RAPHEX Flashcards
Bequerel is SI unit of
Activity ( 1 bequerel is 1 disintegration per second) (1 curie is 3.7 x 10^10 Bq)
Half lives of Iridium 192, Iodine 125, Cesium 137, Pallidium 103
74 days; 60 days; 30 years; 17 days
1 / Effective half life
1/ biological half life + 1/physical half life
Air kerma
air kerma strength x hours x (1/distance)^2
Output of an X-ray machine
increases linearly with tube current and increases with the square of tube voltage
Line focus principle used in diagnostic x-ray tube design in order to
make the apparent focal spot size smaller than the actual focal spot size
(The apparent focal spot size equals the actual focal spot size multiplied by the sine of the anode angle. This allows a larger portion of the anode to be irradiated, thereby increasing the heat dispersion, while keeping the spatial resolution in the images as good as possible.)
source of electrons for accelaration in a linac
Thermionic emission from a heated filament cathode in the electron gun is the source of electrons for acceleration in a linac.
A photocathode uses light, rather than heat, to emit electrons.
A Van de Graff generator uses belts and rollers to collect static electric charge.
Path of photon travel after creation in the target to the patient surface
primary collimator - flattening filter - monitor chambers - jaws
Relation between electron current for photon vs electron beam
Bremsstrahlung production efficiency increases with increasing electron current, therefore, to produce same dose rate, higher electron current is required for low energy x-rays
Salient features of Flattening filter free x-ray beams
a. decrease treatment time
b. has a shallower dmax
c. skin dose can be higher
d. For large treatment fields, the dose uniformity within a treatment field needs to be “modulated” by MLC movements (IMRT) to reduce the higher beam intensity near the central portion of the FFF x-ray beam. Thus, larger MUs are required compared with a conventional x-ray beam
Primary beam transmission (lowest to highest order)
movable jaws (<0.11%)
MLC (1.5 - 3%)
standard thickness alloy blocks (4 - 5%)
TBI geometry solution
(we need to calculate the minimum source-to-patient distance required to cover a patient height of 180 cm using a LINAC field size of 40 cm × 40 cm (at 100 cm SAD), with a collimator angle of 45°)
🧮 Step-by-Step Calculation:
-
Understand the Given Information
- Maximum field size = 40 cm × 40 cm at 100 cm SAD
- Patient height = 180 cm
- Collimator angle = 45°
- Need to project the field to cover 180 cm
-
At 45° Collimator Angle
- A 45° rotation increases the diagonal coverage of the field.
- Diagonal of a square field is:
d = side x square root of {2} = 40 x sqrt{2} approx 56.57 cm
→ So, at 100 cm SAD, the maximum field length is 56.57 cm when rotated 45°.
-
Set Up Proportional Scaling
We now want to find the distanceDat which this 56.57 cm field projects to 180 cm.
Use direct proportion:
56.57/100 = 180/D
Solving for D:
approx 318.2 cm
✅ Correct Answer: B. 318 cm
This is the minimum source-to-patient distance required to cover a 180 cm tall patient using a 40 cm × 40 cm field at 100 cm SAD with a 45° collimator angle.
SI unit for mass attenuation coefficient
m^2/kg
SSD & SAD setup advantages and disadvantages
🔵 Isocentric Setup (SAD = 100 cm):
- The treatment point (isocenter) is fixed at 100 cm from the radiation source.
- The patient’s body is usually closer than 100 cm, which can make it tight or crowded under the machine (especially with accessories, devices, or the couch).
- Good part: Once the patient is positioned, they don’t need to be moved again for different angles of treatment.
🟢 SSD Setup (Source-to-Skin Distance):
- The machine is set up to be further away from the patient, giving more space and comfort.
- Bad part: The patient has to be repositioned for each beam (angle), which takes more time and effort.
- Every repositioning increases the chance of positioning errors.
✅ In short:
- Isocentric = Less movement, less error, but tighter space
- SSD = More space, but more repositioning and more chance of errors
