Non-target dose from radiotherapy:
Magnitude, Evaluation, and Impact
Stephen F. Kry, Ph.D., D.ABR.
Goals
• Compare out-of-field doses from various
techniques
– Methods to reduce out-of-field doses
• Impact of out-of-field doses
– Second cancers, fetal dose, pacemakers
• Techniques to assess out-of-field doses
– Limitations
Outline
• Magnitude of out-of-field doses
• Impact of out-of-field doses
• Assessment of out-of-field doses
Outline
• Magnitude of out-of-field doses
• Impact of out-of-field doses
• Assessment of out-of-field doses
Basics – conventional X-rays
• Sources of out-of-field
radiation:
1.0
– Patient scatter, collimator
scatter, head leakage.
Leakage
Patient scatter
Fraction of Dose
0.8
Collimator scatter
0.6
0.4
0.2
0.0
0
Leakage and Scatter
from machine
10
20
30
Distance from field edge (cm)
Kry et al, 2009, AAPM
Scatter from
within patient
40
50
60
Basics – conventional X-rays
• Doses studied extensively
• Trends in the data  generalizations
Basics – conventional X-rays
 Dose decreases
~ exponentially away
from edge of field
 1% at 10cm
  with field size
 Const. with energy
 Const. with depth
Stovall et al. 1995, Medical Physics
Basics – conventional X-rays
Beam modifiers
• Wedges
– Physical wedges  increase out of field
dose by 2-4 times (Sherazi et al, 1985, Int J Radiat Oncol Biol Phys)
– Dynamic or universal wedges  no increase
(Li et al, 1997, Int J Radiat Oncol Biol Phys)
• MLC
– Secondary MLC  no impact on out-of-field
dose (Mutic et al, 2002, J Appl Clin Med Phys)
– Tertiary MLC is extra shielding  decrease
out of field dose by 30-50% (Stern, 1999, Med Phys)
IMRT
• Near target, dose reduced
– Treat smaller volume
• Farther from target, higher doses
– Modulation increases head leakage
1.0
1.0
Leakage
Patient scatter
0.8
Collimator scatter
Fraction of Dose
Fraction of Dose
0.8
0.6
0.4
0.6
Leakage
Patient scatter
0.4
Collimator scatter
0.2
0.2
0.0
0.0
0
10
20
30
40
50
60
0
10
20
30
40
Distance from field edge (cm)
Distance from field edge (cm)
Kry et al, 2009, AAPM
50
60
IMRT
• Overall: generally higher
doses with IMRT
10000
Kry 18 MV IMRT
Kry 18 MV Conv
Howell 6 MV IMRT
Howell 6 MV Conv
• Extent of modulation
changing
– Modulation factor
decreasing
– Early 2000s: 3-5
Dose (mSv)
1000
100
10
0
10
20
30
40
50
Distance from central axis (cm)
60
(Followill, 1997, Int J
Radiat Oncol Biol Phys. Kry, 2005, Int J Radiat Oncol Biol Phys)
– Clinically (2010): often <2
Kry, 2005, Int J Radiat Oncol Biol Phys
Howell, 2006, Med Phys
70
High-energy photon therapy
Above ~10 MV
neutrons
Mutagenesis in mouse
fibroblast cell line
Hall, 1995
Dicentric chromosome induction in human lymphocytes
Nolte, 2005
•Other measurements have found RBE = 1
•Large uncertainties in neutron RBE
NCRP 104
High-energy photon therapy
This RBE is usually described with:
Radiation weighting factor
1.E-09
20
18
8.E-10
16
Spectrum
14
6.E-10
12
wR
10
4.E-10
8
6
2.E-10
4
2
0.E+00
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
E (MeV)
ICRP 92
1.E-02
1.E-01
1.E+00
0
1.E+01
wR
Neutron Fluence (A.U.)
H = wR×D
High-energy photon therapy
• Neutron dose equivalent  with beam energy
• ~Constant with field size, distance off-axis
Howell 2009 Med Phys
• Dosimetry is difficult – varied results in literature
(Kry 2005, Int J Radiat Oncol Biol Phys)
50
40
30
Testes
Prostate
Colon
Liver
Stomach
Esophagus
Lung
Heart
Thyroid
20
% of total dose
• Neutrons: 25% of out-offield dose equivalent
(18 MV IMRT)
• 10 MV may be optimal
energy for deep tumors
10
0
-10
0
10
20
30
40
Distance from Central Axis (cm)
Kry et al., 2009, Radiother Oncol
50
60
Other photon modalities
• Cyberknife
– Older version  4X higher doses than linac
– Newer versions  added shielding
• Doses more consistent with linacs
(Chuang, 2008, Med Phys)
– Be aware of beams along patient axis
• Gamma Knife
– Doses higher near field,
lower farther away
Zytkovicz, 2007, Phys Med Biol
(Petti 2006, Med Phys)
Other photon modalities
• Tomotherapy (Hi-Art)
– Many more MUs but extra shielding, no
flattening filter
– Mixed/comparable results relative
to IMRT (Ramsey, 2006, J Appl Clin Med Phys)
• Overall:
– Some variation between different
accelerators
– All in the same ballpark (factor of 2)
Proton therapy
• Sources of out-of-field dose equivalent:
– Mostly external neutrons
– Some internal neutrons
– A few photons
Fontenot,
2008, Phys
Med Biol
•
•
•
•
Proton therapy
Neutron dose eq. varies with:
Treatment energy
SOBP and snout size
Air gap
• Small change with distance
or field size
Zheng, 2007, Phys Med Biol
Zheng, 2007, Phys Med Biol
Mesoloras, 2006, Med Phys
Proton therapy
• How much dose equivalent is there?
Variations in beam
parameters
Beam energy, SOBP,
aperture, air gap
Variations in experimental
design
Size and material of
phantom, manufacturer of
accelerator
Challenges in Dosimetry
Lack of high energy
response
Xu, 2008, Phys Med Biol
Unique machines
Protons vs. Photons
Conventional photon therapy
•Photons:
•More dose near
treatment field
•Comparable
dose beyond 1020 cm from field
edge
Xu, 2008, Phys Med Biol
Stovall, 1995, Med Phys
Protons vs. Photons
• Depending on what proton data you use, you
can get very different results:
Hall, 2006, Int J Radiat Oncol Biol Phys
Zytkovicz, 2007, Phys Med Biol
Protons vs. Photons
• Near to field, dose equivalent much lower with p+
– Less lateral scatter
– Less exit dose
• Less total out
of field dose
• Effect more
pronounced at
lower p+ energy
Fontenot, 2008, Phys Med Biol. HT/D as a function of lateral distance
(along the patient axis) from the isocenter from this work compared to
IMRT values collected from Kry et al (2005) and Howell et al (2006).
Scanning Proton Beams
• Much interest in scanning beams
• No external neutrons
• Still internal neutrons, gammas
– Up to half of dose equivalent to near organs
– Negligible dose to distant organs
• Scanning beam is an improvement,
but is not
free from out-offield dose
Outline
• Magnitude of out-of-field doses
• Impact of out-of-field doses
• Assessment of out-of-field doses
How much dose is there?
• Out of field doses vary with treatment
type/parameters.
• Typical numbers, (20 cm from field edge, 60 Gy
treatment):
–
–
–
–
20-60 mSv, scanned proton beam
40-400 mSv, passive scatter protons
250 mSv, Conventional photons
300-450 mSv, IMRT
• What do these doses mean?
How much is this dose?
• Put these doses in perspective
20-60 mSv,
scanned proton beam
40-400 mSv,
passive scatter protons
250 mSv,
Conventional photons
300-450 mSv, IMRT
Courtesy of
David Brenner
5-100 mSv
Late Effects
• Radiation has been shown to increase
the risk of
–
–
–
–
–
Cardiovascular disease
Diabetes
Stroke
Hereditary effects
Second cancers
• Potentially relevant for all patients
Hereditary effects
• Historically a major concern
– Mutant babies
• Gonad dose considered highest risk
• Not supported with data
– Neel Am J Hum Genet 1990, Little Br Med Bull 2003
• Gonad dose no longer considered high risk
Heart Disease
• Increased risk with radiation
– A-bomb survivors
– Left-sided breast patients
• Clinically:
– Minimize dose to the heart
Second Cancers
Female second cancer incidence. Lifetime cases/100k exposures to 0.1 Gy
400
Stomach
Colon
350
Liver
Lung
BEIR VII
Breast
300
Bladder
Other
Thyroid
250
Cases
Leukemia
Uterus
200
Ovaries
150
100
50
0
0
10
20
30
40
50
60
70
Age at exposure
Much higher risk for younger populations
80
Second cancers
• Cancer takes at least 5 years, typically
10+ years to develop after exposure
– Risk continues to be elevated 50 years
later
Absolute Risk
for 10+ year
survivors:
1 in 70
Brenner Cancer 2000
Second Cancers
• Where do second cancers occur?
– 12% within geometric field
– 66% beam-bordering region
– 22% out-of-field (>5 cm away)
Diallo IJROBP 2009
• Get most second cancers in high-dose
regions
– This is also the area we’re trying to treat, so
hard to reduce this dose
• Want to reduce the dose outside PTV
Therapy Symposium
• Late Effects from Radiation Therapy
and Diagnostic Imaging
• Thursday, 12:30-2:20
• Ballroom A
Fetal dose
• Fetus is collection of rapidly dividing
cells
– Very sensitive to radiation damage
• Sensitivity and damage depend on
stage of development
Fetal dose
Stovall et al, 1995
Fetal dose
• Important to put
these risks into
perspective
– Risk of radiation
induced birth
defects vs. normal
birth defects
Stovall et al, 1995
Fetal dose
• Treatment decision will be made by
physician (treat, recommend abortion,
chemo only, etc.)
• Physicists must carefully assess dose
• Impact of this dose may require
discussion as well.
Fetal dose - shielding
• Shielding should always be used
– Even if not necessary
• Pb is not an effective shield against neutrons
– Use BPE
1.0
Leakage
Patient scatter
0.8
Fraction of Dose
• Shielding is more
effective the farther
from the treatment
field you go
~50% reduction
Collimator scatter
0.6
0.4
0.2
0.0
0
10
20
30
40
Distance from field edge (cm)
50
60
Pacemakers/Defibrillators
• Electronics are susceptible to radiation
damage
– Irritation for cameras, CTs etc.
– Patient safety issue with
pacemakers/defibrillators
• Can be transient effects while beam is on,
or can have permanent effects
Pacemakers/Defibrillators
• Inside treatment field is clearly a high dose
• May be sensitive to relatively low doses:
– High intensity imaging procedures
• High LET radiation – high energy X-rays/proton therapy
– Observed failure after 0.9 Gy of neutrons
[Raitt Chest 1994]
Pacemakers/Defibrillators
• Ensure there are policies (and follow them)
– Determine dose to device and compare with
device tolerances
– Monitor functionality of device during/after
patient treatment
– Assess need to move device before treatment
• TG-34: Management of radiation oncology
patients with implanted cardiac pacemakers
• TG-203: Management of radiotherapy patients
with implanted cardiac pacemakers and
defibrillators
Outline
• Magnitude of out-of-field doses
• Impact of out-of-field doses
• Assessment of out-of-field doses
Calculations:
Treatment Planning Systems
Distance from
Field Edge
(cm)
Dosecalc
(cGy)
Dosemeas
(cGy)
Percent
Difference
3.75
3.08
0.61
4.24
0.45
38%
6.25
2.02
0.43
3.01
0.24
49%
8.75
1.16
0.32
2.09
0.14
80%
11.25
0.66
0.33
1.49
0.13
126%
Howell et al, in preparation
• This is for a simple, conventional field.
• As treatment complexity increases, more scatter
contributing to dose, less accuracy
– See this with Mesothelioma cases – even within the
treatment field we are often off by several percent due
to the limitations of the treatment planning system to
handle scatter.
Calculations - simple
• TG-36
• Peridose software
(van der Giessen Ratiother Oncol 2001)
• Accuracy
• Pretty good (±30%)
• Double check with measurement
• Limitations of treatments
• Neutrons, IMRT, electrons, brachytherapy
Additional source
• Fetal dose from a large number of procedures, including
radiotherapy, brachytherapy, nuclear medicine, imaging…
Calculations - Detailed
• Monte Carlo studies
– Benchmark simulations
against measurements
– Use detailed model
Bednarz Med Phys 2008
Measurements
• Ion chamber measurements
Measurements
• In vivo measurements
– Taken directly on/in patient
• Caution: surface dose is elevated (4x)!!
• Cover detector with bolus
TG-36
Kry Med Phys 2006
Additional TLD comments
• TLD very nice dosimeter
• TLD-100 over-responds to neutrons
– Factor of 10 in reported dose for 18 MV
– 2.3 mGy reported as 27.0 mGy Kry JACMP 2007
• Do not use TLD-100 for out-of-field
measurements for E>10 MV
• (OK inside treatment field)
• Use TLD-700, ion chamber
Other considerations
• Energy spectrum is much different
outside the treatment field
• Energy dependent dosimeters will
have a different response
• TLD, diodes….
• Even ion chambers
Perspective and final
thoughts
• Out of field doses are not negligible
• Risk is justified
• Must be cognizant of, and work to minimize, stray
radiation
• Be aware of dose assessment pitfalls
• Approach to assess out-of-field dose should be
based on the required accuracy of the information.
Thank you!