Small Field Dosimetry
&
Radiochromic film
Michael Kissick, Ph.D., Assistant Professor,
Medical Physics & Human Oncology &
Affiliated with the
Morgridge Institute for Research
North Central AAPM Chapter Symposium, 10/10/2013
DEPARTMENT OF MEDICAL PHYSICS, UNIVERSITY OF WISCONSIN-MADISON
• I have no potential conflicts of interest.
Outline and Thesis/Objectives:
I. Small Field Dosimetry – review
II. Radiochromic film (RCF) in general – overview
III. QC/QA for high dose hypofractionated radiotherapy – A few
concerns*, many other reasons not to be concerned.
IV. Conclusions – this film will continue to be useful
Learning Objectives:
I. Radiochromic film is fascinating. The topochemical process
involved is special in chemistry, and very useful in dosimetry
after the vendor has spent decades perfecting it for our use.
II. The QC/QA of the future will likely use this film even more than it
is used now, because of hypofractionation and the need for
anthropomorphic phantoms with high dose gradients for small
fields. Some concerns I have … mostly, RCF is an excellent
choice.
I will assume most of you know the basic positives/negatives of RCF.
DEPARTMENT OF MEDICAL PHYSICS, UNIVERSITY OF WISCONSIN-MADISON
* Composite fields / fractionation
What is a small field?
According to Alfonso et al [1]
2008, page 5180, first paragraph:
“A small field is defined as a
field with a size smaller than the
lateral range of charged
particles.”
R. Alfonso et al, “A new
Formalism for Reference
Dosimetry of Small and
Nonstandard Fields,” Medical
Physics 35 (11), Novenber 2008,
5179-5186.
1νh
1νh
2νh
2νh1T
1T
2T
2T
Loss of charged particle equilibrium
for the small field � We need a
dosimeter with resolution better than
the range of the average charged
particle: FILM IS SUCH A DOSIMETER!
If charged particle equilibrium:
TKDose enc ∝∝∝ µ
If no charged particle equilibrium:
DEPARTMENT OF MEDICAL PHYSICS, UNIVERSITY OF WISCONSIN-MADISON
Then we are sensitive to unknown
aspects of the charged particle spectrum.
RCF does not have the same small field issues
that ion chambers have – it does not have an air
filled cavity.
1. If one attempts to make a small ion chamber for resolution, and the media are not perfectly
matched, then delta-ray equilibrium can be a problem.b
2. Photon energy determines everything, but the proportionality of a ratio of readings to a ratio of
absorbed dose is generally robust to the photon spectrum.a
3. This is because it is well-known that ratios of stopping powers are robust to the charged
particle energy spectrum.b However, we really need charge particle equilibrium for this.
4. Because RCF is not a gas, the energy dependence resulting from a density effect mismatch is
not a concern.
5. It is the tissue equivalence of this film that is a major help for us.
DEPARTMENT OF MEDICAL PHYSICS, UNIVERSITY OF WISCONSIN-MADISON
a. F. Verhaugen et al., “Monte-Carlo dosimetry study of a 6 MV stereotactic radiosurgery unit,”
Phys. Med. Biol. 43 (1998) 2755-2768.
b. Radiological Physics and Dosimetry, Frank Herb Attix, Wiley-VCH, Germany, 2004 edition
RCF has the spatial resolution needed, and it does
not itself change equilibrium.
(It is tissue equivalent also)
4 mm beamlet From:
N. Hardcastle et al. “High dose per fraction dosimetry of
small fields with Gafchromatic EBT2 film,” Medical Physics
38 (7), July 2011, 4081-4085.
DEPARTMENT OF MEDICAL PHYSICS, UNIVERSITY OF WISCONSIN-MADISON
So, we can image the penumbra from lateral electron motion.
In the context of radiotherapy’s future, small
fields pose the following challenges for
dosimetry*:
1. Some innovative machines with small fields CANNOT provide the standard reference field conditions, i.e., they cannot provide the 10 cm X 10 cm uniform field at 100 cm distance. This is true of helical tomotherapy and the CyberknifeTM.
2. Also, there is no standard for composite fields, i.e., helical tomotherapy inherently provides dose with composite fields.
3. For RCF, for composite fields, an issue for me concerns repeated applications of dose – new polymerizations applied on top of previous polymerizations – Note: we have seen fractionation effects, confirmed by David Lewis.
DEPARTMENT OF MEDICAL PHYSICS, UNIVERSITY OF WISCONSIN-MADISON
* Alfonso et al Med Phys 35 (2008) 5179.
How does RCF work? It is made up of
special crystals: LiPCDA.
DEPARTMENT OF MEDICAL PHYSICS, UNIVERSITY OF WISCONSIN-MADISON
From David Lewis
talk, Oct. 20, 2010
Polydiacetelyenes in hair-like crystals:
The crystals are made up of stacks of
monomers that polymerize in place:
topochemical phase change, remarkable!
DEPARTMENT OF MEDICAL PHYSICS, UNIVERSITY OF WISCONSIN-MADISON
CC
CC*
R
R
*C
CCC
R
R
CC
CC
R
R
**C
CCC
*
R
R
*C
CCC
R
R
CC
CC
R
R
**
H. Gross et al., Chem. Phys. Lett. 95 (1983), p. 584
The polymerization process is inherently
nonlinear: the activation energy barrier
could respond to local strain for example:
DEPARTMENT OF MEDICAL PHYSICS, UNIVERSITY OF WISCONSIN-MADISON
Polydiacetylenes, Editor: Hans-Joachim Cantow, Springer-Verlag, ‘84
These barriers could
change size as the
fraction polymerized
changes – understudied!
Some complications … known for a long
time … Not perfectly topochemical –
strain develops!
DEPARTMENT OF MEDICAL PHYSICS, UNIVERSITY OF WISCONSIN-MADISON
From: R.W. Carpick et al., Langmuir 16 (2000), page 4639
Notice the
strain this
causes!
And
rotational
strain too:
Both can
cause OD
Changes!
This local strain, if different from the
surrounding crystal, could cause OD
changes at low dose …
DEPARTMENT OF MEDICAL PHYSICS, UNIVERSITY OF WISCONSIN-MADISON
* N.V. Klassen et al,
Med. Phys. 24 (1997) 1924.
•R.H. Baughman, J. Chem Phys. 68 (1978) 3110.
There are also temperature phase
changes due to the hydrogen bonding to
the substrates:
DEPARTMENT OF MEDICAL PHYSICS, UNIVERSITY OF WISCONSIN-MADISON
Q. Huo, et al., Langmuir 15 (1999) 3972-3980
U. G. Hofmann, et al., Langmuir 17 (2001) 1518-1524
We saw even more worrisome issues:
fractionation effects! – scanner or film?
DEPARTMENT OF MEDICAL PHYSICS, UNIVERSITY OF WISCONSIN-MADISON
Fractionation Effects ...
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0 10 20 30
total accumulated dose
Inc
rem
en
tal s
en
itiv
ity
: d
(OD
)/d
(do
se
)
1Gy fract., 5Gy pre
0.5Gy fract., 5Gy pre
2Gy fract., 5Gy pre
1Gy fract., 0Gy pre
0.5Gy fract., 0Gy pre
2Gy fract., 0Gy pre
5 Gy fract., 0 Gy pre
5 Gy fract., 5 Gy pre
E.Y. Hirata et al., “Low Dose Fraction Behavior
of High Sensitivity Radiochromic Film,” Med.
Phys. 32 (2005) 1054HS film
Biggest
issues
for small
doses atop
big doses !
David Lewis made me aware of the issue
that the field flatness depends on density
in the Epson 10000XL flatbed scanner:
DEPARTMENT OF MEDICAL PHYSICS, UNIVERSITY OF WISCONSIN-MADISON
From David Lewis
talk, Oct. 20, 2010 … he claims this is due to polarization of light through film !
Note: I avoided
the issue by using
small pieces of film
here! – benefit
for small fields and
RCF !
Now with EBT2,3, there are dyes:
DEPARTMENT OF MEDICAL PHYSICS, UNIVERSITY OF WISCONSIN-MADISON
Film looks yellow – red/yellow is NOT absorbed when film UNexposed,but blue is always absorbed.
Film looks green – red/yellow and blue BOTH absorbed – leaving only green to get through when exposed.
But, we have seen the scanner
Behave oddly for one channel,
But not another!
McCaw et al Med. Phys. 38 (2010) 5771-7
The UW ADCL has developed (Ben Rosen)
a new scanning laser densitometer system
(LDS)* that eliminates many scanner
issues!
DEPARTMENT OF MEDICAL PHYSICS, UNIVERSITY OF WISCONSIN-MADISON
From: Ben Rosen talk at the
AAPM 55th Annual Meeting
Scientific: Therapy: Dosimetry,
Patient Safety and QA Procedures:
Calibration WE-E-141-3
8/7/2013 2:00-3:50 pm
* This will be patented by WARF
{Would be good to have
dual energy, but the laser
is adjustable in frequency}
I have used RCF in anthropomorphic phantoms to explore
interplay for helical tomotherapy:
need film for this (small wiggles)!
DEPARTMENT OF MEDICAL PHYSICS, UNIVERSITY OF WISCONSIN-MADISON
The Phantoms of Medical
and Health Physics,
L. DeWerd &
Michael Kissick Ed.
Springer, 2013
M. W. Kissick et al., “A
phantom model demonstration
of tomotherapy dose painting
delivery, including managed
respiratory motion without
motion management,”
Phys. Med. Biol. 55 (2010)
2983-2995.
Of great concern to me about the use of film
for DQA… can it be used to match DVH type
metrics?
DEPARTMENT OF MEDICAL PHYSICS, UNIVERSITY OF WISCONSIN-MADISON
H. Zhen et al., “Moving from gamma
Passing rates to patient DVH-based
QA metrics in pretreatment dose QA,”
Med. Phys. 38 (10) Oct. 2011, 5477-89.
•We have to be careful about
how we use film for QC/QA
on 3D volumes sampled with
2D dosimetry (film).
There is good reason to drop
the use of ‘gamma’ as a useful
metric for patient-specific QA!
Only this case showed correleations
Between DVH based and gamma based
Metrics, and it was weak.
In the end, I like this film! It is improving!
DEPARTMENT OF MEDICAL PHYSICS, UNIVERSITY OF WISCONSIN-MADISON
a. high dose capable so competes with EDR film (which has lots of high Z)
b. great spatial resolution because its film
c. tissue equivalent
d. develops itself (solid state polymerization ‘in place’ (topochemical))
e. not very sensitive to visible light, but it is to UV like from sun
f. now it is mainly energy independent to a large degree
a. sensitivity varies with temperature, UV, (and low dose, fractionation issues).
b. polarized densitometer light affects OD reading (but EBT model improving this)
c. developing takes ~ days, but “saturates” at ~ weeks really:[initial phase = starts < 100 µsec, slow phase = days – weeks, ~log(time)]
d. An issue is the nonlinearity at low doses (10% of maximum): seems to be fraction dependent!
e. needs to be handled very consistently
f. orientation and position dependencies on most scanners – scanners are a big issue!
Pros Cons
1. Gafchromatic film is only made by one vendor.
2. It was invented by David Lewis.
3. It was first used for an image in 1984 for an electron beam
recorder.
4. The film activates at about 10-3 J/cm2 versus.
5. Silver halide film activates at 10-10 – 10-8 J/cm2.
6. The new yellow dye for EBT2,3 is brilliant!
I am on the new TG-235, representing
the UW ADCL.
DEPARTMENT OF MEDICAL PHYSICS, UNIVERSITY OF WISCONSIN-MADISON
(I am writing the QA for Tomotherapy and Cyberknife, and MV IGRT QA parts.)
Conclusions:
DEPARTMENT OF MEDICAL PHYSICS, UNIVERSITY OF WISCONSIN-MADISON
1. The diacytelene tomochemical phase transition process in radiochromic
film is fascinating!
2. There are still unanswered questions from: i. the film, ii. the scanner, and
iii. its use for modern QC/QA., i.e., fractionation / composite fields
issues.
3. The future appears to be very bright for small field dosimetry with
radiochromic film !!