Raphex 18-21

Question 1

Why are there no naturally occurring isotopes w/ Z > 92?

Answer 1

Z > 92 isotopes can be produced artificially, but they are all radioactive and decay w/ short t_1/2s 2/2 repulsive forces b/w protons

Question 2

What and how does the e^- interact with during bremsstrahlung production?

Answer 2

• Interacts w/ atomic nucleus
• Inelastic collision (energy lost as bremsstrahlung)

Question 3

What’s 1 amu?

Answer 3

• Def: 1/12^th the mass of a C-12 atom
• 1.66 × 10^-27 kg
• 931 MeV

Question 4

What parameters of the gantry-mounted XR tube are automatically 𝛥 to produce the best brightness

Answer 4

• mA
• kVp

Question 5

How do Linacs minimize MLC leakage?

Answer 5

By having the jaw track along w/ the MLCs

Question 6

Beem-steering electronics in a Linac maintain which aspects of the beam?

Answer 6

• These electronic monitor two halves of the monitor chamber and steer the beam to compensate for consistent output
• Primarily:
  – Symmetry
• Secondarily:
  – Dose rate
  – Output
  – Quality

Question 7

What’re the different functions of a Magnetron and a Klystron?

Answer 7

• Magnetron: Generate RFs to accelerate e^-s
• Klystron: Requires an RF source, and amplifies the RFs to accelerate e^-

Mnemonic: Make e^- accelerate

Question 8

For a Compton interaction, what’re all the possible scatter angles for the scattered photon?

Answer 8

0 (forward scatter) - 180 (backscatter)

Question 9

If two LInac treatments deliver the same output (cGy/MU) but different dose rates (ΜU/min), how does the dose delivered differ?

Answer 9

• For a Linac, the dose does not change with dose rate (Μ/min) as long as the output remains the same.
  – This is why dose rate is not a part of the Μ/Dose equation!
  – The Linac automatically adjusts the treatment time to accord w/ the dose rate

Question 10

After exposure, how do the OSLDs fade?

Answer 10

• Fade significantly in the first 10 mins
• Stable afterwards

Question 11

How does the energy of neutrons (photonuclear disintegration) from a Linac (>10 MV) depend on beam energy?

Answer 11

• The energy of neutrons is independent of beam energy
• The # of neutrons, however, increases w/ beam energy

Question 12

How do TMR and PDD vary w/ increasing SSD?

Answer 12

• TMR remains the same
• PDD increases (takes into account i/r²)

Question 13

How does penumbra vary with beam energy?

Answer 13

• ≤ 6 MV: ↓ penumbra w/ ↑ energy
• > 6 MV: ↑ penumbra w/ ↑ energy

Question 14

What’s the main purpose of a Linac QA program?

Answer 14

To ensure that machine characteristics do not deviate from the baseline values determined during machine install

Question 15

To send/share DICOM images, what do you need to know about the recipient computer?

Answer 15

• IP address
• Port number
• application entity (AE)

Question 16

What’s the utility of using a large focal spot for XR machines?

Answer 16

• Helps w/ heat dissipation
• Required when high mA is required
• Has an inferior image quality as compared to a small focal spot

Question 17

How does T1 relaxation compare to T2 relaxation?

Answer 17

• T1 ≥ T2
• T1 relaxation is in the z-direction
• T2 relaxation is in the x- y-direction

Question 18

For two plans for the same treatment, which plan would require more beam modulation based on its ΜUs?

Answer 18

The plan w/ more MUs → smaller aperture sizes → more modulation

Question 19

For castration-resistant prostate cancer, how is the dose for ²²³Ra calculated?

Answer 19

• 1.49 μCi/kg
• We use mass to calculate the dose

Question 20

After HDR source exchange, the activity of the source should be within what % of the manufacturer’s certificate?

Answer 20

±3%

Question 21

What’s the max allowable deviation between measured and intended dwell positions and step-size spacing b/w dwell positions?

Answer 21

±1mm

Question 22

For which 2 LDR sources does the inverse square (1/r²) law not hold within ± 5 cm?

Answer 22

• Sources
  – Pd-103
  – I-125
• Reason?
  – Due to their low energy, their attenuation is much larger than the scatter, so it does not cancel out.
  – For other sources, their attenuation (↓ dose) and scatter (↑ dose) are equal within the first 5 cm, so they follow the 1/r² law

Question 23

How does the thickness of lead compare to water(tissue) for e^- range calculations?

Answer 23

Th_lead = 1/10 × Th_med

Question 24

How does the penumbra of a PBS proton beam compare to that of double scattered beam?

Answer 24

PBS has a less sharp penumbra 2/2 the absence of a physical aperture

Question 25

What phenomenon causes the formation of **horns** in an XR beam after it passes through the flattening filter?

Answer 25

FF attenuates the center of the beam a lot more, decreasing the central fluence more than the peripheral fluence → horns

Question 26

How does the mass attenuation coefficient of lead compare to that of water in the Compton range?

Answer 26

They are ~ equal

Question 27

Which instruments are the gold standard for measuring surface dose?

Answer 27

Extrapolation chambers

Question 28

How does the beam quality of FFF beams compare to flattened beams?

Answer 28

Flattened beams have a 5% higher quality 2/2 increased energy and penetration

Question 29

When you use lower photon energy, how do the MUs required to deliver the dose compared to the MUs used for a higher-energy beam?

Answer 29

More MUs are required for lower energies because lower energy beams are less penetrating.

Question 30

How often do you need to check light and radiation field coincidence for QA purposes?

Answer 30

Monthly

Question 31

Which regulatory authority oversees the sale of medical linacs?

Answer 31

FDA

Question 32

What property of a material determines US wave echoes or reflections?

Answer 32

**Acoustic Impedance:** Z × Speed of sound in the medium