Lecture 3: Clinical Photon Beams

Depth Dose Distribution (PDD)

Describes dose fall off as a function of depth at constant SSD, normalized to maximum dose (dependent on energy and field size).

What is surface dose measured with?

Parallel-plate ionization chambers

What contributes to surface dose?

scattered photons, electrons produced from photon interactions in air and shielding, backscattered photons from patient

Does surface dose increase or decrease with energy?

Decrease

Does surface dose increase or decrease with field size?

Increase

Beam Profile: Central Region

Dose profile does not change significantly. Depends on inverse-square dose fall-off, flattening filter, energy of electrons, and target

Beam Profile: Shoulder

Photons begin scattering out of the field more than in (disequilibrium)

Beam Profile (Penumbra):

Characterized by 20/80% width. Depends on beam energy, source size, SCD, and depth

Transmission Penumbra

Due to transmission through jaws

Geometric Penumbra

Due to finite source size

Scatter Penumbra

due to inpatient x-ray scatter

Beam Profile: Umbra

Due to outscattered photons and transmission through jaws

Beam flatness (F) is required to be less than what percentage using the variance method?

3%

What causes horns in beam profiles

Over-flattening at z_max and under-flattening as z increases

What percent symmetry change from baseline is allowed for 2009 TG142?

1%

Off-axis ratio (OAR)

the ratio of dose at an off-axis point to the dose on the central beam axis at the same depth in phantom

PDD: Buildup Region (z < z_max)

CPE does not exist (D < K_c); secondary electrons are released and deposits kinetic energy at larger depths

When is CPE reached on a PDD (D = K_c)

z = z_max (~range of secondary charged particles)

PDD: TCPE (z > z_max)

D and K_c decrease due to photon attenuation resulting in transient CPE

PDD: Does z_max decrease or increase with beam energy?

increase