image-Guided Neurosurgery,
Figure 9. Flowchart summarizing patient preparation, imaging, treatment planning, and irradiation procedures
followed during Gamma Knife-based stereotactic radiosurgery. AVM= arteriovenous malformation, GK= Gamma
Knife, MRI= magnetic resonance imaging, CT= computed tomography, and DVH= dose-volume histogram.

for generously granting permission to reproduce data collected on her institution’s recently installed Model B Gamma Knife in this
publication.
†One Gy (gray) is defined as one joule of energy absorbed per kilogram mass of energy-absorbing medium (i.e., tissue), where
one joule is the energy required to accelerate a 1-kg mass at a rate of 1 m/sec2 for a distance of 1m.
References
1. Flickinger JC, Lunsford LD, Kondziolka D, et al: Potential human error in setting stereotactic coordinates for radiosurgery:
Implications for quality assurance. Int J Radiat Oncol Biol Phys 27:397-401, 1993
2. Goetsch SJ: Stereotactic radiosurgery using the Gamma Knife, in Palta J, Mackie TR (eds): Teletherapy: Present and Future.
Madison, WI: Advanced Medical Publishing, 1996, pp 611-642
3. Heydarian M, Hoban PW, Beddoe AH: A comparison of dosimetry techniques in stereotactic radiosurgery. Phys Med Biol
41:93-110, 1996
4. Higgins PD, Sibata CH, Siskind L, et al: Deconvolution of detector size effect for small field measurement. Med Phys 22:1663-
1666, 1995
5. Johns HE, Cunningham JR: The Physics of Radiology. Springfield, IL: Charles C Thomas, 1983, pp 557-669
6. Kondziolka D, Lunsford LD, Flickinger JC: Current concepts in Gamma Knife radiosurgery. Neurosurg Quarterly 3:253-271,
1993
7. Larson DA, Bova F, Eisert D, et al: Current radiosurgery practice: Results of an ASTRO survey. Task Force on Stereotactic
Radiosurgery, American Society for Therapeutic Radiology and Oncology. Int J Radiat Oncol Biol Phys 28:523-526, 1994
8. Leksell L: The stereotaxic method and radiosurgery of the brain. Acta Chir Scand 102:316-319, 1951
9. Maitz AH, Wu A, Lunsford LD, et al: Quality assurance for gamma knife stereotactic radiosurgery. Int J Radiat Oncol Biol
Phys 32:1465-1471, 1995
10. McKenzie MR, Souhami L, Podgorsak EB, et al: Photon radiosurgery: A clinical review. Can J Neurol Sci 19:212-221, 1992
11. Podgorsak EB, Pike GB, Pla M, et al: Radiosurgery with photon beams: Physical aspects and adequacy of linear
accelerators. Radiother Oncol 17:349-358, 1990
12. Task Group 21, Radiation Therapy Committee, American Association of Physicists in Medicine: A protocol for the
determination of absorbed dose from high-energy photon and electron beams. Med Phys 10:741-771, 1983
13. Wu A, Lindner G, Maitz AH, et al: Physics of gamma knife approach on convergent beams in stereotactic radiosurgery. Int J
Radiat Oncol Biol Phys 18:941-949, 1990
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