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scatter
When the main radiation beam hits objects (like the patient, collimator, or other materials) and changes direction.
• What It Is: Consists of photons or electrons that spread out, not in the intended path.
DMAX
The depth where the radiation dose is strongest before it starts decreasing. In tissue
radiation dose is the strongest before it
starts to decrease. It happens because radiation interacts with tissues, releasing and
absorbing energy.
Factors that impact dmax
Beam energy
Field size
Treatment distance
Tissue/Density of patient
How does beam energy effect dmax
• High-energy (MV): Dmax is deeper inside the body.
• Low-energy (kV): Dmax is near the skin surface.
How does dmax effect field size
Larger fields produce more scatter, possibly pushing Dmax deeper.
How does dmax affect treatment distance
Increasing the distance from patient or decreasing can shift Dmax deeper or shallower.
How does dmax effect patients tissues
Different tissues (e.g., bone vs. muscle) can affect where Dmax occurs.
What is skin sparing
When the highest radiation dose (Dmax) is below the skin surface, protecting the skin.
• When It Happens: With high-energy (megavoltage, MV) photon beams, delivering more dose deeper into the tissue.
Define output.
Refers to the dose rate (cGy/MU) produced by a treatment machine -
the dose rate produced by the treatment machine measuring rad exposure at a reference field size (usually 10 x 10 cm) and a specific distance (typically at isocenter).
It is measured without tissue in the way, typically in air or a phantom, to get a baseline dose rate.
Factors may impact output
Field Size – Bigger field = more scatter = higher output. Smaller field = less scatter = lower output.
Distance – The farther from the source, the less radiation reaches the patient (because of the inverse square law).
Collimator Setting – Changing the collimator changes scatter radiation, which affects output.
Patient/Phantom Scatter – Scatter inside the patient or phantom adds to the total dose
Beam Energy – Different beam energies can change how output behaves.
Higher output = More radiation delivered per monitor unit.
What is output factor?
number that tells us how much the dose rate changes when we change the field size on the treatment machine.
A 10 × 10 cm field is the standard → its output factor is 1.00.
ratio that compares the dose rate of a specific field size to the dose rate of a standard 10 x 10 cm field size.
• Purpose: It adjusts for changes in scatter radiation when the collimator settings are changed.
• Bigger Field Size: More scatter = Higher output factor HIGHER dose more radiation
What is a tissue absorption factor?
How much energy a radiation beam loses as it
travels through the body - the more tissue it travels through, the more it attenuates.
based on depth, field size, and energy.
Describe Percentage Depth Dose.
way to measure how much radiation reaches a certain depth inside the patient compared to the maximum dose (Dmax). It is expressed as a percentage
PDD relationship w distance
As distance from the source of radiation to patient surface increases, the PDD increases - More of the dose reaches deeper tissues from farther away.
Why distance do that
When the machine is farther away, the radiation beam spreads out and penetrates deeper, resulting in a higher PDD because more radiation reaches deep tissues instead of being absorbed at the surface.
PDD relationship with depth
PDD decreases as depth in tissue increases because dose is deposited in tissue as it travels - a smaller percentage of original dose available at greater depths.
Attenuation:
The tissues act like a barrier, gradually weakening the beam.
How does Energy impact PDD?
Higher energies are more penetrating so a greater percentage of dose is delivered at a specific depth compared to lower energies
How does Depth impact PDD?
PDD decreases as depth in tissue increases because of
attenuation as the beam travels.
How does Field size impact PDD?
As field size increases, more scatter is added to deposited beam, increasing PDD.
Small Field: Less tissue interaction, less scatter, lower PDD.
Large Field: More tissue interaction, more scatter, higher PDD.
Depth (Definition)
Distance beneath the skin surface where the
prescribed dose is to be delivered.
Separation
Measurement of patient thickness from point of beam entry to point of beam exit.
Dmax
Depth of maximum dose; depth of maximum equilibrium of electrons. 100% beam dose deposited
Buildup region |
Region before Dmax where | |
secondary electrons are | |
formed. Skin sparing effect | |
Percentage Depth
这
Dose (PDD)
Developed for
SSD setups (nonisocentric); ratio
dependent on four factors:
Energy, Depth, Field size, SSD.
