Protons & Electrons

What is the practical range of an electron?

• where the “falling off” part of the dose curve runs into the depth axis

• the extent to which the electron treatment is clinically useful

• Rp=E/2

What does the tail in an electron dose curve represent?

• bremmstrahlung x-ray contamination in the beam

• between 0.5% and 3% of dmax dose

What are the rules of electron cutouts?

• the width of the cutout in cm should be at least as wide as half of the electron energy in MeV

what are cutouts made of?

• cerroben: bismuth, lead, tin, cadmium

does the inverse square law apply to electrons?

• no

• we use virtual source distance

what electron energies are used clinically?

• 6-20 MeV

How do electrons interact?

ionization or excitation

what is the rate of energy loss with depth in water?

2 MeV/cm

How does energy impact dose falloff?

lower energy will have sharper dose fall off

how is depth distribution determined?

by ion chambers, diodes, and film

how does energy impact surface dose?

surface dose increases with increasing energy

How does beam obliquity impact the dose?

• increases dose at dmax

• decreases deeper dose

if an electron and photon field abut, which will have the hotspot?

• photon side because hotspot scatters laterally

where does bolus have to be placed?

on patient’s skin surface

What is the minimum thickness of lead required for shielding?

E/2 in mm of lead

what do we need to be aware of with internal shielding?

• backscatter

• shouldn’t shoot electron beam directly through lead

what is needed in electron arc therapy to define the treatment field and sharpen dose distribution at field edges?

custom shielding (molded on to the surface)

what are the differences between electrons and superficial x-rays?

• superficial x-rays deposit higher dose into bones

• electrons have less skin sparing and spare underlying tissue more

• electron beam on has greater output and therefore faster treatment

what parameter is used as a beam quality specifier for electron beams?

R50

where do hotspots occur when boosting a sinus with electrons?

• along the air cavity interface

How are protons produced?

• by cyclotrons

• by synchrotrons

what is the difference between a cyclotron and a synchrotron?

• cyclotrons have fixed energy, energy degraders to create the spread-out Bragg Peak at any depth

• Synchrotrons produce beams of variable energy

What is the RBE (relative biological effectiveness) of protons?

• 1.1

Why should there not be multiple beams stopping in an OAR in a proton plan?

because LET and RBE increase near the end of the proton’s range

what is range uncertainty in proton planning?

• results from mapping of the CT-based HU to the stopping power of protons

How are proton beams delivered?

• passive beam spreading or pencil beam scanning

• pencil beam scanning involves magnets that shape the beam

what are proton dose calculation algorithms?

• pencil beam

• convolution/superposition

• monte carlo

why use protons?

• give conformity comparable to IMRT with less integral dose (dose to body)

why do flattening filters cause dose horns?

• they overcompensate at the surface to obtain flatness at 10cm depth

geometric penumbra formula

what is geometric penumbra?

• dose decreases on sides of beam bc they’re far from central axis

why is geometric penumbra not the best measure of penumbra?

• bc it is not the only cause of penumbra

• with reduced side scatter, we also have physical penumbra (between 90 and 20% IDLs)

what IDL does the light field coincide with?

• 50%

what is the Clarkson method?

• a way of calculating dose to an irregularly shaped target by dividing it into pieces that are easier to add together

within what percentage should isodose curves in the TPS be accurate?

2%

what tools can be used to measure isodose curves?

• ion chambers

• solid state detectors

• radiographic films

what is the ICRU?

international commission on radiation units. Named targets

what is the gross tumor volume?

demonstrated tumor

what is the clinical target volume?

• GTV + sub-clinical disease

what is the internal target volume?

• CTV + margin for motion

What is the planning target volume?

• CTVs + ITV + margin for variations in setup, positioning, geometry

what is the treated volume?

• volume that receives a dose that is important for local cure or palliation

what is the irradiated volume?

tissue volume that receives a dose that is significant compared to normal tissue tolerance

what is a differential DVH?

• represents the volume receiving dose in the corresponding dose bin

• shows how homogenous dose is

• spiky: not homogenous

• smooth: homogenous

what is a cumulative DVH?

• represents volume receiving greater than or equal to the value in the corresponding dose bin

• this is what we use

how do you read this?

33% of volume should get less than 60 Gy

what is the conformity index?

• rx isodose volume/target volume

• if you stack your rx isodose level over target volume, how closely will they fit

• perfect score = 1

• further from 1, less conformal

what is the homogeneity index?

• ratio between the max dose in target volume to rx dose

• Assesses hot and cold spots in target volume

• further from 1, less homogenous

• larger number = more inhomogenous