SRS
2013
Leaning objectives
Available teqs.
 Radiobiology.
 Review the clinical use.
 Side effects.
 Planning evaluation.

STEREOTACHTIC
STEREO:FROM GREEEK WORD
STEREO MEANING SOLID AS IN 3D
 TACHTIC:
ARRANGEMENT,ORIENTATION

RADIOSURGERY:
High dose Radiation delivered in generally
one fraction.
 Considered ablative.
 Generally yields high rate of local control.

Modern definition

The delivery of stereotactically directed,
highly focal, large single dose of radiation
to inactivate tumor growth or obliterate
a vascular lesion. (ASTRO 2009).
Developed by, Lars Leksell a
neurosurgeon in late 1940s
 Using Protons, Leksell and larsson studied
SRS on humans and animals in Uppsala,
Sweden 1950s
 Developed gamma knife in 1960s.

Basic Principles
Inverse relationship between dose and
volume to respect tolerance.
 Thus larger target volume must receive
lower dose
 Lower dose=Lower probability of
obliteration or tumor control
 larger single dose damaging to late
responding tissue

Radiobiology
Treat with larger single fraction of radiation
1- The radiobiological response of benign
tumors and vascular lesion is closer to the
surrounding normal tissue than malignant tumor.
2- High dose single fraction works better in
malignant tumor than previously believed. (effect
on vasculature)
3-By treating tumor with no extra margin, you
can give higher dose of radiation
Improved conformality by piecemeal irradiation
Radiosurgery can replace surgical intervention

A classic example of in vivo Chinese Hamster cell survival illustrating the linear
tail (on a log-linear plot) of the cell survival curve at high dose where cell killing
becomes purely exponential.
Effect of single dose on low and high
α/β

Comparison of single-dose effect curves (A) and
fractionated dose-effect curves (B) for low and high
α/β tissues. The small advantage seen in the low-dose
region sparing low
 α/β tissues (A) is amplified through dose fractionation
(B).

SRS delivery system
Gamma Knife
 Linear Accelerator
1-Standared Linac.
2-Cyberknife.
3-Tomotherapy.
4-Rapid Arc.
Charged particle
1-Protons.
2-Carbon Ions.

Radiation
Dose Prescription
to Periphery of
Target
GammaKnife
Cone-Base
MLC Based
Nuclear
Linac
Linac
50% Isodose
70-90%Isodose
Usually 80%
Dose
Homogeneity
Peripheral dose =50% of max
dose
Peripheral dose 70-90%
of max dose
Conformality
Gamma-Knife better than cone
based Linac treatment
Comparable to GammaKnife
Linac Vs Gamma
Gamma -Knife
Periphery dos =50%
Maximal dose=100%
Linac Based (MLC)
Periphery dos =80%
Dose delivery
SRS dose is prescribed to isocenter as well as to tumor
periphery.
 Tumor periphery dose often quoted as percent of
maximal dose e.g. (tumor completely covered by 80%
isodose line).
 Dose at the target periphery is the clinically relevant
dose, since this generally reflects the minimum dose
delivered to the target.

Peripheral dose 50-80%
Central dose 100%
SRS Plan Evaluation
100% target coverage is desirable.
 RTOG:Entire target must be covered by
90%

Why 80%
SRS plan evaluation
PI/TV=planning isodose volume to target
volume ratio
 PI/TV=Radiation dose volume/taget
volume
 Ideal conformity PITV=1.0
 PI/TV<2.0 meets RTOG SRS guidelines
 Rule of thumb PITV=1.2 is good

HI= Dmax ⁄ DRx ≤ 2
 Acceptable <3.5.

SRS needs steep dose gradient out side of
the target
 Better than 60% per 3 mm.
 Generally we worry about distance
between prescription isodose shell and
1/2of prescription isodose shell

What is treated with SRS?
Brain metastasis <4 cm
as a boost or salvage
 Primary brain malignancy <4 cm.
generally as a boost or salvage
 Acoustic neuroma <3 cm.
 Menengioma <3cm.
 Pitutary adenoma residual or recurrent.
 Arteriovenous malformation.
 Trigiminal neuralgia(target is verve root).
 Parkinson’s disease and sezures.

Think About
Irradiate the target safely
Ablate the tumor/target.
Minimize the risk of toxicity to normal
organ.
 Relevant Variables:
Dose
Volume
Location/neighboring critical structures

What are the critical structures
Brain Stem
 Normal Brain Tissue
Hippocampus
temporal lobes
Insula
Motor cortex
Language area
 Cranial Nerves
 Cochlea

Review the literature
Mostly retrospective
 Single institution
 Late toxcicity.

SRS Toxicities
Acute(1-2%)
Sezures mostly occures in the first 24 h
(cortial structure, prior sezures, anti
epileptic medications
H/A, nausea, vomiting, aphasia, motor
neuropathy
Late:
Radiation necrosis(2-3%)
2ndy malignancy
Death extremely rare
Optic Nerve and Chiasm

Cavernous sinus meningiome

Pituitary adenoma
Risk of Optic Neuropathy
Max ose
<8
8
U.Pitt
0% (0/35)
U.Pitt
0%
(0/31)
k.Franzen .U
10
12
1 case
Mayo clinic
2% (1/58)
U.Maryland
0% (0/20)
>15
24% (4/17)
67% (2/3)
27% (6/22)
U.Pitt
15
78% (9/13)
3 cases of 2400
2% (1/58)
7%
Acturial incidence @3 years
SRS=Fractionated RT in case of Optic neuropathy
Tishler et al , IJOBP 27 215-21,1993
Duma et al, Neurosurgery 32:699-740,1993
Girkin et al, Opthalmology.104 1634-43,1997
Leber et al,J. Neurosurgery.88:43-50,1998tishle
Stafford et al IJOROBP 55:1177-81,2003
Ove et al .InT J Cancer9 90:343-50,2000
Shrieve et al , J neurosurgery S3:390-5,2004
Radiation induced Optic Neuropathy
occurs with in 3 years after SRS.
 Is Rare.
 No Dose constrains based upon:
length of optic nerve
Volume of Optic apparatus
Location along the nerve
Prior external beam
Combined illness

Which one is safer?
120
100
Max dose=8Gy
V8=15%
V10=0
80
60
1
2
Max dose=10
V8=5%
V10=negligable
40
20
0
0.00
4.00
8.00
12.00
Max dose=13
V8=negligable
V10=negligable
14.00
16.00
3
Uncertanity
? DVH what parameter we should use to
predict toxicity.
 Precision of contouring.
 Regional variation in dose suitability
volume of the nerve?
prepheral vs central?
Chiasm versus nerve?

Hearing
Date from retrospective studies of benign
tumors (Aquestic neuroma, cerebellopontine angle meningioma)
 Hearing loss associated with the dose
prescribed to the tumor periphery

Hearing
Relevant Questions
 Is hearing loss from:
1-Radiation effect of the nerve VIII?
2-Radiation effect of vasculature of the
nerve?
3-Radiation effect on the brain stem?
4-Radiation effect on the cochlea?
5-Tumor effect

Hearing
Relevant variables impacting risk of hearing loss
1-Tumor related
Size
Location (intra-canlecular Vs CP angle)
2-Patient related
Age
Pretreatment hearing status.
3-Treatment related variables
Dose
Volume
4-Assement related variables
Frequency of hearing assessment

Hearing Loss : acoustic neuroma
dose
Peripheral
dose
<8 8
U.Pitt
5 year preserved hearing =75%
5year survivable hearing is 95%
50*
60%*
Komak iCity
Hearing preservation=68%
13% #
U.Pitt
10 years5 year preserved hearing
=44%
10year survivable hearing is 85%
* SRS
10
dose de-escalation 18-20 Gy
to 16-18 to 14-16 .
# p=<.001
12
15
>15
Gy
Hearing outcomes after Stereotactic
Radiosurgery for unilateral shwannoma
(IJORO,2013 seol national university)
60 patients tumor in the canal
 All treated with RT as initial treatment
 All patients treated with cyberknife
 Max dose to tumor 24.3 Gy
 Marginal dose 12.2Gy
 Isodose line used 50%
 Maximal dose to chochlea 8 Gy and mean is
4.2Gy.
 Acturail hearing loss 70,63 and 55 at 1,2 5
years.

Hearing :Cochlea Dose
Stable/improved vs. worse hearing
1-median cochlear dose 3.7 vs. 5.3, p=0.0005
in 82 patients
2-lower radiation dose to cochlea volume over
range of
2-11Gy, P=0.0001

Messager et al.J.neurosurg .107:733-9,2007
U.Hospital Erasme,Belgium
3-cochlear point dose 4.6 vs 4.8 (NS) in 175patients
Gabertet al. Neurochirurgie 50:350-7,2004
U.de latimon,France
4-cochlear max 9.1vs 7.8 (NS) in 25 patients
Peaket al.cance 104:580-90, seol,France ,205
Hearing loss
FSRT vs SRS
Between 97-2008
42 patients
Dose is 54 Gy/27-30
Acturial Tumor control at 2, 4,10 were
100,91,85%
Servivable hearing loss at 2 years almost
50 % and reaching to 100%by 10 years.

To improve Hearing
Prescribe marginal dose of 12-13Gy
 Contour brain stem
 Contour cochlea
 Delineate tumor and nerve exclude Viii
from GTV.


CT images of normal right inner ear anatomy: (a–f) axial superior
to inferior images. 1, internal auditory canal (IAC); 2, superior
semicircular canal (SCC); 3, lateral SCC; 4, posterior SCC; 5,
vestibule; 6, basal turn of cochlea; 7, apical turn of cochlea; 8,
vestibular aqueduct; 9, facial nerve.

Figure 3fNormal anatomy of the temporal bone. Axial high-resolution
(a–e) and coronal MPR (f) multidetector CT images of the temporal
bone show the external auditory canal (EAC), carotid canal (CC) and
jugular bulb (JB), malleus (M), facial nerve (FN), cochlea (C), semicircular
canals (SCC), internal auditory canal (IAC), incus (I), vestibule (V), vestibular
aqueduct (VA), and mastoid air cells (MAC).
Trigeminal and facial nerve
Variables impacting CN V and VII toxicity
Prescribed Radiation Dose
length and volume of nerve irradiated
modern imaging
prior resection
Brain Stem maximal dose
Trigeminal and facial Nerve

University of Florida retrospective study
of 149 patients treated with SRS for AN
1- Prior SX confined to 5 folds increased
in toxicity.
2-Brain stem Maximum dose p<0.0001
BS max dose > 17.5 Gy resulted in 50
fold
incresed risk
3- Univariate analysis CN V,VII significant
for the nerve length.
Brain Stem
Accepted dose 10-12 Gy max ,(Loffler
NSJ 2008).
 Confounding factors
 Tumor type
 Regional variation

Brain Stem

Retrospective study of 38 patients SRS
for Benign and low grade lesions
7 developed adverse radiation imaging
effect
4 new neurologic deficits
3 no neurologic deficits
3 developed permanent neurologic
defecits
Sharma et al.Neurosurgery J ,U.Pitts 2008

Thank you