Radiation Physics Illustrations for Radiation Therapy applications.

1. Advanced Computer Simulation & VisualizationAdvanced Computer Simulation & Visualization
Tools for Enhanced Understanding ofTools for Enhanced Understanding of
Core Medical Physics ConceptsCore Medical Physics Concepts
Shahid A. Naqvi,Shahid A. Naqvi,
Saint Agnes Cancer InstituteSaint Agnes Cancer Institute
Baltimore, MDBaltimore, MD
Presented atPresented at
AAPM 2014, Austin, TexasAAPM 2014, Austin, Texas

2. DisclosuresDisclosures
Nothing to disclose

3. Radiation PhysicsRadiation Physics
 Basic physics connects dose to
the fundamental properties of
electrons, photons, atoms and
molecules.
 The physics at the microscopic
level us physical insight, but it
doesn’t seem very useful when it
comes to treating patients.
D = Φelectron x S/ρ

4. Radiation Physics (clinical version)Radiation Physics (clinical version)
 Collapses physics into
a few empirical terms
 Difficult to visualize
physics
 But easy to calculate!
 And then we happily
treat our patient
( )2
TMR I
M
NV
U
ref c pS S
D
K × × × ×
=
(calc pt)

5. Beam calibration: Physics or protocol?Beam calibration: Physics or protocol?
 As we swtiched to TG51,
Physics was reduced to a single
term kQ
 Look up tables for various ion
chambers for our beam energy
to get a perfect calibration
 Many students do not feel the
need to understand Bragg Gray
or Spencer-Attix theory.
I’ll just look it up for
my favorite chamber

6. Basic Physics vs. Clinical formalismsBasic Physics vs. Clinical formalisms
 Formalisms such as TPR based MU calculations based on
empirical data achieve results
– Conveniently
– Accurately
– Reproducibly
 But the physicist becomes more proficient in clinical
calculations, the basic physics continues to fade from
memory.
 Question: Do we really care about the basic physics
concepts when our main goal is to get the patient treated
effectively and safely?

7. When is basic physics useful?When is basic physics useful?
 Not essential in routine clinical calculations and checks
 Deeper physical understanding is useful in evaluating
– New types of detectors
 Do I need buildup?
 Is it water equivalent?
 Is it sensitive to low energy photons?
 Do I need stopping power ratio corrections?
– New dose calculation algorithms
 What approximations are involved?
 Model based or correction based?
 How does it handle electron transport at interface regions?
 Could lead to commissioning and QA errors, which may affect
patient care.

8. Physics Teaching ToolsPhysics Teaching Tools
 TOOL 1: A Monte Carlo code (“Athena”) is developed
– with a strong educational component
– to facilitate explicit visualization of radiation physics
– to make explicit connections with clinical physics
 TOOL 2: A “particle in electromagnetic field” simulator to
illustrate devices such as
– linacs
– cyclotrons

9. Relating attenuation & InteractionsRelating attenuation & Interactions
 Interactions occur randomly at all depths (exponential probabality)
 Each interaction removes a primary photon, which reduces the
number downstream (attenution).
 Hence interactions & beam attenuation are seen as flip sides of the
same coin
Primary photons paths and primary photon interactions shown only

10. Relating photon interactions withRelating photon interactions with
electrons releasedelectrons released
this e- deposit
primary dose
this e- deposits
scatter dose
primary
photon
interaction
Fewer photon interactions
and tracks downstream
due to attenuation

11. Electron tracksElectron tracks →→
ionization & excitationionization & excitation
 Ionization and excitation
related to passage of electron
through medium (collisional
losses)
 Density of dots proportional
to ionization density (related
to stopping power).
tracks
Ionization/excitation

12. Understanding electronic buildupUnderstanding electronic buildup
and physical penumbraand physical penumbra
Cartoon
simplification
Real tracks
Illustrates how buildup
and penumbra related to
the forward and lateral
range of electrons
respectively

13. Dose vs.Dose vs. KKcc
 Interaction density
(proportional to fluence) peaks
at surface
 Collision kerma (proportional
to interaction density)
 Dose does not peak at the
surface
D
Kc

14. Interfaces (tissue-lung-tissue)Interfaces (tissue-lung-tissue)
Primary photons, and interaction points [red] Photon and electron tracks
tissue lung tissue

15. Interfaces (isodose and tracks)Interfaces (isodose and tracks)
Illustrates how spreading of isodose lines ) in lung
correlates with spreading of electron tracks.
Physical penumbra increases in lung.

16. Separating tracks by starting regionsSeparating tracks by starting regions
Tracks due to primary interactions in 1
Tracks due to primary interactions in 2
Tracks due to primary interactions in 3
1
2
3

17. Color coding track and doseColor coding track and dose
componentscomponents
total dose1 2 3

18. Studying lateral buildupStudying lateral buildup

19. Visualizing a kernelVisualizing a kernel
real beam
Collapse laterally Collapse to a point
pencil beam kernel point kernel

20. Primary and scatter componentsPrimary and scatter components
of point kernelof point kernel
These e- and e+ tracks
makethe scatter kernel
These e- and
e+ tracks make
the primary
kernel
5MeV photons interacting at red dot TERMA
dose

21. Color coding electron tracksColor coding electron tracks
No coding Energy
Color code
(red=20MeV)
Energy loss rate
(color coding)
20MeV e- incident

22. Color coding application [e-]Color coding application [e-]
 Illustrates how electron energy
spectrum changes with depth
in electron beams.
 Explains need of water-air
stopping power ratio in PDD
measurement for e- beams with
ion-chambers.
electron energy colored
20 MeV incident

23. Secondary electron spectrum inSecondary electron spectrum in
photon beamsphoton beams
 Shows secondary
electron spectrum hardly
changes with depth in
photon beams.
 Explains no stopping
power correction is
needed for photon PDD
measurement.

24. Tool #2Tool #2
Simulation of Particle in external electromagnetic fieldSimulation of Particle in external electromagnetic field
 Simulation of relativistic
particles in external EM field
 Runge-Kutta method to propagate
momentum and position
 Can be used to simulate
particle in a wave in a linac
which has an axial electric field
)cos(
)(
0 kztEE
q
dt
d
x −=
×+=
ω
BvE
p

25. Linac kinematics (18MV)Linac kinematics (18MV)
Injection
120 keV
3 GHz Microwaves in
a linac waveguide
Accelerating electrons
Final energy 18MeV

26. Example: Bending Magnet tuningExample: Bending Magnet tuning
 Can help physicist appreciate
what checks are needed when the
engineer tweaks the machine
 Example.
– bending magnet current can change
the spectrum of energies getting
through the energy slit,
– Hence the energy of the photon
beam must be checked

27. ConclusionConclusion
 In this work, the teaching tools described
– help to elucidate the physics by breaking the physical
processes into layers of complexity
– Help in making connections with clinical calculations
– develops physical insight and deeper understanding so that
new situtations such as a new dose calculation algorithm can
be evaluated with sound judgement
– helps to appreciate the beauty of the Physics
– Can reduce commissioning errors for safer patient treatment.