An Introduction to Gamma Knife Radiosurgery - Cedars-Sinai

An Introduction to
Gamma Knife Radiosurgery
Amin Mirhadi, Mirhadi,
M.D.
Dept. of Radiation Oncology

What is Radiation
Therapy?

• Purpose is to
deliver high energy
X-rays or gammagammarays (XRT) to a
target while
minimizing delivery
to normal tissues.

What is Stereotactic
Radiosurgery (SRS)?
• The delivery of multiple intersecting
beams of XRT to a radiographically
discrete treatment volume using precise
localization in order to ablate a target.
• This results in a rapid fall-off of dose at
the edge of the target volume and a
clinically insignificant dose to adjacent
normal tissue.

What is Stereotactic
Radiosurgery (SRS)?
• The delivery of multiple multiple intersecting
beams beams of of XRT XRT to a radiographically
discrete treatment volume using precise
localization in order to ablate a target.
• This results in a rapid fall-off of dose at
the edge of the target volume and a
clinically insignificant dose to adjacent
normal tissue.

What is Stereotactic
Radiosurgery (SRS)?
• The delivery of multiple intersecting
beams of XRT to to a a radiographically
discrete discrete treatment volume volume using
precise localization in order to ablate a
target.
• This results in a rapid fall-off of dose at
the edge of the target volume and a
clinically insignificant dose to adjacent
normal tissue.

What is Stereotactic
Radiosurgery (SRS)?
• The delivery of multiple intersecting
beams of XRT to a radiographically
discrete treatment volume using using precise precise
localization in order to ablate a target.
• This results in a rapid fall-off of dose at
the edge of the target volume and a
clinically insignificant dose to adjacent
normal tissue.

• The patient’s patient s head is
immobilized in a
stereotactic head frame.
• This ensures that the
patient’s patient s head postion in
space is reproducible
between treatment
planning and the actual
treatment.

This is accomplished by placement of a
Stereotactic headframe for 3-D localization…
localization

…which which allows correlation of internal
anatomy with external markers
• The head frame
fiducials are visible on
the MRI.
• This allows us to
determine the target
“coordinates
coordinates” in an
X,Y,Z plane.
• We can therefore
orient our beamlets in
a conformal fashion
External markers giving 3D localization

What is Stereotactic
Radiosurgery (SRS)?
• The delivery of multiple intersecting
beams of XRT to a radiographically
discrete treatment volume using precise
localization in order to to ablate ablate a a target. target
• This results in a rapid fall-off of dose at
the edge of the target volume and a
clinically insignificant dose to adjacent
normal tissue.

Different Ways that XRT kills
cancer cells… cells
1. Causes Mitotic Death
2. Causes Apoptotic Death
3. Causes Necrosis of cells
4. Alters molecular signaling pathways
(p53, RB, ROS, WT-1, …)

• Conventional XRT is delivered over several
smaller fractions to take advantage of the
biological differences b/w normal and malignant
tissue.
• I.E. – the smaller XRT dose allows normal tissue
to recover during therapy, but is just high
enough to cause a mitotic cell death (or
apoptosis) in rapidly dividing cells.

• Radiosurgery is considered ablative b/c it
kills all cells in its target volume by
necrosis, and its dose is above the
threshhold that would allow normal cells
an opportunity to repair.
• It relies on geometric accuracy accuracy rather
than biology biology to elicit it’s it s effect and needs
a discrete target volume.

What is Stereotactic
Radiosurgery (SRS)?
• The delivery of multiple intersecting
beams of XRT to a radiographically
discrete treatment volume using precise
localization in order to ablate a target.
• This This results results in in a a rapid rapid fall-off fall-off of of dose dose
at at the the edge edge of of the the target target volume volume and and
a a clinically clinically insignificant dose dose to to
adjacent adjacent normal normal tissue.
tissue.

What are the different
ways to deliver SRS?

1. X-knife

• Has a single beam source (non(nonradioactive) • Arm rotates into various positions to
deliver XRT
• Beam is ‘shaped shaped’ by movable MLC’s MLC s rather
than circular collimators
• Has ability to fractionate if necessary.

2. Cyberknife

3. Proton Beam

4. Gamma Knife

How does Gamma Knife
work?
• The Gamma
Knife unit has
201 radioactive
cobalt-60
sources…
sources

…housed housed in a very thick lead unit…
unit

The sources are arranged
concentrically around a ‘helmet helmet’… ’…

…and and are oriented to deliver their radiation to a
single point in space called the ‘isocenter isocenter’.

The radiation is modified by circular
‘collimators
collimators’ inside the helmet, which have
various diameters…
diameters

Benign Tumors – (acoustic neuroma)
Vascular Malformations – (AVM)
Brain Mets
Glioma - (low and high grade)
Functional Disorders - (Trigeminal
Neuralgia)

Specific Disorders Managed
With SRS

Acoustic Neuroma

Acoustic Neuroma:
A Non-Invasive CP
Angle Tumor
and a
Radiosurgical
Target

Acoustic Neuromas: Radiosurgery vs Microsurgery?

Acoustic Neuromas: Clinical-radiographic Diagnosis
Intracanalicular
Extracanalicular
47 y/o female speech pathologist, opera singer
5 mo tinnitus, cheek numbness, slight decreased hearing
Retention of useful hearing, no facial palsy

Multiple Isocenters - An Attempt at “Dose Shaping”