Electron Dosimetry Exam Actual
Questions and Answers 2026
Explain electron R90 and R50. How is the RP (practical range)
determined? -
correct answer ✅A straight line is drawn between R90 and R50
and used to calculate extrapolation values. Extrapolating back to
100% gives the Rq , while extrapolating forward to 0 % gives the Rp.
R max is the maximum range of electrons, after which dose is
entirely due to Bremsstrahlung x-rays.
Why is surface dose less than 100% in electron beams? -
correct answer ✅Multiple scattering causes an increase in dose at
depth, so the max dose is higher than the surface dose. Higher
energy electrons scatter less, so surface dose increases with energy,
unlike with photons.
What causes electron dose tail and why does it increase in higher
energy electron beams? -
correct answer ✅Higher electron energies have a longer range,
but they suffer from a less sharp distal dose falloff and a larger
Bremsstrahlung X-ray "tail".
What causes electron beam straggling (reduction before meeting
medium)? -
, Electron Dosimetry Exam Actual
Questions and Answers 2026
correct answer ✅Loss of energy in air, scattering foil, monitor
chamber leading to slightly less surface dose.
What is the primary measure of electron beam quality? -
correct answer ✅E0 = Mean energy at patient surface
Define Rp for Electrons. -
correct answer ✅E/2 cm
Define R80 for Electrons. -
correct answer ✅E/2.8 cm
Define R90 for Electrons. -
correct answer ✅E/3.2 cm
How do low isodose lines appear at depth for electrons, in terms of
side profile. -
correct answer ✅High isodose lines (95, 90 %) "pull in" at depth,
while low isodose lines (50, 20 %) "bow out" at depth.
What do air gaps between cone and patient result in? -
correct answer ✅Poorly defined field edge and wider penumbra.
Questions and Answers 2026
Explain electron R90 and R50. How is the RP (practical range)
determined? -
correct answer ✅A straight line is drawn between R90 and R50
and used to calculate extrapolation values. Extrapolating back to
100% gives the Rq , while extrapolating forward to 0 % gives the Rp.
R max is the maximum range of electrons, after which dose is
entirely due to Bremsstrahlung x-rays.
Why is surface dose less than 100% in electron beams? -
correct answer ✅Multiple scattering causes an increase in dose at
depth, so the max dose is higher than the surface dose. Higher
energy electrons scatter less, so surface dose increases with energy,
unlike with photons.
What causes electron dose tail and why does it increase in higher
energy electron beams? -
correct answer ✅Higher electron energies have a longer range,
but they suffer from a less sharp distal dose falloff and a larger
Bremsstrahlung X-ray "tail".
What causes electron beam straggling (reduction before meeting
medium)? -
, Electron Dosimetry Exam Actual
Questions and Answers 2026
correct answer ✅Loss of energy in air, scattering foil, monitor
chamber leading to slightly less surface dose.
What is the primary measure of electron beam quality? -
correct answer ✅E0 = Mean energy at patient surface
Define Rp for Electrons. -
correct answer ✅E/2 cm
Define R80 for Electrons. -
correct answer ✅E/2.8 cm
Define R90 for Electrons. -
correct answer ✅E/3.2 cm
How do low isodose lines appear at depth for electrons, in terms of
side profile. -
correct answer ✅High isodose lines (95, 90 %) "pull in" at depth,
while low isodose lines (50, 20 %) "bow out" at depth.
What do air gaps between cone and patient result in? -
correct answer ✅Poorly defined field edge and wider penumbra.
