RadOncQuestions Q banks

Question 1

what is the minimum energy required for pair production?

Answer 1

minimum incident photon energy of 1.022 MeV (twice the rest energy of electron)

Question 2

At what energy does pair production becomes the dominant photon interaction in water or tissue?

Answer 2

25 MeV

Question 3

what is the photon energy range where compton scattering is dominant?

Answer 3

25 keV - 25 MeV

Question 4

Describe the process of pair production

Answer 4

Question 5

how can the effect of tissue inhomogeneities on electron beam can be estimated by?

Answer 5

By calculating the water-equivalent effective depth

Question 6

Bone, air cavities, and other tissue inhomogeneities (relative to water) can alter the dosimetry of electron beams. What is the correction factor that is used to convert a given thickness of inhomogeneous tissue to an equivalent thickness of water?

Answer 6

The coefficient of equivalent thickness (CET)

Question 7

CET (coefficient of equivalent thickness) is related to the electron density of a given tissue. Describe the CET of the lung vs spongy bone vs cortical bone.

Answer 7

Lung tissue is low-density and therefore has low CET (CET ~ 0.25), spongy bone CET is ~ 1, and cortical bone CET is ~ 1.65)

Question 8

Explain the coefficient of effective tissue (CET) and effective depth.

Answer 8

As CET increases effective depth also increases. In means that electron beam will be less attenuated and travel longer distance in lung, but will be more attenuated by dense cortical bone and travel a shorter distance.