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Proton Beam Range Verification using Proton Activated Fiducials and Off-site PET
J Cho, G Ibbott, M Gillin, C Gonzalez-Lepera, U Titt and O Mawlawi
The University of Texas MD Anderson Cancer Center, Houston, TX
AAPM Best in Physics, Indianapolis, 2013
Talk session: Novel Imaging Tech & Appl 4:40PM August 7th (Wed) at 500 Ballroom
In-Air Irradiation
Motivations
Embedded Irradiation: Lung tissue
30 min PET scan after 126 min delay
The conventional proton range verification using PET
takes advantage of endogenous tissue activation in
combination with Monte Carlo simulation. However, this
approach has the following limitations:
 Weak tissue activation at the end of the proton
beam
 Perfusion driven activity washout
 Short decay half-life (need of costly in-room PET)
 Monte Carlo uncertainties in
 Elemental tissue composition conversion
 Nuclear cross sections
 Biological washout model
The purpose of this work was to develop a novel proton
range verification method that is not subject to the above
limitations. By taking advantage of patient-implantable
fiducial markers that are strongly activated by low energy
protons and decay with relatively long half-lives, a proton
range verification can be realized using commonly
available off-site PET scanners.
(a)
(a)
(a)
(b)
Fig. 2: 68Zn and 63Cu have large cross sections at low proton
energies and decay with relatively long half-lives. As a result,
they are activated strongly at the proton distal fall-off region
where proton energy is low. Endogenous tissue elements (12C
and 16O) are shown for comparison.
Fig. 4: 68Zn (98% enriched) and Cu foils and polycarbonate
sheets (as a tissue substitute) are placed at 4 distal fall-off
depths and irradiated by a proton beam. Cu has 69% 63Cu.
(b)
Fig. 7: (a) Balsa wood (as lung substitute) with embedded 68Zn
and Cu foils at 4 distal fall-off depths and irradiated by a proton
beam. (b) CT scan and treatment plan of the balsa wood show
isodose curves relative to each depth.
Fig. 9: (a) Foil locations relative to PDD. (b) Foil at depth 4 is
marginally activated and foil at depth 5 is not activated.
Therefore, the proton range can be estimated with ±5mm
uncertainty.
(b)
Fig. 11: A coronal plane view with a row of 25mm3 Cu foils. (a)
PET/CT fusion images. (b) Treatment plan generated isodose
curves with respect to CT image. With the borderline of 95%
isodose curve, Cu foils located at higher dose are activated.
Embedded irradiation: Soft tissue
Parodi et al, NIH public access 2007
30 min PET scan
after
48 min delay
Background
Fig. 5: Depths of PC (polycarbonate), 68Zn and Cu relative to
PDD and their PET/CT fusion images. Only 68Zn and 63Cu show
PET signals at deeper depths (not PC). 68Zn and 63Cu signals
are strong despite its volume is 7.6 times smaller than PC.
(a)
Fig. 1: Proton range uncertainty can result in overdosage in
critical organs or underdosage in tumor. It is crucial to verify the
proton range accurately.
(a)
Conclusions
(b)
Fig. 3: Hypothetical images of proton activated fiducial markers
made of 68Zn or 63Cu implanted in a patient prior to proton
therapy. (a) One marker is implanted near the proton distal falloff and one just outside the proton range. (b) The marker
implanted near the distal fall-off is activated (red) and the other
marker outside the proton range is not activated (green). In this
regard, the proton range can be approximated to be fall
somewhere between the two fiducials.
Signal reduced
(b)
Fig. 6: Comparison of measured PDD and activity (PET signal)
with Monte Carlo simulation. Activity fall-offs follow the dose
fall-off with 1~2 mm offsets.
Fig. 8: PET/CT fusion images of each depth in the beam’s eye
view. Signals from foils are much stronger than balsa wood.
Signals are reduced with depth. Foil is marginally activated at
depth 4.
Fig. 12: A coronal plane view with a row of 25mm3 68Zn foils.
With the borderline of 50% isodose curve, 68Zn foils located at
higher dose are activated. Foil 1 is not activated because
protons activating this foil is too high (see fig. 2).
Fig. 10: (a) Soft-tissue phantom made of beef cut diagonally is
embedded with 68Zn and Cu foils and is irradiated by a proton
beam. Each of 5 different coronal planes contains a different
row of foils. (b) Locations of embedded foils relative to PDD.
Higher activation of 68Zn and Cu at the proton dose distal
fall-off region and their long half-lives indicate the
possibility of using those materials as patient implantable
fiducial markers for proton range verification using off-site
PET scanners.