This document discusses stereotactic radiosurgery (SRS) as a precise form of radiation therapy used to target tumors and vascular lesions in the head and spine, defining radiosurgery according to the American Association of Neurologic Surgeons. It covers the mechanisms of ionizing radiation, treatment doses, and the differences between SRS and traditional radiotherapy, highlighting the types of radiosurgery such as photon and proton beam therapies. The document also outlines clinical indications, contraindications, and specific treatment procedures involved in SRS, emphasizing its application for small lesions and cautioning against its use in larger or infiltrative tumors.

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Dr. Suresh Bishokama, MS
MCH Neurosurgery resident
Department of Neurosurgery, Upendra Devkota Memorial National Institute of
Neurological and Allied Sciences
Bansbari, Kathmandu
STEROTACTIC RADIOSURGERY

2.  Radiation therapy is the use of high-energy photons or charged particles that takes
advantage of the ionizing radiation portion of the electromagnetic spectrum to induce
specific biologic changes for the treatment of various pathologic entities including tumors
and vascular lesions.
 Radiosurgery, as defined by the American Association of Neurologic Surgeons (AANS) is
“a distinct discipline that utilizes externally generated ionizing radiation in certain cases to
inactivate or eradicate a defined target(s) in the head or spine without the need to make an
incision.”
DEFINITION

3.  Ionizing radiation contains enough energy to result in the removal of electrons from their
atoms, and thus leads to the creation of reactive species that cause damage to cells.
 DNA replicative failure:
 The most critical form of damage is DNA strand break. Double strand breaks are more
difficult for cells to repair, and the repair process can generate aberrant chromosomes that
result in mitotic catastrophe, or mutations that result in reduced replicative fitness.
 Vascular endothelium dysfunction:
 Endothelial cell apoptosis, endothelial cell death may lead to direct hypoxic necrosis of the
tumor or may secrete signaling molecules that result in tumor cdl death. However, other
studies have shown that at even higher
MECHANISM OF THERAPEUTIC
RADIATION

4.  Clinical radiation dose, measured in grays (Gy), represents the energy deposited by
ionization in material per unit mass of the material.
 Maximizing the dose of radiation to the target and minimizing the dose to normal tissue is
the central goal of radiosurgery.
DOSING

5.  1 Gy=100 rads
 Cells are most sensitive to radiation during the G2/M phase of the cell cycle and resistant
during the late S phase and G1.
FACTS

6. S
N
TUMOR SRS DOSE (Gy) TRADITION XRT DOSE
1 MENINGIOMA 12-16 Gy 55-60 Gy
2 LGG NA 54Gy infraction of 1.8Gy
3 EPEDYMOMA NA 59.4Gy
4 MEDULLOBLASTOMA NA 35-40Gy whole CSI+10-15Gy tumor bed and spinal
boost, Fractionated over 6-7wks
5 VESTIBULAR SCHWANNOMA 12.5-14Gy
MENINGIOMA 55-60Gy
7 PCNSL 40-50Gy 1.8-3 Gy daily fraction
8 CRANIOPHARYNGIOMA 8 Gy 54 Gy in 30 fractions
9 PITUITARY ADENOMA 40-50 Gy over 4-6weeks NA
10 DAVF 16-20Gy NA
11 AVM 15-25Gy NA
12 PINEAL TUMOR NA 55Gy in 1.8-Gy daily fractions, with 40Gy to the
ventricular system and an additional 15 Gy to the
tumor bed
13 TECTAL GLIOMA <14Gy 45-55Gy
DOSAGE

7.  SRS typically consists of a single high-dose treatment of radiation delivered using a
precise localization system.
 Conventional radiotherapy typically involves lower doses delivered in daily fractions to
reduce the effects of radiation on normal tissue.
Radiation delivery system
Fractionation VS radiosurgery (SRS)

8. 1. Re-oxygenation of hypoxic areas, resulting in increased sensitivity of malignant cells that
were previously hypoxic.
2. Re-assortment of cells in the cell cycle, as cells are most sensitive to radiation during the
G2/M phase of the cell cycle and resistant during the late S phase and G1. Allows resistant
phases of the cell cycle to move to more sensitive phases of the cell cycle during subsequent
fractions. (Late S phase and G1 G2/M).
3. DNA repair to occur, which favors normal cells that retain the full complement of DNA
repair proteins.
4. Re-population of tumor cells during the therapy.
FRACTIONATION INDUCES 4R OF RADIOBIOLOGY:
“OARP”

9.  Differential sensitivity of tissues to fractionation (/ ratio): which is a radio biologic
concept based on a model of radiation response.
 Tissues with a high / ratio respond quickly to radiotherapy and are sensitive to smaller
fraction sizes.
/ RATIO

10. 1. Photon beam radiosurgery: (X-ray/Gamma)
2. Proton beam radiosurgery: (Helium or Proton)
TYPES OF RADIOSURGERY

11.  Photon particle: no mass: speed of light: carries the energy present in all electromagnetic
radiation, including microwaves, visible light, ultraviolet light, and x-rays. Photon is the
most common form of ionizing radiation used in radiotherapy.
 Mechanism:
 Photons interact with matter predominantly via the photoelectric effect. Ionizing radiation
results in the ejection of electrons from the atoms and scattering of the photon. The
scattered photon has a change in its energy. Photons interact with a molecule of water,
resulting in the production of superoxide, hydroxyl radicals, and other reactive oxygen
species, which damage the DNA and result in replicative failure. Radiotherapy is believed
to be more effective in the presence of oxygen, and it is thought that hypoxic areas of
tumors may be less sensitive to the effects of ionizing radiation.
PHOTON BEAM RADIOSURGERY

12.  Three main categories (based on sources of radiation) are:
 Gamma knife
 Linear accelerator based, and
 Heavy charged particle radiosurgery.
PHOTON BEAM RADIOSURGERY

13.  The Gamma Knife is specifically designed for cranial and upper cervical lesions, and is
best suited for smaller lesions (<3 cm diameter).
 The unit, as currently constructed, contains 201 fixed sources of 60Co (Half-life:5.3 years)
distributed in a hemisphere, each of which is a thin rod with the long axis oriented along
the radius of the sphere, converging on a single point, called the treatment isocenter.
Because there are no moving parts during treatment, there is a high degree of setup
accuracy.
 The GK cobalt-60 sources decay to nickel-60 with the release of two energies of gamma
rays at 1.117 and 1.33 MeV, which are the main treatment energies.
GAMMA KNIFE RADIOSURGERY

14.  Linear accelerator-based radiotherapy (LINAC) system generates photon x-rays by
accelerating electrons to a high speed using microwave energy to deliver SRS onto the
target. LINAC produces 6-MeV x-rays. Fixed to the LINAC platform: true stereotactic
frame, noninvasive facemask, and dental mold .
 The x-ray source and beam collimation is mounted on a rotating gantry, creating a fixed
isocenter in space. Beam convergence is achieved via rotating arcs with the isocenter
fixated on the target.
 Cyberknife is a miniaturized linac that is mounted on a robotic arm with 6 degrees of
rotational freedom (rather than an isocentrically-mounted linac). The linac and robotic arm
combination can deliver multiple small beamlets of radiation from many different angles to
produce a conformal dose plan.
LINEAR ACCELERATOR-BASED RADIOSURGERY
(LINACS)

15.  Proton a nucleus of hydrogen atom are positively charged, subatomic particles of ~ 1
atomic mass unit (protons, helium, and carbon, to name a few) produced by stripping
electrons from molecular hydrogen gas, before being accelerated to therapeutic energies in
a synchrotron or cyclotron. As a particle travels through the medium it loses its energy in a
myriad of these collisions and finally comes to a full stop. Because more energy is lost the
slower the particle goes, a useful feature arises-a peak right at the end of the particle's
travel ("Bragg peak").
HEAVY CHARGED PARTICLE RADIOSURGERY
(PROTONS OR HELIUM IONS)

16.  Cyclotron is used to generate high charged particle for use in radiosurgery.
 Proton production starts by stripping the electron from molecular hydrogen gas, and the
resulting protons are then accelerated to a therapeutic energy level using alternating
magnetic fields in a cyclotron or synchrotron.
 Unlike high-energy photons (gamma and x-rays), which deposit the majority of their
energy upon entrance into tissue and continue to deposit decreasing amount of energy as
they travel through the body, heavy charged particle beams have a shorter, bounded range
of penetration wherein particles sharply increase energy deposition near the terminal depth
of penetration (Bragg peak effect).
 Particle radiosurgery achieves a well-localized volume of high dose radiation by taking
advantage of cross firing of a number of beams as well as the Bragg peak. Due to the
expense and increased complexity of heavy charged particle SRS, this therapy is only
available in a few centers in the world.

17.  Absence of dose beyond the target and the decrease in dose proximally.
 To cover lesions, which extend in depth, multiple Bragg peaks, originating from protons
with different initial energies, are superimposed to create a spread-out Bragg peak (SOBP).
ADVANTAGES OF
PROTONS OVER PHOTONS

18.  Physical uncertainty and cost.
 Given the sharp dose fall-off, it is critical to be able to calculate and deliver proton dose
precisely.
 Large accelerators are needed to generate proton (football field size); compact technology
is under development)
DISADVANTAGE OF
PROTONS OVER PHOTONS

19. 1. VASCULAR LESIONS
○ AVMs (including dural arteriovenous
fistulas)
○ Cavernous malformations
2. TUMORS
○ Metastases
○ Vestibular schwannomas
○ Meningiomas
○ Pituitary adenomas
○ Gliomas
○ Others: craniopharyngioma, pineal tumors.
INDICATIONS OF RADIOSURGERY
3. FUNCTIONAL DISORDERS
○ Trigeminal neuralgia
○ Intractable chronic pain: thalamotomy
○ Movement disorders: pallidotomy for
Parkinson’s disease or thalamotomy for tremor
○ Psychiatric diseases (e.g. obsessive
compulsive disorder)
○ Epilepsies
4. SPINE: PLIF, TLIF: to shave the end plates.
SRS is useful for well-circumscribed lesions less than approximately
3 cm diameter (in general)

20.  Compressive tumors of the spinal cord, brainstem or optic structures:
 (Even with the sharp fall of radiation dose, there remains radiation delivered within a few
millimeters of the margins of the isocenter).
 This, together with post-radiation swelling, might create significant risk of neurologic
injury.
CONTRAINDICATIONS

21.  Treatment unlikely to result in functional improvement or clinically meaningful disease
stabilization, not otherwise achievable.
 Patients with wide-spread cerebral or extra-cranial metastases with limited life expectancy
unlikely to gain clinical benefit within their remaining life.
 Patients with poor performance status (Karnofsky Performance Status less than 40 or
ECOG Performance greater than 3)
 Essential tremor, coverage should be limited to the patient who cannot be controlled with
medication, has major systemic disease or coagulopathy, and who is unwilling or unsuited
for invasive surgical procedure. Coverage should further be limited to unilateral
thalamotomy.
SRS is not considered medically necessary

22. 1. Cranial nerves: Optic nerve cant tolerate >8Gy radiation within 2mm.
2. Damage to small nutrient vessels and Schwann cells or oligodendroglia are the possible
mechanisms of radiation injury to cranial nerves.
3. Special sensory nerves (optic, vestibulocochlear) are the most radiosensitive
4. SRS treatment may also have a deleterious effect in structures sensitive to swelling,
such as brainstem.
5. Additionally critical radiation sensitive structures include: optic vitreous, nerve, and
chiasm, brain stem, pituitary gland, and cochlea.
RED FLAG ZONE

23.  The treatment procedure includes placement of stereotactic frame (in framed-based SRS),
obtaining stereotactic images, target definition, treatment planning and execution of
treatment.
TREATMENT PROCEDURE

24.  Methylprednisolone 40 mg IV and phenobarbital 90 mg IV immediately after the radiation
dose to patients with tumors or AVMs to reduce these adverse effects
PREMEDICATION

25. 1. Position stabilization (attachment of a frame or frameless).
2. Imaging for localization (CT, MRI, angiography, PET, etc.).
3. Computer-assisted tumor localization (i.e., “image guidance”).
4. Treatment planning – number of isocenters; number, placement and length of arcs or
angles; number of
5. beams, beam size and weight, etc.
6. Isodose distributions, dosage prescription and calculation.
7. Setup and accuracy verification testing.
8. Simulation of prescribed arcs or fixed portals.
9. Radiation treatment delivery.
COMPONENT OF SRS PROCEDURES

26. 1. SRS is best accepted for the treatment of small to moderate-sized (< 3 cm) AVMs that
are deep or
2. Border on eloquent brain and have a “compact” (i.e. sharply demarcated) nidus.
3. The radiation induces endothelial cell damage, smooth muscle cell proliferation,
thickening of the vascular wall, and ultimately obliteration of the lumen over a period
of 2-3 years (latency period). AVFs pose high risk of hemorrhage. SRS is of no benefit
for venous angiomas. SRS for cavernous malformations remains controversial.
ESPECIAL CONSIDERATION
AVM AND VASCULAR LESIONS

27. ● Total tumor number ≤ 10
● Total tumor volume ≤ 15 cm3
● Single tumor volume is <10 cm3, and
● No leptomeningeal disease present.
BRAIN METASTASES:

28.  Indications are: poor surgical candidates (due to poor medical condition and/ or advanced
age, some use >65 or 70 years as a cutoff), patient refusing surgery, bilateral VS.
VESTIBULAR SCHWANNOMA

29.  SRS is generally not indicated as a primary treatment for infiltrating tumors, e.g. gliomas
INFILTRATING TUMORS

30. 1. Focal deficits, seizures, or headache.
2. Radiation necrosis and permanent deficits ( Mechanism: glial cell damage, breakdown
of blood brain barrier or early venous thrombosis
3. Premature venous thrombosis or occlusion before obliteration of AVM nidus can
produce venous hyperemia or intracranial hemorrhage.
4. Vasculopathy
5. Cranial nerve deficits (Incidence is higher with tumors of CPA or skull base):
Mechanism is due to damage of small nutrient vessels and Schwann cells or
oligodendroglia)
6. Radiation induced tumors: Astrocytoma
DRAWBACKS OF SRS

31. NATIONAL INSTITUTE OF NEUROLOGICAL AND ALLIED SCIENCES, BANSBARI, KATHMANDU
STEROTACTIC RADIOSURGERY
Dr. Suresh Bishokama, MS
MCH Neurosurgery resident
Department of Neurosurgery, Upendra Devkota Memorial National Institute of Neurological and
Allied Sciences
Bansbari, Kathmandu
drsureshbk@gmail.com