64. RADIOTHERAPY: ISODOSE MEASUREMENT

Question 1

1. What is important when we look at X-Ray beams?

Answer 1

• their dose distribution
• this is produced by the X-Ray beam
• it is produced within the tissue

Question 2

2. What is the importance of the Dose Distribution?

Answer 2

• it is used to determine the dose at a specific patient
  volume

Question 3

3. How do we measure the dose at a specific patient volume?

Answer 3

• we use a Tissue Equivalent Phantom

Question 4

4. What is a Tissue Equivalent Phantom?

Answer 4

• it is a machine
• it has the same atomic number as the tissue
• it has the same electron density as the tissue
• it has the same density as the tissue

Question 5

5. What is a necessary factor of the Phantom?

Answer 5

• we need to be able to move ionisation chambers within
  the phantom

Question 6

6. What material is placed inside the Phantom?

Answer 6

• water

Question 7

7. How do we plot the Dose Distribution?

Answer 7

• it is plotted in the form of Isodose charts

Question 8

8. What are the Isodose lines in these charts?

Answer 8

• they are lines that show a constant dose
• they are expressed as a percentage of the maximum
  dose

Question 9

9. What is the relation between the dose and the depth?

Answer 9

• the dose will fall with increasing depth
• this means that they are inversely proportional

Question 10

10. Why dose the dose fall with depth?

Answer 10

• the beam is absorbed by the tissue
• it follows the inverse square law
• Compton scattering also takes place

Question 11

11. What does the Compton scattering result in?

Answer 11

• the edges of the beam are diffused
• this happens at low energies

Question 12

12. What happens to the edges of the beam at higher energies?

Answer 12

• they are better defined
• this gives rise to something known as the Skin-Sparing
  effect

Question 13

13. What is the Skin Sparing effect?

Answer 13

• this is the build up of electrons at depths under the
  skin

Question 14

14. What happens at higher energies?

Answer 14

• the 1–% dose line is below the surface of the phantom

Question 15

15. What is the depth of the peak in this image?

Answer 15

• it is roughly a quarter of the photon energy
• this is in Mega Volts

Question 16

16. What can be said about the depth of the peak when we are looking at a 4 MV X-Ray?

Answer 16

• the peak is approximately 1 cm below the surface

Question 17

17. What is the Surface dose percentage for most teletherapy units?

Answer 17

• it is between 20% and 50%
• this is one of the main advantages of high-energy
  therapy