beam modifiers week 7

Beam modifiers

Any device placed in the path of the beam

u Four Main Types

u Shielding

u Compensation

u Wedge Filtration

Flattening

What is Shielding?

Goal: Minimize radiation exposure to critical structures and shape the beam.

• Purpose:

• Protects healthy tissues.

• Creates complex radiation field sizes and shapes.

Types of Shielding

Lead Blocks:

• Custom-made blocks to protect critical structures.

• Can be positive or negative shapes.

• MLCs (Multileaf Collimators):

• adjust collimator jaws or asymmetrical fields used to shape and modulate the beam dynamically.

But this stops that from changing isocenter duh

• you wanted to block one side more than the other, you’d have to shift the isocenter to keep the target in the beam

• Provides more precise and automated beam shaping.

Shielding positive/negative blocks

Critical structures like heart island

Negative- peripheral surrounding structures are treated

Lead Block Composition

BLT w CHEESE

Bismuth: 50%

• Lead: 26.7%

• Tin: 13.3%

• Cadmium: 10%

What is Tray Transmission Factor?

measures how much radiation passes through the block tray.

• Used when custom blocks are mounted on a plastic tray

Purpose of Tray Transmission Factor

To account for the attenuation (reduction) of the beam by the tray.

• Ensures accurate dose delivery when blocks are used.

What Are Compensators?

Devices used to account for surface irregularities and differences in tissue thickness.

• Ensures a uniform dose across the treatment area

Purpose of Compensators

To modify the radiation beam to maintain an even dose to the target tissue.

• Protect critical structures and reduce hot and cold spots

Types of Compensators

Bolus

• Purpose: Acts as tissue-equivalent material to bring the dose closer to the skin.

• When Used:

• Treating superficial tumors.

• Reducing Dmax (maximum dose depth) for surface treatments

Bolus Reduces Dmax by shifting the maximum dose closer to the skin.

• This ensures better dose coverage for superficial tumors

Wedges

• Wedges reduce beam intensity on one side.

• They tilt isodose curves.

• Reduce dose overlap

• Tilt depends on wedge angle (15°, 30°, 45°, 60°).

• Made of tungsten, lead, brass, or steel

What kind of wedges

• Physical wedge = placed in beam path.

• Dynamic wedge = made by moving collimator jaws.

• Wedge transmission factor (WTF) = how much radiation passes through.

• WTF depends on material and angle (steeper angle = lower WTF).

Flattening filter

• Located in the head of the LINAC.

• Used in photon beams (not electrons).

• Flattens the beam to make dose more even across the field.

• Reduces central axis (CAX) dose (beam is more intense in the middle without it).

• Dose rate is lower with the filter, especially at the CAX.

What Electron Beams Are Used For:

• Treat tumors close to the skin (superficial tumors)

• Great when you don’t want the beam to go deep

• Dose builds up quickly near the surface

• Reaches a peak dose (Dmax)

• Then drops off fast — so deeper tissues get very little dose

Electron Beam Isodose Curves

• The dose lines look like a balloon or bubble shape going into the body.

• The sides spread out (lateral scatter), especially near the surface.

• With higher energy, the shape is less bubbly and more straight.

Describe the practical range in centimeters in tissue (mean energy at surface). How is it used?

• It tells you how deep the electrons go before the dose basically stops.

• It’s the spot where the dose is only a tiny bit left (a few percent).

• Used to pick the right energy for how deep the tumor is

• 80% line

• used for most treatments
  → It tells you how deep you can treat safely
  → Formula: Depth = MeV / 3

• 90% line

• used for very shallow tumors
  → It’s closer to the surface
  → Formula: Depth = MeV / 4

• These help you pick the right energy so the tumor gets the full dose — not too deep, not too shallow.

• External Shielding

• 
  

  • Lead or Lipowitz cutouts

  • Placed on skin or cone

  • Shapes the beam

• Internal Shielding:

• 
  

  • Lead or tungsten

  • Placed inside the body near sensitive areas

• Bolus

• A compensator
  

  • Wax or acrylic

  • Used to shape the dose or even out the skin surface