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.