Photon Dosimetry Concepts and Calculations

Patient Alignment

Use of external landmarks (eyes, nose, sternal notch, xiphoid process, navel, legs) as well as the sagittal laser through middle of body can help to ensure that the patient is straight on the table. The spine can be an effective way to straighten a patient when imaging is used. Adjustment of the patients hips or shoulders may be required.

Localization

identifying the position and extent of the tumor volume using a three-dimensional coordinate system

Coordinate System

used to describe the location of any point with respect to another known point (origin) along the x, y, and z axes

Triangulation

three landmarks whose intersection marks a single reproducible point within the patient. Three reference points can confirm patient alignment in three planes (x, y, z). Shifts can be made from this location to another location within the patient.

Isocenter

fixed point in space where the central axes of the radiation beams intersect from every angle. The gantry, table and collimators all rotate around this point. Positioning lasers are aligned to intersect at this point. Generally set at 100 cm from the radiation source for modern linear accelerators. The field size for a treatment unit is defined at the isocenter of the machine.

Gray

Absorbed dose is measured in _____.

100

1 Gy is equivalent to _____ rads

1

1 Gy is equivalent to ___ J/Kg

100

1 rad is equivalent to ___ erg/g.

depth

distance beneath the skin surface where the prescribed dose is to be delivered

Separation

Measurement of the patient thickness from point of beam entry to point of beam exit. Also known as midplane.

• Measured with calipers, optical distance indicator (ODI)

Source-to-Skin Distance (SSD)

Distance from source or target of machine to surface of patient. Also called Target-to-______ distance.

• Measure with ODI

• Continually changes as gantry rotates around patient.

Source-to-axis distance (SAD)

Distance from source of photons to the isocenter. Also called Target-to-_____ distance.

• A fixed distance that will not change as gantry rotates around patient (usually 100cm).

SAD

An isocentric set up also refers to a ________ set up.

Isocentric

The isocenter is established at some reference point inside the patient. The gantry (source) rotates around this point (axis). Since the field size is defined at the isocenter, the collimated field size is defined by the size set inside the patient and not the field size on the skin surface. This is referred to as an ________ setup.

Backscatter

Radiation that is scattered back toward the surface of the patient.

• Can lead to a “double dose” to the patient

Depth of maximum equilibrium of electrons

Point at which the energy of the electrons coming to rest equals the energy of electrons being set into motion by new photon interactions

Buildup region

Region between the surface and the point of max dose

Skin-sparing effect

When depth of Dmax occurs below 1 cm.

200 kv

What is the beam energy at 0.0 cm depth of Dmax?

1.25 MV

What is the beam energy at 0.5 cm depth of Dmax?

4 MV

What is the beam energy at 1.0 cm depth of Dmax?

6 MV

What is the beam energy at 1.5 cm depth of Dmax?

10 MV

What is the beam energy at 2.5 cm depth of Dmax?

18 MV

What is the beam energy at 3.5 cm depth of Dmax?

24 MV

What is the beam energy at 4.0 cm depth of Dmax?

Output (Dose Rate)

The dose rate of the machine. Amount of radiation exposure produced by a treatment machine or source as specified at a reference field size (usually 10cm x 10cm) at a specified reference distance (100cm from source).

• Increases with increased field size

• Decreases with increased SSD

• Measured in tissue equivalent material

Output Factor

Ratio of the dose rate of a given field size (collimator) to the dose rate of the reference field size.

Dose maximum (dmax)

the depth of maximum buildup where 100% of the dose is deposited. This the depth of the electronic equilibrium (amount of energy lost is equal to the energy gained)

Sc

Collimator scatter factor (unknown output, know field size)

Sp

Phantom or patient scatter factor (known output for given field size)

Sc,p

Combined collimator and phantom scatter

Collimator Scatter (Sc)

the ratio of the dose rate of a given field size to the dose rate of the reference field size measured in air.

• Allows for the change in scatter as the collimator setting changes.

• Usually normalized or referenced to a 10cm x 10cm

Phantom Scatter (Sp)

Scatter from the patient. This factor is derived by measuring machine output in a phantom (tissue equivalent).

• If the known collimator scatter factor is removed from the dose measured with a reference field size (10cm x 10cm) and the new field size, the remaining change in dose is due to ___________.

increase

If the energy increases, the Percentage Depth Dose (PDD) will ….

decrease

If the depth increases, the percentage depth dose (PDD) will…

increase

If the field size increases, the percentage depth dose (PDD) will …

Percentage Depth Dose (PDD)

The ratio of absorbed dose at a given depth to the absorbed dose at a fixed reference point (dmax).

• Used with SSD techniques

Tissue-air ratio

Ratio of the absorbed dose at a given depth in phantom to the absorbed dose at the same point in free space.

• Used to perform calculations for SAD treatments involving low-energy treatment units (ex: cobalt-60, 4MV)

increase

If beam energy or field size increase, the tissue-air ratio will ….

decrease

If the depth increases past dmax, the tissue-air ratio will …

dmax

TAR can be defined at any depth where BSF is only defined at _____

Scatter-air ratio

ratio of the scattered dose at a given point to the dose in free space

Backscatter Factor

ratio of dose rate with scattering medium to dose rate in air at dmax.

high

BSF is _____ for low energies

lower

BSF is ____ in high energies (8-10MV)

Tissue-Max Ratio

Also known as Tissue-Phantom Ratio; most commonly used patient attenuation factor

Tray Transmission Factor

defines how much of the radiation is transmitted through a block tray

• numerical measure by physics

• use when custom blocks are mounted on a plastic tray