Special Calcs

In couch/collimator problems, is the collimator associated with the cranial field or the spinal field?

Spine

In couch/collimator problems, is the couch associated with the cranial field or the spinal field?

cranial

what is the formula for the couch angle?

what is the formula for the collimator angle?

What is the formula for breast tangent?

if need to calculate tangent in reverse direction, add tan instead of subtract (if you’re given LAO instead of RAO)

what is the formula for transmission?

• transmission gives what percentage survive

As the x-ray energy increases from 1MV to 20MV, what happens to the half-value layer in lead?

initially decreases bc of increased compton interaction, then increases as pair production becomes more prevalent at higher energies

What is the linear attenuation coefficient?

• the fractional number of photons removed from a beam per cm of absorber

what is transmission penumbra?

The region irradiated by photons which have gone through part of the collimator

what is geometric penumbra?

• region irradiated by primary photons, coming directly from the source

What is the formula for geometric penumbra?

• S: source size

• d: depth

• SDD: source to diaphragm distance

What does the width of penumbra depend on?

beam energy

How do you calculate the timer setting calculation?

• be mindful of inverse square, units, and PDD

• fractional dose = dose at dmax

How does increasing frequency impact the energy of a photon?

• increases it

• E=h (Planck’s constant) x frequency

• frequency = c (speed of light) / wavelength