4/28/2025 Attempt Flashcards
(25 cards)
PET
axial spatial resolution - 3-4 mm
OER
OER = D(anoxic)/D(oxic)
Neutron OER = 1.5
RBE
Influenced by:
- biological endpoint
- fractionation
- dose rate
- LET
Protons - 1.1
Proton Therapy
Field penumbra - Distal penumbra is smaller than lateral penumbra
Primary source of neutron dose - patient
Robust optimization - used to maintain CTV coverage and OAR sparing in presence of range uncertainty and setup variations
Concrete shielding thickness - 2-3m
Ion Chamber Corrected Readings
M = M(raw)C(T,P)
C(T,P) = (273.2+T/295.2)(760/P)
Permanent Implant Dose
Dose remaining = 0.5 ^(days implanted/half life)
Dose delivered = 1 - dose remaining
Treatment margin size
impacted by:
- proximity of normal tissue to target
- probability of tumor cells located outside the tumor volume
- tumor cell radiosensitivity
- density of tumor cells
Luminescent Detectors
imperfections are added to luminescent detector crystals to create trap centers for charge carriers
Electron beam
- Exiting treatment head, the energy spectrum is approximately Gaussian
- Rp in water (cm) = E (MeV)/2
- Rp in air (m) = E (MeV)/4
- parallel plate ion chamber is best for measuring low energy electrons when the cutout is much larger than the chamber volume
TG-51
minimum 5 measurements required to perform photon dosimetry
- 2 to determine kQ
- 1 at 300V
- 1 at 150V
- 1 at -300V
P(ion) cannot be under 1
MRI
Propeller sequence - used to reduce motion artifacts
FLAIR - reduces intensity of CSF in brain MRIs
Free Air Ionization Chambers
- collect charge from air ionizations
- measures exposure
NCRP 116
10% risk/Sv to embryo/fetus
Spatial Resolution for Imaging Modalities
Mammography 0.03 - 0.1mm
CT - 0.3mm
MRI - 1mm
PET - 5mm
SPECT - 7mm
Radiographic vs Radiochromic
Radiochromic is:
- more tissue equivalent
- its response is more linear
- does not require post processing
TG-142
Imaging and treatment coordinate system should be checked daily
TG-180
MV EPID: 1-4cGy per orthogonal pair (1-2cGy for 2.5MV)
MV CT: 1-3cGy
MV CBCT: 2-12cGy
kV DR: 0.1-1cGy per orthogonal pair
kV CBCT: 0.1-3cGy
TG-43
Anisotropy factor decreases when:
- r decreases
- encapsulation thickness increases
Scatter and Shielding
Leakage radiation will typically dominate scattered in shielding requirements
Exposure
1 R = 2.58e-4 C/kg
TG-56
Code of practice for brachytherapy physics
TVD
TVD for a maze = 3 * sqrt (height * width)
Water equivalent dosimeters
- radiochromic film
- plastic scintillation dosimeters
SRS cones
Cones give sharper penumbra than MLCs
