Therapy Physics FUNdamentals
The dominant photon interaction in the clinical MV range.
Compton
Fig 1. Y-axis label.
Atomic number (Z)
The mean energy of an MV photon beam.
~1/3 the max energy
The maximum field size at 100 cm SSD.
40 x 40 cm
The annual effective dose limit for a nuclear energy worker.
50 mSv
100 mSv over 5 years
The isodose line at which field size is normally defined.
50%
Fig 2. What is this?
FFF beam profile.
A 1 cm setup error (shift in SSD) results in this % dosimetric error at 5 cm depth, 95 cm SSD (TOH reference depth).
~2%
What component of the linear accelerator is this?
Flattening filter.
3 methods to reduce exposure to radiation.
Time (decrease)
Distance (increase)
Shielding
List 3 dose calculation algorithms.
Pencil beam
Convolution superposition
Monte Carlo
Fig 3. What photon interaction is demonstrated here?
Photoelectric effect?
Fat appears bright in this MR T-type weighted image.
T1
The energies that bremsstrahlung photons can have.
0 to max electron energy
List 5 safety systems present in a bunker.
LPO (including timeouts)
A/V monitoring
Motion sensors
Door interlocks
Linac touchguards
ESBs
In MV photon beams, this is the depth at which Charged Particle Equilibrium is established.
dmax
Fig 4. Label these lines.
KERMA
Dose
Electron beam energy [MeV] loss per cm.
2 MeV/cm
(Dividing energy by 2 gives you the 'practical range' of the beam).
(1) Common linac target material and (2) bremsstrahlung efficiency [%]
(1) Tungsten
(2) ~2-3%
The effective dose from a CTSim (mSv).
Head: ~2 mSv
Chest: ~5-8 mSv
Pelvis/Abdo: ~10-20 mSv
The PTV is a geometric concept, commonly calculated as a function of two types of error. State the two types of error and give an example of each.
Systematic (poorly delineated GTV) - carried throughout entirety of treatment (all fractions)
Random (intra-fraction organ motion) - approximately sums to zero over the course of all fractions
Example recipe (Van Herk et al. 2000):
PTV = 2.5(SIGMA) + 0.7(sigma)
where SIGMA = SD of systematic errors
sigma = SD of random errors
Fig 5. What is this graph showing?
6 MV photon energy spectrum.
The ~ rate of decay per day for an Ir-192 HDR brachy source.
~1%/day
Nominal dose rate [MU/min] of a 6 MV FFF beam [Elekta].
~1400 MU/min [6 MV]
~2400 MU/min [10 MV]
(Flattened is ~600 MU/min)
The physical property that Hounsfield Units are measuring.
Linear attenuation coefficient mu (electron density of the medium).