Download Special procedures and techniques in radiotherapy

Special procedures and techniques
in radiotherapy
講者：蕭安成
物理師
References:
1. The physics of radiation therapy / Faiz M. Khan. - 4th ed.
2. AAPM REPORT NO. 23 TOTAL SKIN ELECTRON THERAPY: TECHNIQUE
AND DOSIMETRY
3. AAPM REPORT NO. 17 THE PHYSICAL ASPECTS OF TOTAL AND HALF
BODY PHOTON IRRADIATION
4. Stereotactic body radiation therapy: The report of AAPM Task Group 101. Med.
Phys. 37 (8), August 2010, 4078-4101
5. AAPM REPORT NO. 54 STEREOTACTIC RADIOSURGERY
2
Special procedures and
techniques in radiotherapy
Total Body Irradiation
Total Skin Irradiation
Stereotactic Radiosurgery/Radiotherapy
Stereotactic Body Radiation Therapy
Total Body Irradiation
(TBI)

Clinical indication
– Total body irradiation (TBI) with
megavoltage photon beams is most
commonly used as part of the conditioning
regimen for bone marrow transplantation,
which is used in the treatment of diseases
such as leukemia, aplastic anemia,
lymphoma, multiple myeloma, autoimmune
disease, inborn errors of metabolism, and so
on.
Total Body Irradiation
(TBI)

Clinical indication
– The role of TBI is to destroy the recipient’s
bone marrow and tumor cells, and to
immunosuppress the patient sufficiently to
avoid rejection of the donor bone marrow
transplant
– Usually the patient undergoes a
chemotherapy conditioning program before
the TBI and bone marrow transplant.
Total Body Irradiation
(TBI)

Techniques and equipment
– The choice of a particular technique depends
– the available equipment, photon beam
energy, maximum possible field size,
treatment distance, dose rate, patient
dimensions, and the need to selectively
shield certain body structures
– Compensators are required to achieve dose
homogeneity of  l0%, excluding extremities.
Total Body Irradiation
(TBI)

Techniques and equipment
Total Body Irradiation
(TBI)

Techniques and equipment
Total Body Irradiation (TBI)
Techniques and
equipment

Beam energy
FS = 10X10 cm,
SSD= 100 cm.
15%
AP/PA
Lateral
Total Body Irradiation (TBI)
Techniques and
equipment

Initial Dose Build-up
– Dose buildup data obtained at normal SSD
(e.g., 100 cm) does not apply accurately at TBI
distances (e.g., 400 cm) because of the longer
distance and the intervening air
– most TBI protocols do not require skin sparing
– a bolus or a beam spoiler is specified to
bring the surface dose to at least 90% of the
prescribed TBI dose.
Total Body Irradiation (TBI)
Techniques and
equipment

Initial Dose Build-up
A large spoiler screen of 1- to 2-cm-thick acrylic is sufficient to meet these
requirements, provided the screen is placed as close as possible to the patient surface
Total Body Irradiation (TBI)
Techniques and
equipment

Patient Support/Positioning Devices
– Important criteria include
– patient comfort, stability, and
reproducibility of setup and treatment
geometry that allows accurate calculation
and delivery of dose in accordance with the
TBI protocol.
Total Body Irradiation (TBI)
Bilateral Total Body Irradiation
Sagittal laser or floor marks
to define setup distance
Arms shadow the
lung to provide
protection
Compensator
s used to
compensate
body parts of
small
thickness
such as
head/neck,
legs, etc.
Total Body Irradiation (TBI)
Techniques and
equipment

Patient Support/Positioning Devices
– To achieve dose uniformity within
approximately  10% along the sagittal axis
of the body, compensators are designed for
head and neck, lungs (if needed), and legs.
– The reference thickness for compensation is
the lateral diameter of the body at the level of
the umbilicus
– Compensators can be designed out of any
material, but at the University of Minnesota
they are custom-made out of aluminum.
Total Body Irradiation (TBI)
AP/PA Total Body Irradiation
Standing position for adults
Laying position for children
50% transmission
block to protect the
lungs, chest wall
boost by electron
beams
Total Body Irradiation (TBI)
Dosimetry data



The dose calibration should be performed using
the principles and methodology of AAPM TG21
The calibration is best made under geometric
and phantom conditions that most nearly
represent the actual treatment geometry.
“dose rate to a small mass” > the value
calculated from the IVSL, because of scatter
contribution from the walls or the floor. This is
especially true for Co-60 and lower energy
linacs. In air calibrations not be performed.
Total Body Irradiation (TBI)
Dosimetry data

Direct output calibration
SAD+311cm
isocente
r
Ion chamber
measuring dose/MU using Farmer-type ion chamber placed in a water
phantom of dimensions approximately 40 x 40 X 40 cm3.
A table of output factors is generated as a function of depth (TMR) that can
be used to calculate MU for-a patient of given at the prescription point.
Total Body Irradiation (TBI)
Dosimetry Data- Calculation formalism
Dose rate under standard calibration conditions, e.g. 1cGy/MU
Collimator setting at isocenter distance
Field size equivalent to patient field size
D / MU  k  S c (rc )  S p (rp )  TMR (d , rp )   f f '2  OAR(d )  TF
Prescription depth, typically patient
midline depth at the umbilicus
Source to chamber distance under standard
calibration conditions
Source to body axis distance for TBI treatment setup
Off-axis ratio for the prescription point at treatment depth d
Transmission factor for the tray, spoiler screen
Total Body Irradiation (TBI)
Dosimetry Data- Calculation formalism
D / MU  k  S c (rc )  S p (rp )  TMR (d , rp )   f f '2  OAR(d )  TF

k.  S c (rc )  S p (rp )   f f '  Dose/MU ( f , d max , rTBI ),
2
 .measuring using Farmer - type ion chamber according to TG - 21
Dose/MU ( f , d max , rTBI )  IVSL  Dose/MU ( f , d max , rTBI ) measurement

TMR/TPR for large fields (e.g., >30  30 cm2) are
not very sensitive to field dimensions.
Total Body Irradiation (TBI)
Dosimetry Data- Calculation formalism

The TMR data obtained under standard
conditions (at isocenter) must be checked
for their validity at the TBI distance.

In addition, the inverse square law factor
must also be verified for the TBI distance.

Alternatively, D/MU calculated by Equation
may be compared with directly measured
output factors (D/MU) at the TBI distance.
Total Body Irradiation (TBI)
Compensator Design

Compensator design for TBI is complicated
because of




Large variation in body thickness,
lack of complete body immobilization, and
internal tissue heterogeneities.
The thickness of compensator required depends on




the tissue deficit
material of the compensator (e.g., its density),
distance of the compensator
field size, and beam energy
Total Body Irradiation (TBI)
Compensator Design

The thickness of a compensator, to at any point in
the field is given by:
t c  TD    c 
Where
tc = compensator thickness,
TD = tissue deficit,
 = thickness ratio ~0.7
c = compensator physical density
Total Body Irradiation (TBI)
Compensator Design
Alternatively:
I T ( AR , d R )  OARd
  eff t
e

I0
T ( A, d )
t
1
 eff

T ( A, d )
ln
 T ( AR , d R )  OARd



where I0, and I are the doses administered before and after the compensator
is added, T(AR,dR) and T(A,d) are the TPRs or TMRs for the reference body
section and the section to be compensated for equivalent fields AR and A at
midline depths dR and d, OARd is the off-axis ratio at depth d relative to the
prescription point, and eff is the effective linear attenuation coefficient for
the compensator material measured under TBI conditions.
Total Body Irradiation (TBI)
Compensator Design

compensator material should be selected so that the
compensator is not too bulky or of too high a
density that small errors in machining would
amount to large errors in dose.
t c  TD    c 

pb = 11.3 g/cm3, Al = 2.7 g/cm3
Simple 1D compensators for variable
body thicknesses
Simple one-dimensional compensator
used for lateral field irradiation technique.
The compensator corrects for tissue
variations along one line only. The
numbers shown in this figure are direct
dose measurements. The numbers in
parenthesis are calculated from the
entrance and exit surface measurements.
AAPM report no.17 (1986)
Total Body Irradiation (TBI)
In-vivo Patient Dosimetry

it is recommended that an in vivo dosimetry check be
performed on the first 20 or so patients.

TLDs surrounded by suitable buildup bolus, may be placed
on the patient's skin at strategic locations and doses
measured for the actual treatments given.


An agreement of 5% between the calculated and measured
doses is considered reasonably good.
An overall dose uniformity of 10% is considered
acceptable for most protocols.
Total Skin Irradiation



Electrons in the energy range of 2 to 9 MeV have
been found useful for treating superficial lesions
covering large areas of the body, such as mycosis
fungoides and other cutaneous lymphomas.
At these energies, electron beams are
characterized by a rapid falloff in dose beyond a
shallow depth and a minimal x-ray background
(1% or less).
Thus, superficial skin lesions extending to about
1 cm depth can be effectively treated without
exceeding bone marrow tolerance.
Total Skin Irradiation

Translational technique:


The patient lies on a motor-driven couch and is
moved relative to a downward-directed beam at a
suitable velocity
Alternatively, the patient may be stationary and the
radiation source translated horizontally
• a 24-Ci 90Sr  source, in the form of a 60-cm
linear array, is used
• The maximum energy of the  particles emitted
by 90Sr is 2.25 MeV
• the effective depth of treatment in this case is
only a fraction of a millimeter
Total Skin Irradiation

Large Field Technique



can be produced by scattering electrons through
wide angles and using large treatment distances
The field is made uniform over the height of the
patient by vertically combining multiple flields or
vertical arcing.
The patient is treated in a standing position with four
or six fields directed from equally spaced angles for
circumferential coverage of the body surface.
Total Skin Irradiation
Large Field Technique

Field Flatness
• Low-energy electron beams are considerably
widened by scattering in air.
 a 6-MeV narrow electron beam, after
passing through 4 m of air, achieves a
Gaussian intensity distribution with a 50%
to 50% width of approximately 1 m
• A proper combination of more such fields or
a continuous arc can lead to a larger uniform
field, sufficient to cover a patient from head
to foot
Total Skin Irradiation
Large Field Technique

Field Flatness
Combination of three beam
intensity profiles along the
vertical axis to obtain a
resultant beam profile.
The central beam is
directed horizontally,
whereas the others are
directed at 18.5 degrees
from the horizontal.
is a weighting factor used
in an equation developed by
Holt and Perry.
Total Skin Irradiation
Large Field Technique

Field Flatness
Vertical beam profile at
the treatment plane for a
stationary single field and
an arcing field
Total Skin Irradiation
Large Field Technique

X-ray Contamination
• a limiting factor in total skin irradiation
• contributed by brermsstrahlung interactions
produced in the exit window of the
accelerator, scattering foil, ion chambers,
beam-defining collimators, air, and the
patient
• The bremsstrahlung level can be minimized
if the electron beam is scattered by air alone
before incidence on the patient
Total Skin Irradiation
Large Field Technique

X-ray Contamination
•
•
In the Stanford technique the electron beam, after
emerging from the accelerator window, is scattered
by a mirror (0.028-inch Al), an aluminum scatterer
located externally at the front of the collimator
(0.037-inch Al), and about 3 m of air before
incidence on the patient
The x-ray contamination incident on the patient is
reduced by angling the beam 10 degrees to 15
degrees above and below the horizontal. Because the
x-rays produced in the scatterers at the collimators
are preferentially directed along the central axes,
they largely miss the patient
Total Skin Irradiation
X-ray contamination
X-ray contamination
along the beam
central-axis
Reduce x-ray contamination by angling the central axis away from the patient
Total Skin Irradiation
Field arrangement
acrylic scatter plate ( 1 cm in thickness)
10~15°
10~15°
Total Skin Irradiation
Large Field Technique

Dose Distribution
•
•
•
•
For an oblique beam, the depth-dose curve and its
dmax shift toward the surface
a dose uniformity of ± 10% can be achieved over
most of the body surface using the six-field
technique, areas adjacent to surface irregularities
vary substantially due to local scattering.
Areas such as inner thighs and axilla, obstructed by
adjacent body structures, require supplementary
irradiation
The total bremsstrahlung dose in the midline for the
multiple field tech. is  twice the level of a single
field.
Total Skin Irradiation
Large Field Technique

Modified Stanford technique
•
•
•
adopted the Stanford technique in principle without
making alterations in the accelerator hardware
the electron field is collimated by a special wide
aperture insert attached at the end of the collimator.
• Wider jaw setting and a specific electron energy,
selected for high dose rate mode of operation
• high dose rate mode is installed to allow an
output of more than 2,000 MU/min
an acrylic scatter plate ( 1 cm in thickness) in front
of the patient to provide additional scatter to the
electron beam
Total Skin Irradiation
Modified Stanford technique

Dual field angle
•
widened in size by scattering:
• 9-MeV electron beam, after transversing 4 m of
air and an acrylic scatter plate, attains a Gaussian
dose profile measuring a 90% to 90% isodose
width of about 60 cm, cover a patient's width.
• Along the height of the patient, two fields are
angled such that in the composite dose
distribution a 110% dose uniformity can be
obtained over a length of about 200 cm.
Total Skin Irradiation
Modified Stanford technique

Dual field angle
11°
11°
Total Skin Irradiation
Modified Stanford technique

Calibration
•
•
A thin window ( 0.05 g/cm2) p-p chamber is a
suitable instrument for measuring depth dose
distribution for the low-energy beams used for this
technique
• calibrated by intercomparison with a calibrated
Farmer-type chamber, using a high-energy (10
MeV) electron beam
The AAPM (Report No. 23) recommends that the
total skin irradiation dose be measured at the
calibration point located at the surface of the
phantom and the horizontal axis
Total Skin Irradiation
Modified Stanford technique

Calibration
•
A p-p chamber, embedded in a polystyrene phantom,
is positioned to first measure the depth dose
distribution along the horizontal axis for the single
dual field
• the depth dose distribution can also be measured
by a film sandwiched in a polystyrene phantom
and placed parallel to the horizontal axis
Total Skin Irradiation
Modified Stanford technique

Calibration
•
The surface dose measurement is made at a depth of
0.2 mm. Suppose M is the ionization charge
measured; the calibration point dose to polystyrene,
(DP)Poly is, given by:
=1, p-p chamber
•
calibration point dose to water,
1, close to the surface
Total Skin Irradiation
Modified Stanford technique

Calibration
•
•
The treatment skin dose, (Ds)poly, is defined by the
AAPM (Report No. 23) as the mean of the surface
dose along the circumference of a cylindrical
polystyrene phantom 30 cm in diameter and 30 cm
high that has been irradiated under the total skin
irradiation conditions with all six dual fields.
(Dp)Poly is the calibration point dose for the single
dual field, then:
B ranges between 2.5 and 3 for the
Stanford-type technique
Total Skin Irradiation
Modified Stanford technique

Calibration
30 cm
30 cm
(Dp)Poly
(Ds)Poly
Total Skin Irradiation
Modified Stanford technique

Calibration
•
The composite depth dose distribution for the six
dual fields may be determined by sandwiching a
dosimetry film
Total Skin Irradiation
Modified Stanford technique

In Vivo Dosimetry
•
•
•
•
regions of extreme nonuniformity of dose on the
patients skin.
Excessive dose (e.g., 120-130%) can occur in areas
with sharp body projections, curved surfaces, or
regions of multiple field overlaps
Low-dose regions occur when the skin is shielded by
other parts of the body or overlying body folds.
From in vivo measurements, areas receiving a
significantly less dose can be identified for local
boost.
Total Skin Irradiation
Modified Stanford technique

In Vivo Dosimetry
•
•
•
If eyelids need to be treated, internal eye shields can
be used, but the dose to the inside of the lids should
be assessed, taking into account the electron
backscatter from lead.
TLD (<0.5 mm) are most often used for in vivo
dosimetry.
TLD may be calibrated in a polystyrene phantom
using an electron beam of approximately the same
mean energy as in the in vivo measurement
conditions.
Stereotactic
Radiosurgery/Radiotherapy



The term radiosurgery was coined by a
neurosurgeon Lars Leksell in 1951
SRS
 single-fraction radiation therapy procedure
 treating intracranial lesions
 using a combination of a stereotactic apparatus
and narrow multiple beams delivered through
noncoplanar isocentric arcs
SRT
 same procedure, multiple dose fractions
SRS/SRT



High degree of dose conformity and high
accuracy of beam delivery
The best achievable mechanical accuracy in terms
of isocenter displacement from the defined center
of target image is 0.2 mm  0.1 mm, although a
maximum error of 1.0 mm is commonly
accepted
Types used in SRS and SRT:
 heavy-charged particles,
 Cobalt-60 Gamma rays
 megavoltage x-rays.
SRS/SRT
SRS TECHNIQUES

X-RAY Knife
 linac-based SRS technique
 multiple noncoplanar arcs of circular (or
dynamically shaped) beams converging on to
the machine isocenter
 dose distribution be shaped to fit the lesion,
• Blocking, shaping with MLC
• changing arc angles and weights,
• using more than one isocenter,
• combining stationary beams
SRS/SRT
SRS TECHNIQUES

Stereostatic frame
Leksell
Riechert-Mundinger
Todd-Wells
Brown-Robert-Wells (BRW)
SRS/SRT
SRS TECHNIQUES

Stereostatic frame
CT localizer
head ring with
posts and pins
angiographic
localizer
patient-positioning
mount
SRS/SRT
SRS TECHNIQUES

Stereostatic frame
angiographic
localizer
CT localizer
DELINEATION OF BRAIN AVMs ON MR-ANGIOGRAPHY
FOR THEPURPOSE OF STEREOTACTIC RADIOSURGERY
DENNIS R. BUIS et al. Int. J. Radiation Oncology Biol.
Phys., Vol. 67, No. 1, pp. 308–316, 2007
SRS/SRT
SRS TECHNIQUES

The CT localizer frame is equipped with nine
fiducial rods, which appear as dots in the
transaxial slice image
 any point in the image can be defined in terms
of the frame coordinates.
SRS/SRT
SRS TECHNIQUES

patient docking device couples the frame to the
accelerator through the patient support system (pedestal
or couch-mount bracket)
SRS/SRT
SRS TECHNIQUES

The magnetic resonance imaging (MRI) localizer is a
slightly modified version of the CT localizer and is
compatible with MRI.
SRS/SRT
SRS TECHNIQUES

A special relocatable head ring, Gill-Thomas-Cosman
(GTC), has been designed for fractionated SRT
 uses a bite block system,
SRS/SRT
SRS TECHNIQUES

Linac isocentric accuracy


An essential element of the SRS procedure is the
alignment of stereotactic frame coordinates with the
linac isocenter
Acceptable specification of linac isocentric accuracy
within a sphere of radius 1.0 mm with any
combination of gantry, collimator, and couch
rotation.
SRS/SRT

Stereotactic accuracy

using the phantom base to check the alignment of
radiation isocenter with the target point defined by
the coordinates set on the BRW pedestal.
SRS/SRT

Overall accuracy




Before the SRS system is declared ready for patient
treatments, the entire radiosurgery procedure should
be tested for geometric accuracy
This can be accomplished by using a suitable head
phantom
The comparison of these coordinates with the known
coordinates of these points in the phantom gives the
geometric accuracy.
localization error, LE
SRS/SRT

Beam collimation



SRS or SRT is normally used for small lesions
requiring much smaller fields
the geometric penumbra (inversely proportional to
SDD) must be as small as possible
attachment of long cones below the x-ray jaws
extends the SDD, thus reducing the geometric
penumbra.
SRS/SRT

Beam collimation

SRS fields can be shaped with cone or MLC
SRS/SRT

Gamma knife



201 Co-60 sources are housed in a hemispherical
shield
Source to focus distance of 40.3 cm
The central axes of all 201 beams intersect at the
focus with a mechanical precision of 0.3 mm.
SRS/SRT

Gamma knife
照野孔大小分別為直
徑 4, 8, 14 及 18 mm
4 組不同大小照野孔的
準直頭盔
治療時固定於病患頭部的立體定位頭架與治療床定位架接合，準
直頭盔再與治療床外部固定結構接合
SRS/SRT

Gamma knife




Selected channels can be blocked with plugs to
shield the eyes or to optimize the dose distribution.
The plugs are made of 6-cm-thick tungsten alloy
Stereotactic CT, MRI, or angiography can be used
for target determination
Most users of Gamma Knife technology have
restricted lesion size to a mean spherical diameter of
35 mm (and usually less)
The accuracy of dose delivery using the Gamma
Knife as tested at the University of Pittsburgh was
found to be approximately 0.25 mm
SRS/SRT

Dosimetry


Three quantities of interest:
• central axis depth distribution (%dd or TMR),
• cross-beam profiles (off-axis ratios),
• output factors (Sc,p or dose/MU)
Measurement of these quantities is complicated by
two factors:
• detector size relative to the fieId dimensions
• a possible lack of charged particle equilibrium
• the sensitive volume of the detector must be
irradiated with uniform electron fluence (e.g.,
within 0.5%).
SRS/SRT

Dosimetry





Types of detector systems have been used in SRS
dosimetry:
• ion chambers, film, TLD, and diodes
ion chamber is the most precise and the least energydependent system but usually has a size limitation;
film has the best spatial resolution but shows energy
dependence and a greater statistical uncertainty (e.g.,
3%);
TLD show little energy dependence and can have a
small size but suffer from greater statistical
uncertainty as the film;
diodes have small size but show energy dependence
as well as possible directional dependence
SRS/SRT

Dosimetry

Cross-beam profiles
• a detector size of 3.5 mm diameter, circular fields
in the range of 12.5 to 30.0 mm in diameter can
be measured accurately within 1 mm.
• little change in the photon energy spectrum
across small fields, diodes and film are the
detectors of choice
AAPM Report No. 54
Cross-beam profiles
3x3,CC13 VS TPS
3x3, CC01, Film VS TPS
2x2, Film VS TPS
1x1, Film VS TPS
69
SRS/SRT

Dosimetry

Depth dose distribution
• For field sizes of diameter 12.5 mm or greater, it
has been shown that the central axis depth dose
can be measured correctly with a p-p ionization
chamber of diameter not exceeding 3.0 mm
• Film or diodes can also be used for central axis
depth dose distribution, especially for very small
field sizes
SRS/SRT
Dosimetry

Output factors
1.2
1.0
0.8
ROF

0.6
0.4
0.2
ROF-TPS
ROF-Pinpoint
ROF-Semiflex
ROF-Farmer
ROF-5 mm
ROF-2 mm
ROF-1 mm
ROF-0.5 mm
0.0
0.0
1.0
2.0
3.0
4.0
5.0
FS (cm)
6.0
7.0
8.0
9.0
10.0
SRS/SRT

Dosimetry

Output factors
 for fields of diameter 12.5 mm and larger,
clindrical or p-p chambers of 3.5 mm diameter
allow the output factors to be measured
accurately to within 0.5%
Stereotactic body radiation therapy
References:
1. Stereotactic body radiation therapy: The report of AAPM Task Group 101.
Med. Phys. 37 (8), August 2010, 4078-4101.
2. Technical Basis of Radiation Therapy, Practical Clinical Applications. Fifth
Edition
Introduction

SBRT refers to an emerging radiotherapy
procedure that is highly effective in
controlling early stage primary and
oligometastatic cancers at locations
throughout the abdominopelvic and
thoracic cavities, and at spinal and
paraspinal sites.
Introduction

The major feature that separates SBRT
from conventional radiation treatment
◦ large doses in a few fractions, which results
in a high biological effective dose (BED)

In order to minimize the normal tissue
toxicity, conformation of high doses to
the target and rapid fall-off doses away
from the target is critical.
Introduction

In SBRT, confidence in this accuracy is
accomplished by the integration of
◦ modern imaging,
◦ simulation,
◦ treatment planning,
◦ delivery technologies

into all phases of the treatment process;
from treatment simulation and planning,
and continuing throughout beam delivery.
Introduction
History and rationale for SBRT



Over 4000 publications have affirmed the
clinical usefulness of SRS in the treatment
of benign and malignant lesions, as well
as functional disorders.
The radiobiological rationale for SBRT is
similar to that for SRS.
The clinical outcomes of SBRT for both
primary and metastatic diseases compare
favorably to surgery with minimal adverse
effects.
Current status of SBRT-patient
selection criteria

The majority of patients treated with SBRT
◦ lung, liver, and spinal tumors.

Most investigators limit eligibility to
◦ well-circumscribed tumors
◦ maximum cross sectional diameter of
up to 5 cm.

The use of SBRT as a boost in addition to
regional nodal irradiation.
Current status of SBRT-patient
selection criteria



assessment of patient eligibility should
include a careful evaluation of normal
tissue function and dose distribution.
Typically, pulmonary function and the
volume of normal liver that is irradiated
are the most immediate considerations.
Tumors proximal to mainstem bronchi,
trachea, esophagus, gastric wall, bowel,
blood vessels, or spinal cord should be
approached with great caution.
Simulation imaging

SBRT requires precise delineation of
patient anatomy, targets for planning, and
clear visualization for localization during
treatment delivery.
◦ CT or 4DCT for visualizations and dose
calculation
◦ MRI and PET images assist in target and
visualization
◦ Dynamic contrast-enhanced CT is the most
sensitive study for the hepatic system
Simulation imaging

Recommendation:
◦ A typical scan length should extend at
least 5–10 cm superior and inferior
beyond the treatment field borders.
◦ For noncoplanar treatment techniques,
the scan length may further be extended
by 15 cm inferior/superior beyond the
target borders
Simulation imaging

Recommendation:
◦ all OAR should be included and covered
by the selected scan length so they can
be considered by the TPS and evaluated
with DVH
◦ tomographic slice thickness of 1–3 mm
though the tumor site is recommended
Abdominal
compression
plate

Activ e breath control (ABC)
JOURNAL OF APPLIED CLINICAL MEDICAL PHYSICS, VOLUME 14, NUMBER 6,
2013
Evaluation of two synchronized external surrogates for 4D CT sorting
Carri K. Glide-Hurst,a Megan Schwenker Smith, Munther Ajlouni,
Indrin J. Chetty
Data acquisition for mobile tumors

Techniques to image moving targets
include
◦ slow CT, breath-hold techniques, gated
approaches, 4DCT used in conjunction
with maximum-intensity projection,
respiration-correlated PET-CT
Imaging artifacts


slower acquisitions to characterize the
movement of the target can also lead to
motion artifacts
Motion-related artifacts may be improved
by immobilization and patient
cooperation.
Imaging artifacts
a spherical object (R=1.2 cm), CT
scanned while periodically moving on a
sliding table (A=1 cm, T=4.4 s).
different artifacts
obtained by standard
axial CT scanning.
imaged with 4DCT
Four-dimensional computed tomography: Image formation
and clinical protocol
Eike Rietzel, et al. Med. Phys. 32 (4), April 2005
Imaging artifacts

Recommendation:
◦ If target and radiosensitive critical
structures cannot be localized on a
sectional imaging modality with sufficient
accuracy because of motion and/or metal
artifacts, SBRT should not be pursued as a
treatment option
Gated CT (phase-50% )
MIP CT
Treatment planning

prescribing dose for SBRT
◦ A limited volume of tissue, containing
the gross tumor and its close vicinity, is
targeted for treatment through
exposure to a very high dose per
fraction, and hotspots within the target
are often deemed to be acceptable.
Treatment planning

prescribing dose for SBRT
◦ The volume of normal tissue receiving
high doses outside the target should be
minimized. Dose fall-off outside the
target should be sharp.
Treatment planning


In SBRT (especially for metastatic lung,
liver, and paraspinal cases), GTV = CTV
Typical SBRT margins for defining the
minimal distance separating the CTV and
PTV surfaces are 0.5 cm in the axial planes
and 1.0 cm in the inferior/superior
directions for treatments that were
performed in conditions that suppressed
respiratory motion.
Treatment planning


Dose prescriptions in SBRT are often
specified at low isodoses (e.g., 80%
isodose) and with small or no margins for
beam penumbra at the target edge
Hot spots within the target volumes are
generally viewed to be clinically desirable,
as long as there is no spillage into normal
tissue.
Treatment planning

parameters that affect the dose fall-off
◦ multiple nonoverlapping beams
◦ beam energy. A 6 MV photon beam provides a
reasonable compromise between the beam
penetration and penumbra characteristics for
SBRT lung applications.
◦ the resolution of beam shaping (e.g. MLC leaf
width). The 5 mm MLC leaf width has been
found to be adequate for most applications,
with negligible improvements using the 3 mm
leaf width MLC for all but the smallest lesions
(< 3 cm in diameter).
Beam selection and beam geometry


the avoidance of sensitive organs,
mechanical constraints imposed by the
equipment and short beam paths
In general, a greater number of beams
yields better target dose conformity and
dose fall-off away from the target.
◦ limit the number of beams or arcs
◦ avoiding beam overlaps
Calculation grid size



The calculation grid resolution used in the
TPS affects the accuracy of the dose
distribution calculated.
a 2.5 mm isotropic grid produces an
accuracy of about 1% in the high-dose
region of an IMRT plan consisting of
multiple fields.
an accuracy of 5% for an isotropic grid
resolution of 4 mm
Calculation grid size


dose difference of 2.3% of the prescribed
dose for 2 mm calculation grids as
compared to 1.5 mm grids, rising to 5.6%
for 4 mm grids.
Recommendation:
◦ The use of an isotropic grid size of 2 mm or
finer.
◦ The use of grid sizes greater than 3 mm is
discouraged for SBRT.
Bioeffect-based treatment planning


SBRT involves the application of high
fractional doses in a range not studied in
prior decades.
To evaluate the possible biological effect
of a SBRT treatment plan
◦ BED concept,
◦ the normalized total dose (NTD) concept,
◦ the equivalent uniform dose (EUD) concept.
Normal tissue dose tolerance


Normal tissue dose limits for SBRT are still
quite immature. should not be directly
extrapolated from conventional
radiotherapy data.
radiobiological factors, particular attention
should be paid
◦ fraction size,
◦ Total dose,
◦ time between fractions,
◦ overall treatment time,
Normal tissue dose tolerance
Normal tissue dose tolerance
Because of the sparseness of long-term follow-up for SBRT, it should
be recognized that the data in both Table III and the published reports
represent, at best, a first approximation of normal tissue tolerance.
Localization, tumor-tracking, and gating
techniques for respiratory motion management

Image-guided techniques
◦ Fluoroscopy
◦ gated radiographs
◦ cone beam imaging of soft tissue

Cone beam scans can have an acquisition time
60 s or more, capturing the average tumor
position over 15 or more breathing cycles, which
may correspond well to the planning ITV as
obtained from 4DCT.
Respiratory gating techniques



dose is delivered only in particular phases of the
respiratory cycle with the goal of reducing the
probability of delivering dose to normal tissue
and underdosing the target
Respiratory gating increases treatment time as
compared to nongated treatments; published
duty cycles (ratio of beam on to total beam
delivery time) range from 30% to 50%
the benefit of gated beam delivery is minimal
with motion amplitudes smaller than 2 cm
Special dosimetry considerations


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Heterogeneity correction becomes extremely
important in situations where the target is
surrounded by low-density tissue such as the
lungs.
Pencil-beam algorithms accounting for only 1D
scatter corrections are not recommended for
accurate estimate of the dose in such tumors and
in general for any lung tumors
convolution/superposition perform adequately in
most clinical situations
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