Dosimetry midterm Exam Actual
Questions and Answers 2026
what is a beam profile? -
correct answer ✅the dose variation or dose rate variation along a
line perpendicular to the central axis of the beam at a certain
depth.
what are the three regions of a beam profile? -
correct answer ✅1) central: where doses are greater than 90% of
the dose on the central axis
2) penumbra: where doses are between 80 and 20% of the dose on
the CAX
3) umbra: where doses are lower than 20% of the dose on the
central axis
what is photon intensity? -
correct answer ✅number of photons per unit area. NOT the
number of photons in a beam
what is the horns phenomena? -
correct answer ✅the two bumps seen on beam profiles at depths
near the surface. The bumps are in the periphery, where the dose is
higher than that on the CAX
, Dosimetry midterm Exam Actual
Questions and Answers 2026
what causes horns? -
correct answer ✅caused by the flattening filter and the photon
energy dependence of the attenuation coefficient
how does the horns effect change with beam energy? -
correct answer ✅horns are more pronounced for higher energy
beams
how does the horns effect change with field size? -
correct answer ✅horns effect is more pronounced for larger field
sizes
at which depth is the beam flatness specified? -
correct answer ✅beam flatness is specified at 10 cm depth
what are the three components of penumbra? -
correct answer ✅-geometry, transmission and lateral scattering
geometric penumbra -
correct answer ✅geometric: due to the finite source size. it is
more of a concern in the Co-60 machines where the source is
between 1 and 2 cm size
Questions and Answers 2026
what is a beam profile? -
correct answer ✅the dose variation or dose rate variation along a
line perpendicular to the central axis of the beam at a certain
depth.
what are the three regions of a beam profile? -
correct answer ✅1) central: where doses are greater than 90% of
the dose on the central axis
2) penumbra: where doses are between 80 and 20% of the dose on
the CAX
3) umbra: where doses are lower than 20% of the dose on the
central axis
what is photon intensity? -
correct answer ✅number of photons per unit area. NOT the
number of photons in a beam
what is the horns phenomena? -
correct answer ✅the two bumps seen on beam profiles at depths
near the surface. The bumps are in the periphery, where the dose is
higher than that on the CAX
, Dosimetry midterm Exam Actual
Questions and Answers 2026
what causes horns? -
correct answer ✅caused by the flattening filter and the photon
energy dependence of the attenuation coefficient
how does the horns effect change with beam energy? -
correct answer ✅horns are more pronounced for higher energy
beams
how does the horns effect change with field size? -
correct answer ✅horns effect is more pronounced for larger field
sizes
at which depth is the beam flatness specified? -
correct answer ✅beam flatness is specified at 10 cm depth
what are the three components of penumbra? -
correct answer ✅-geometry, transmission and lateral scattering
geometric penumbra -
correct answer ✅geometric: due to the finite source size. it is
more of a concern in the Co-60 machines where the source is
between 1 and 2 cm size
