IMÁGENES DE FLUORESCENCIA DE RAYOS X IN VIVO

M. Valente
1,2,*
, F. Botta3, G. Pedroli3, P. Pérez2,4
CONICET, Buenos Aires; Argentina.
FaMAF – Universidad Nacional de Córdoba, Argentina.
3
Medical Physics Department, European Institute of Oncology, Milan; Italy.
4
ANPCyT, Buenos Aires; Argentina.
1
2
* Email: [email protected]
Web site: www.famaf.unc.edu.ar/~valente
 Introduction: Nuclear medicine & DPK
 Materials
& Methods:
Radiation matter
interaction & MC simulations
 Application
& Results:
3D Energy deposition &
sDPK for Monoenergetic & β- radionuclides
 Discussion
& Conclusions






Nowadays, there are interests as well as active investigations devoted to
the study and application of radiolabeled molecules, able to selectively
target and irradiate tumoral cells.
Beta nuclides have proved to be appropriate for radioimmunotherapy.
Dosimetric performance of each radionuclide has to be carefully
investigated and characterized.
One usual and practical dosimetric approach is the calculation of dose
distribution about a unit point source of any radionuclide of interest,
which is known as dose point kernel (DPK).
Special requirements arise from complex spatial activity distribution with
extremely non-uniform characteristics.
Absorbed dose distribution results from the contribution of primary and
scattering radiation.
To develop a method capable of performing
nuclear medicine dose distributions while
separately computing primary and scattering
contributions.
Starting point: develop a system capable of computing 3D energy
deposition(E(x,y,z)).
Once E(x,y,z) is attained, scaled Dose Point Kernels become
straightforward.
Radionuclide sDPK can be obtained by means of the corresponding
emission spectrum weights.
Dedicated data handling and image processing subroutines (MatLab®
supported)for calculation analysis.
Due to intrinsic relevant differences between primary and scattering
contributions, it may be considered that their separation may be provide a
suitable mechanisms aimed to perform LET-dependent equivalent dose
calculation.
GOAL as end point: Improved (more realistic) TCP and NTCP distributions
for nuclear medicine applications.

Activity distribution may be determined by means of
different modalities. Nowadays it is mainly measured using
imaging techniques but otherwise it is also possible to infer
it. This information is then incorporated in the treatment
planning system in order to obtain an estimation of the dose
distribution.

The dose distribution about a unit point source of any
radionuclide of interest - known as DPK - has proved to be a
particularly useful tool for the dosimetric calculation.

DPK by means of analytical methods: straightforward only
for homogeneous media.

A method suitable for DPK by means of Monte Carlo
methods: capable of calculating for both homogeneous and
heterogeneous media.
D s  =
1
S  E  s 
4πρ

Nuclear medicine radionuclide characterization.

Irradiated media (phantom) definition.

Radiation transport algorithm: full stochastic
Boltzmann equation iterative resolution
PENELOPE v. 2008 main code.

For a monoenergetic source emitting electrons
(E0), absorbed dose per source transformation (D)
at a distance s that the electron has traveled from
the point source as:
1
D=
S  E s
4 πρ
E0
s≡ ∫
E  s
dE
S  E
D s  =
1
S  E  s 
4πρ

The specific absorbed fraction (Ф) at distance s
from a monoenergetic point source is Ф (E0,
s)=D(s)/E0.

It is usually convenient to introduce the scaled
DPK for beta particles (F) by:
F

s
R CSDA

δE
≡
 s
E0
δs
R CSDA
R
E0
dE
CSDA≡∫ S  E 
0
δs is shell thickness
δE(s) is energy delivered in the shell between s
and s + δs.
D s  =
1
S  E  s 
4πρ

Analytical approaches for presented model implicitly assume
some approximations like straight-line motion and continuous
energy loss for electron interactions. However, well known
departures from continuous slowing down arise from multiple
scattering and energy loss fluctuations, like delta-ray and
Bremsstrahlung production.

Contrary to analytical techniques, MC calculations of DPK are
capable of more realistic approaches. MC are capable of
handling multiple scattering as well as radiative energy
losses. Therefore, taken into account that part of the energy
loss straggling may be carried out to positions far away, even
at distances grater than RCSDA.
D s  =
1
S  E  s 
4πρ

When considering radionuclides instead of
monoenergetic sources, it becomes necessary to
calculate scaled DPK by means of weighting the
corresponding associated spectra, which is
usually accomplished by means of decomposing
the spectrum into M groups according to the
branching probability bi and end-point energy Ei,
as follows:
M
N  E  =∑ p i N i  E 
i=1
where N indicates the channel intensity
1. CSDA Path model:
2.
E
S=∫E− ΔE
dE
=R  E −R  E− ΔE 
SE
E
g
 
−
∫
1
Angular Distribution:
F L  ω = ∑ ℓ P ℓ  ω  e E −ΔE
2
1
ℓ ≥0
g ℓ ≡1−∫ dω P ℓ  ω  p  ω ; ω≡cosθ ; p  ω  PDF
−1
3. Random Hinge:
s A =ξs ; s B =s−s A
dE
ℓ
λs

Specific subroutine developed based on the
PENELOPE v. 2008 main code [Salvat et al].

Calculation system capable of performing scaled
DPK calculation in several (even heterogeneous)
media for both monoenergetic and radionuclides
sources.

The developed subroutine can assess primary
(dose due to particles emitted by the source)
and scattering (due to all kind of dose
components that carry out when scattered, like
secondaries, etc. particles deposit energy within
the shell) contributions.
D(i,j,k) for
I corresponding to a 15mm radious water-equivalente sphere
131
D(i,j,k) for
131
I corresponding to a 15mm radious water-equivalente sphere

Electrons as primary particles: Interaction mechanisms
for phase state change or secondary radiation
generation:
1.
Soft events (energy and angle variations lower than
specific threshold values).
Elastic collisions.
Hard inelastic collisions.
Bremsstrahlung emissions.
Inner-shell (K, L and M) impact ionizations.
Delta interactions.
2.
3.
4.
5.
6.

Scaled DPK simulation within 10cm radius waterequivalent sphere computing energy deposition in
concentric shells of RCSDA/40 thickness.
Applications & Results

1.
2.
3.
4.
•
1.
2.
3.
4.
5.
After preliminary tests, the dedicated MC subroutine has been
used for the calculation of in-water energy deposition of
monoenergetic sources. A set of different monoenergetic
sources:
1keV, 2keV, 5keV
10keV, 20keV, 50keV
100keV, 200keV, 500keV
1MeV, 2MeV, 5MeV
Typical beta-minus radionuclides used in nuclear medicine
treatments:
90
Y - Yttrium
177
Lu - Lutetium
131
I - Iodine
153
Sm - Samarium
89
Sr - Strontium
Applications & Results:
Monoenergetic (total) sDPK
10keV sDPK
20keV sDPK
100keV sDPK
200keV sDPK
Applications & Results:
Radionuclide (total) sDPK
131
Iodine total sDPK
153
Samarium total sDPK
177
Lutetium total sDPK
Applications & Results:
Primary/Scattering contributions
1MeV sDPK
I sDPK
131
90
Y sDPK
177
Lu sDPK
Applications & Results
Final remarks &
Conclusions
✔A dedicated calculation system has been proposed for scaled DPK
assessment.
✔Specific Monte Carlo code has been adapted for 3D energy deposition
and sDPK calculation.
✔Tensor & image data handling software toolkit has been developed for
calculation analysis.
✔The separation of primary and scattering contributions to the total
sDPK has been successfully accomplished. Furthermore, different
kinds of scattering contribution were also identified.
✔The net effect of LET-weighted equivalent dose, TCP and NTCP
calculations has shown to be significant for monoenergetic β- sources,
whereas real radionuclides typically used in nuclear medicine present
almost negligible differences.
✔However, the situation may be significantly different when considering
α-emmitting radionuclides, due to the involved high LET values.
✔Therefore, the presented method may constitute a valuable tool in the
precedent case.
Thanks for your attention
M. Valente
, F. Botta3, G. Pedroli3, P. Pérez2,4
1,2,*
CONICET, Buenos Aires; Argentina.
FaMAF – Universidad Nacional de Córdoba, Argentina.
3
Medical Physics Department, European Institute of Oncology, Milan; Italy.
4
ANPCyT, Buenos Aires; Argentina.
1
2


* Email:
Web site:
[email protected]
www.famaf.unc.edu.ar/~valente