Visualizing Radiation Physics Concepts with photon electron particle tracks
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Transcript of Visualizing Radiation Physics Concepts with photon electron particle tracks
Advanced Computer Simulation & Visualization Advanced Computer Simulation & Visualization Tools for Enhanced Understanding of Tools 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 at Presented at
AAPM 2014, Austin, TexasAAPM 2014, Austin, Texas
DisclosuresDisclosures
Nothing to disclose
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
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
2TMR IM
NVU
ref c pS S
D
K
(calc pt)
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
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?
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.
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
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
Relating photon interactions with Relating photon interactions with electrons releasedelectrons released
this e- depositprimary dose
this e- deposits scatter dose
primaryphoton interaction
Fewer photon interactionsand tracks downstreamdue to attenuation
Electron tracks Electron 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
Understanding electronic buildup Understanding electronic buildup and physical penumbraand physical penumbra
Cartoonsimplification
Real tracks
Illustrates how buildup and penumbra related to the forward and lateral range of electrons respectively
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
Interfaces (tissue-lung-tissue)Interfaces (tissue-lung-tissue)
Primary photons, and interaction points [red] Photon and electron tracks
tissue lung tissue
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.
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
Color coding track and dose Color coding track and dose componentscomponents
total dose1 2 3
Studying lateral buildupStudying lateral buildup
Visualizing a kernelVisualizing a kernelreal beam
Collapse laterally Collapse to a point
pencil beam kernel point kernel
Primary and scatter components Primary and scatter components of point kernelof point kernel
These e- and e+ tracks makethe scatter kernel
These e- and e+ tracks makethe primary kernel
5MeV photons interacting at red dot TERMA
dose
Color coding electron tracksColor coding electron tracks
No coding EnergyColor code(red=20MeV)
Energy loss rate(color coding)
20MeV e- incident
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
Secondary electron spectrum in Secondary 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.
Tool #2Tool #2Simulation 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
qdt
d
x
BvEp
Linac kinematics (18MV)Linac kinematics (18MV)
Injection 120 keV
3 GHz Microwaves ina linac waveguideAccelerating electrons
Final energy 18MeV
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
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.