Cobalt-60 Teletherapy Basics

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Questions and Answers

What is the typical range of square and rectangular radiation fields provided by collimators in teletherapy machines?

5×5 to 35×35 cm2 (correct)

20×20 to 50×50 cm2

10×10 to 40×40 cm2

15×15 to 30×30 cm2

Which type of penumbra is influenced by the transmission through the edge of the collimator block?

Transmission Penumbra (correct)

Thermal Penumbra

Geometric Penumbra

Physical Penumbra

What can be done to minimize geometric penumbra?

Use a smaller source diameter (correct)

Increase the size of the collimator opening

Increase the SSD

Place blocks further from the patient

What does physical penumbra take into account in addition to transmission and geometric penumbra?

Scattered radiation Signup and view all the answers

Which factor can lead to increased transmission penumbra?

Increase in collimator opening Signup and view all the answers

What distance should blocks be placed from the patient to avoid excessive electron contamination?

10 to 20 cm Signup and view all the answers

How can transmission penumbra be effectively reduced?

By shaping the collimator Signup and view all the answers

What is the physical penumbra defined as?

Distance between specified isodose curves Signup and view all the answers

What can be a consequence of increasing the source-to-skin distance (SSD)?

Increase in transmission penumbra Signup and view all the answers

Which of the following statements about penumbra trimmers is true?

They should be placed close to the patient's skin Signup and view all the answers

What is a common effect of increasing the collimator opening in teletherapy?

Increased transmission penumbra Signup and view all the answers

Which type of penumbra is primarily affected by the diameter of the radiation source?

Geometric Penumbra Signup and view all the answers

How can the collimator shape be adjusted to influence penumbra in teletherapy?

By shaping the collimator to minimize transmission penumbra Signup and view all the answers

What determines the location at which the physical penumbra is measured?

Specified isodose curves at a certain depth Signup and view all the answers

What is a benefit of using penumbra trimmers in teletherapy?

They minimize the effects of transmission penumbra Signup and view all the answers

Which of the following describes physical penumbra in radiation therapy?

The distance between isodose curves concerning scattered radiation Signup and view all the answers

When considering transmission penumbra, what is the impact of increasing the source-to-skin distance (SSD)?

It increases transmission penumbra Signup and view all the answers

Why should radiation blocks be placed 10 to 20 cm away from the patient?

To avoid excessive electron contamination Signup and view all the answers

What characteristic of physical penumbra includes aspects of both transmission and geometric penumbra?

The distance between isodose curves including scattering Signup and view all the answers

Study Notes

Cobalt-60 Teletherapy Machine

Collimators provide radiation fields of varying sizes, typically from 5×5 to 35×35 cm², at a distance of 80 cm from the radiation source.
Penumbra refers to the gradient area at the edge of a radiation beam where dose rates change rapidly in relation to distance from the beam axis.

Types of Penumbra

Transmission Penumbra: Arises from the inner surface of collimator blocks aligned parallel to the beam's central axis. This type increases with greater source-to-surface distance (SSD) and larger collimator openings. Reducing collimator size can help minimize transmission penumbra.
Geometric Penumbra: Caused by the finite diameter of the radiation source. This can be reduced by:
- Utilizing a smaller source diameter.
- Positioning penumbra trimmers as close to the patient's skin as possible (ideal at z = 0).
Physical Penumbra: Measured as the distance between specific isodose curves at defined depths (90% and 20% at Dmax). It factors in scattered radiation alongside transmission and geometrical penumbra.

Electron Contamination

Blocks should be positioned at least 10 to 20 cm away from the patient to minimize excessive electron contamination originating from the blocks.

Cobalt-60 Teletherapy Machine

Collimators provide radiation fields of varying sizes, typically from 5×5 to 35×35 cm², at a distance of 80 cm from the radiation source.
Penumbra refers to the gradient area at the edge of a radiation beam where dose rates change rapidly in relation to distance from the beam axis.

Types of Penumbra

Transmission Penumbra: Arises from the inner surface of collimator blocks aligned parallel to the beam's central axis. This type increases with greater source-to-surface distance (SSD) and larger collimator openings. Reducing collimator size can help minimize transmission penumbra.
Geometric Penumbra: Caused by the finite diameter of the radiation source. This can be reduced by:
- Utilizing a smaller source diameter.
- Positioning penumbra trimmers as close to the patient's skin as possible (ideal at z = 0).
Physical Penumbra: Measured as the distance between specific isodose curves at defined depths (90% and 20% at Dmax). It factors in scattered radiation alongside transmission and geometrical penumbra.

Electron Contamination

Blocks should be positioned at least 10 to 20 cm away from the patient to minimize excessive electron contamination originating from the blocks.

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Description

This quiz covers the essential concepts of Cobalt-60 teletherapy machines, including radiation field sizes and the concept of penumbra. Learn about the different types of penumbra—transmission, geometric, and physical—and how they impact therapy. Perfect for students studying radiation therapy techniques.