Transcript Out-of-field Dosimetry for Secondary Cancer Studies: Past Experience and Future Needs

Out-of-field Dosimetry for
Secondary Cancer Studies: Past
Experience and Future Needs
David Followill, Ph.D.
Radiological Physics Center
U. T. M. D. Anderson Cancer Center
Introduction
As we have all known for a long time:
Patients undergoing radiation therapy are
exposed to secondary radiation (radiation out
of the treatment field).
Secondary radiation is composed of photons,
and at high treatment energies (above 8
MV), neutrons, which are produced in the
accelerator head.
Introduction - Photons
Secondary photon radiation composed
of scatter and leakage.
Scatter from within patient and off of collimators
is dominant source near the treatment field.
Leakage through the accelerator head is the
dominant source away from the treatment field.
Leakage and Scatter
from machine
Scatter from
within patient
Introduction - Neutrons
Neutrons are produced primarily by
photons striking the primary collimator,
jaws, and target.
Neutrons are important because of their
high RBE. ????????
Radiation Type
Energy
Quality Factor
X and gamma Rays
All
1
Neutrons
< 10 keV
10 keV to 100 keV
100 keV to 2 MeV
2 MeV to 20 MeV
> 20 MeV
5
10
20
10
5
Why all this Concern?
There are now new treatment techniques and
devices being used:
–
–
–
–
Proton machines
Tomotherapy units
CyberKnife units
IMRT delivery
The new treatment techniques and/or devices
are designed to deliver high dose gradients such
that the target gets a high dose and the
surrounding normal tissues get a lower dose.
The great dose distributions sometimes come at
a cost!
Why all this Concern?
Amount of secondary radiation is a function of the
amount of beam-on time.
Some IMRT treatments may require up to 4 times
as many MU’s to deliver as conventional
treatments.
For deep treatment sites, low energy treatments
typically require more MU’s than high energy
treatments.
High energy x-rays and protons produce neutrons
Bottom Line: More MU’s mean more
secondary radiation.
Why are we Concerned?
LNT – BEIR VII
Linear Exponential (Gray 1965, Schneider et al. 2005)
– model suggested from human, animal, and in vitro data
Linear Plateau (Ron 1998)
– derived from human epidemiological studies of radiationinduced breast, bladder, and stomach cancers
Hall 2006
This is what started it ALL
Likelihood of Secondary Fatal Malignancy
(%)
Technique
6 MV
18 MV
25 MV
Conventional
0.3
1.8
3
MLC
modulated
1.0
5.1
8.4
Serial
Tomotherapy
2.7
14.9
24.4
Calculated Risk estimates
Followill et al (1997)
Where are we Concerned?
Secondary radiation in the “Patient Plane”!!
Let’s First Worry about Photons
Early measurements – early 80’s
– Ion chambers in large water phantoms
Large volume ion chambers (0.3 – 30 cc)
Scanning tanks
More Measurements
Phantoms began to more closely
approximate actual patient geometry
– Using cylindrical ion chambers
More Measurements
Solid geometric phantoms also used
– Using TLD, diodes and 0.6 cm3 ion
chambers
Mutic et al (1998)
More Measurements
Solid geometric phantoms also used
– Using cylindrical small volume ion chambers
Klein et al. (2006)
Most Recent Measurements
Anthropomorphic Rando phantom with TLD -100
and 700 depending on x-ray energy at 10 specific
organ sites.
3 TLD at each location.
Kry et al (2005)
Adult Procedure
Adult Prostate Treatment with
TomoTherapy and CyberKnife units
– Same prescription for all treatment devices
– Common TLD placement in phantom organ locations
(2 TLDs)
Pediatric Procedures
Pediatric TomoTherapy Cranio-Spinal Irradiation
(CSI) and CyberKnife GBM treatment
– Same prescription for
3D and Tomotherapy treatment plans
IMRT and CyberKnife treatment plans
– TLD and EBT film placement in pediatric phantom
– Organ doses from TLD-100
– EBT film validation of TPS calculations
Photon Measurement Cautions
1. Biggest issue: low doses = very low rdgs.
– Increase in uncertainty of measurements
– Long exposure times
2. Need for multiple rdgs. at each point.
3. Rdg. location (air vs. phantom)
or at what depth?
4. Point vs. volume measurements.
5. Neutron component for high X-ray energies
Photon Dose Equivalent as a percent of
dose at dmax vs. Distance
Out of Field Photon Dose for Varian Accelerators
1
6 MV V
10 MV V
Dose as Percent of Dmax on Central Axis
15 MV V
18 MV V
Mutic /Klein
Stern
0.1
0.01
0
10
20
30
40
Distance From Central Axis (cm)
50
60
Neutron Measurements
Neutron fluence measured with gold foils.
–
197Au(n,g)198Au
Count the g,b emissions of the foils, convert to
neutron fluence by NIST traceable conversion
factor:
Gold foils detect thermal neutrons.
– Bare gold foils measured the thermal neutron fluence.
– Fast neutrons are thermalized by moderators. Gold
foils placed in moderators thereby measure the fast
neutron fluence.
Neutron Measurements
Determining neutron
dose equiv. comprises
several steps
– Obtain NIST traceable
calibration
– Measure neutron fluence
– Calculate neutron dose
equivalent at dmax
– Calculate neutron dose
equivalent at depth
Neutron Measurements
Bonner sphere system to measure the
fluence from which the neutron spectrum
is deconvolved.
Howell et al (2006)
Neutron Measurements
Bubble detectors
or
neutron meters
Neutron Contribution (%) to
Secondary Dose
Organ Site
Colon
Liver edge
Stomach edge
Liver center
Stomach center
Esophagus edge
Lung edge
Lung center
Esophagus center
Thyroid
Bone Marrow
Percent of Total Dose Equivalent from Neutrons
18 MV C 10 MV V 15 MV V 15 MV S 18 MV V
20
23
43
0.7
13
28
28
49
1.1
16
28
1.3
30
54
18
36
1.1
35
58
21
38
1.2
35
59
22
31
29
54
0.7
18
55
2.3
52
72
29
55
1.3
46
71
22
58
1.2
49
72
23
80
6.1
74
85
36
50
3.3
53
70
32
Data from S. Kry
Neutron Fluence
Fast neutron fluence measured on CAX and out of field.
Fast neutron fluence out of field varied by less than the
uncertainty in the dosimeter. Fast neutron fluence assumed
constant out of field.
For each distance from central axis, the neutron fluence was
broken down into 12 components to account for energy and
Direct
geometry.
Scattered
Thermal
X
Neutron fluence was examined at the same 10 points as where
the photon dose was measured.
Neutron Measurement Cautions
1.
Gold foil activation – not for everyone.
–
–
2.
NIST traceability
Still the “gold” standard
Low doses = very low rdgs.
–
–
Increase in uncertainty of measurements
Long exposure times
Need for multiple rdgs. Along patient plane.
4. Measurement variability among the different
neutron dosimeters
5. Difficulty measuring the neutron dose at depth in
a patient
3.
Dose Equivalent per Complete
Prostate Treatment
(photon and neutron)
Dose Equivalent (cSv) per complete Adult Prostate Treatment
Organ Site
Thyroid
Lung center
Esophagus center
Liver center
Stomach center
Trans. Colon
Treatment Plan. Sys.
18 MV
3D CRT
12.7
12.6
9.6
24.2
23.2
48.2
Pinnacle
6 MV
6 MV
6 MV
IMRT
TomoTherapy
CyberKnife
5.3
2.4
34.4
15.3
6.7
5.1
27.0
12.0
6.2
4.2
25.4
11.3
15.5
12.2
27.7
12.3
19.9
15.6
30.3
13.4
33.3
27.6
19.0
42.8
- 42.8
Pinnacle HiArt plan station Multiplan
18 MV
IMRT
54.6
44.7
35.1
69.3
68.7
101.4
Corvus
Data from Kry et al, S. Lazar, M. Bellon
Risk (%) of Secondary Cancer per
Complete Prostate Treatment
(photon and neutron)
Lifetime Risk (%) of Secondary Cancer per Prostate Treatment
Organ Site
Thyroid
Lung center
Esophagus center
Liver center
Stomach center
Trans. Colon
Treatment Plan. Sys.
18 MV
3D CRT
0.00
0.12
0.06
0.03
0.03
0.24
Pinnacle
6 MV
6 MV
6 MV
IMRT
TomoTherapy
CyberKnife
0.00
0.00
0.00
0.06
0.05
0.25
0.11
0.04
0.02
0.15
0.07
0.02
0.01
0.03
0.01
0.02
0.02
0.03
0.01
0.16
0.14
0.09
0.21
- 0.21
Pinnacle HiArt plan station Multiplan
18 MV
IMRT
0.00
0.42
0.20
0.08
0.08
0.50
Corvus
Data from Kry et al, S. Lazar, M. Bellon
Dose Equivalent to Edge of Stomach
Dose Equivalent
Equivalent to
to Center
Center of
Dose
of Stomach
Stomach
1000
1000
900
900
800
Neutron
Equivalent
Neutron
DoseDose
Equivalent
Photon
Equivalent
Photon
DoseDose
Equivalent
Dose
(mSv)
DoseEquivalent
Equivalent (mSv)
800
700
700
600
600
500
500
400
400
300
300
200
200
100
100
00
18
18MV
MVCRT
CRT
Pinnacle
Pinnacle
6MV
6MVVV
Corvus
Corvus
6MV
6MVVV
Pinnacle
Pinnacle
6MV
6MVSS
Corvus
Corvus
10MV
10MV VV
Corvus
Corvus
15MV
15MV V
V
Corvus
Corvus
15MV
15MV S
S
Corvus
Corvus
TreatmentApproach
Approach
Treatment
18MV
18MV V
V
Corvus
Corvus
6MV
6MV Tomo
Tomo
Plan.
Plan. Sta.
6MV
6MV CK
CK
Multiplan
Multiplan
Dose Equivalent and Risk (%) per
Complete Pediatric GBM Treatment
Dose Equivalent
Lifetime
Risk (%) (cSv)
per complete Pediatric GBM Treatment
Organ Site
Thyroid
Lung center
Breast
Liver center
Trans. Colon
Stomach
center
Ovary
Treatment Plan. Sys.
6 MV
IMRT
5.5
0.04
3.7
0.10
2.5
0.05
1.5
0.01
1.1
0.02
1.0
0.00
Pinnacle
MV CyberKnife
CyberKnife
66 MV
version 1.5
version 2.1
32.2
19.3
0.24
0.15
60.2
39.1
1.59
1.03
12.3
8.0
0.26
0.17
9.2
6.0
0.03
0.02
8.5
5.5
0.12
0.08
8.1
5.3
0.04
0.02
Multiplan
Multiplan
New optimization and tuning tools as well dose homogeneity
Now let’s include Proton Treatments
Seems that every
other day there is a
new proton facility
being built
Is there any Secondary
Radiation to worry about?
YES!
Proton beams generate neutrons by
interacting with the scattering
systems, range modulator wheel,
collimators and even the patient
How significant can this be?
Stray Radiation Exposure from Different RT Facilities
Hall, Harvard, Passive,
Normalized to 10x10 cm2
Yan, Harvard, Passive, 8 cm
SOBP, FS=5x5 cm2
Zheng, PTCH, Passive, 8 cm
SOBP, FS=10x10 cm2
Hall, 4 Field CRT, 6 MV
1000.00
H/D (mSv/Gy)
100.00
Zheng, PTCH, Passive, Pristine
peak
Hall, IMRT, 6 MV
10.00
Schneider, PSI, Scanning,
Pristine peak
Mesolaras, MRPI, Passive
1.00
0.10
0.01
0.00
0
50
100
Distance from Field Edge (cm)
150
Polf, Harvard, Passive, Large
Field, 8 cm SOBP
Tayama, PMRC, Passive, 10 cm
SOBP, FS=11 cm2
Fontenot, Harvard, Radiosurgery,
pristine peak
Yan, Harvard, Passive,
Radiosurgery
Yan, Harvard, Ocular, 2.3 cm
SOBP
SlideHall
coutesy
(2006)of J Fontenot
Neutron Measurements
Measured and calculated data is sparse
Results are not consistent
Measurement techniques are not consistent
Many different factors affect neutron
production
– Machine type (synchrotron vs cyclotron)
– Proton energy
– Range modulation
– Field size
– Lateral scattering technique
Bonner Sphere Extension
•The BSS and BSE response function from thermal to 15
MeV is verified and corrected using AmBe & Cf-252
source at Georgia Tech.
•The response function from 15 MeV up to 800 MeV is
corrected using the 800 MeV neutron beam at LANSCE
Slide courtesy of Rebecca Howell
Experimental Setup
Head
Hip
Shoulder
Neutron Measurement Cautions
1. Neutron energies much higher than
observed around electron accelerators
2. Is your neutron calibration technique
calibrated appropriately for these
“neutrons”?
3. Long exposure times
4. Moderators used on electron accelerators
are not adequate.
Comparison of the doses in the pediatric CranioSpinal case
treated by 3D-conventional, tomotherapy, and proton therapy
Organ site
3D 6 MV
(cGy)
Thyroid
2797 (12)
362 (12)
152 (9)
437 (6)
Heart center
2957 (41)
865 (16)
Heart edge
2345 (18)
438 (16)
Lt. Lung center
226 (13)
907 (43)
Lt. Lung Edge
242 (33)
446 (9)
Liver Center
2583 (18)
1107 (124)
Liver edge
217 (14)
545 (16)
Lt. Kidney
221 (6)
748 (83)
Bladder
195 (11)
77 (2)
86 (4)
528 (30)
322 (60)
135 (22)
Lt. Breast Bud
Pelvic
Lt. Ovary
TomoTherapy
Proton
(cGy)
(120-180 MeV)
(cSv)
Data from S. Lazar and Z. Wang, MDACC
22
21.8
17.7
Summary
No consensus as to the best measurement
technique. Calculations will play a larger role in the
future.
Increases in the secondary dose is highly
dependent on the number of MUs and photon
energy.
Measurement of the secondary dose requires an
established NIST traceable technique and
characterization of the dosimeters at the
appropriate energies.
New treatment planning software with better
optimization routines have reduced the number of
MUs per treatment reducing the secondary doses.
Neutron dose may play less of a role than
previously thought.
Take Home Message
Measurement techniques are maturing.
Patient models and dose calculations are
more sophisticated and accurate.
Biggest uncertainty is the RISK estimate
There are many factors to be considered
in the treatment of a patient and the risk of
a secondary cancer is only one of many.
I believe this risk to be small regardless of the
treatment technique