Part 10: Beam Shaping devices: Coll. & Field Blocks

Beam Shaping Devices aka

Beam Limiters

Beam Shaping Devices purpose

they alter the physical shape of the beam

List Beam Shaping Devices

1. Collimators

2. Field Blocks

3. MLC’s

4. Lead Strips

5. Lead Cutouts

Part 1. Collimators

aka

Beam-Restricting Diaphragms

Collimators

1. What are they made of?

2. How many HVLs thick are they?

3. Types:

1. Tungsten

2. 10 HVL thick

3. (1) Multi-Leaved (2) Asymmetric (x,y blocks)

Part 2. Field Block

aka

Custom Blocks

Field Blocks are made out of:

Lead

or

Lead Alloy

What problem can occur with field blocks that are too close?

Secondary Electron Contamination

term. Secondary Electron Contamination

When ↑E x-rays hit metal, they knock off e-.

If you knock off e- they will dissipate in the air at a certain distance ___.

If you are any closer than that specific distance:

electron contamination = ↓ skin sparing

What is the minimum distance between the patient and any accessory device on the treatment head (to avoid electron contamination)?

15 cm

term. Electron Absorbers

Materials with a z# ≤ 50, absorb e- (instead of producing them)

What is the ONLY METAL that is an electron absorber?

Tin (Z# 50)

Any other metal will produce electrons

Materials for construction of custom blocks (Brand Names):

1. Cerrobend- most popular

2. Ostalloy

The actual name of the alloy used for custom blocks?

Lipowitz Metal

aka Metal 158 (it’s melting point in F°)

Lipowitz ingredients

BLT with Cheese

50% Bismuth

26.7% Lead

13.3% Tin

10% Cadmium

__ fumes of Lipowitz alloy are carcinogenic

cadmium

Frequency of blood tests for mold room tech

semi-annual

True or False: Cadmium-free blocks are now available

True, but they are very pricey

Primary advantage of cerrobend

Low Melting point (FYI: So we can make molds with them)

Block Cutter (brand name)

Huestis (hint: huesitis)

In order to assure correct block size, the block cutting system must match the machine’s:

1. TFD

2. STD (source to tray distance)

Cutting a block into the treatment field

positive blocking

e.g. A heart block

Cutting the treatment field into the blockend area

Negative blocking

e.g. e- blocks

Examples of negative blocking

1. electron blocks

2. PAB: Posterior Axillary Boost

3. H/N

4. Mantle Field (debatable)

How should you pour in the blocking alloy when making a custom block?

slowly to avoid air bubbles

Why do we use a ___spray on Styrofoam when making custom blocks?

Silicone

1. for easy release

2. to keep metal from being contaminated with Styrofoam

Sources of error in block cutting:

1. loose wire (fyi: the wire that cuts the Styrofoam)

2. air holes (in the cerrobend)

3. Incorrect block parameters: SFD & STD

Custom blocks are ___ HVLs thick

5

term. transmission blocks

thinner blocks that allow a known amount of radiation through

How many HVLs thick are transmission blocks?

1. 1-2

The most common use for transmission blocks:

Tx the whole abdomen for ovarian cancer

Whole abdomen tx includes (anatomy):

diaphragm to bottom of ischium

Why are transmission blocks used for ovarian whole abdomen tx?

The dose exceeds the tolerance dose of the kidneys and liver

Transmission filter reduces the dose to these structures while allowing a therapeutic dose to transmit through the filter

transmission block (filter)

transmission blocks on kidney and liver for whole abdomen ovarian tx