Principles of Treatment Planning

Question 1

What are the main ICRU reports and what do they discuss? (9 marks)

Answer 1

ICRU 50 - covers basic 3D planning with newly inverted computed controlled treatment machines and CT. Volumes were introduced.

ICRU 62 - conformal planning, invention of MLCs, more complex beam shapes available. Margins were introduced.

ICRU 83 - VMAT, inverse planning, fly-by wire delivery which is computer planned and check by physicists.

Question 2

What are the ICRU volumes? (9 marks)

Answer 2

Gross target volume - palpable or demonstrable extent of tumour.

Clinical target volume - GTV plus microscopic spread, indicated by the clinician and varies based on type of tumour.

Internal treatment volume - CTV plus internal margin to account for movement of organs such as bladder filling and breathing.

Planned treatment volume - ITV plus set-up margin for errors in patient positioning.

Treated volume - volume planned to receive the dose required for treatment, may not exactly match the PTV but should be as close as possible.

Irradiated volume - volume of tissue which receives a significant amount of dose in comparison to normal tissue tolerance.

Question 3

What is an OAR and PRV? (2 marks)

Answer 3

Organ at risk is an organs with a radiosensitivity which can affect how the plan is designed. There are dose limits to OARs.

Planning Organ at Risk Volume = OAR plus internal and set-up margin

Question 4

What is the ICRU reference point? Dose specifications for reporting? (9 marks)

Answer 4

Need to report:

• Dose delivered to ICRU reference point. Ref point chosen so that it is representative of dose throughout the PTV. Not in a rapidly changing dose gradient. Easy to define. Isocentre is usually as suitable choice.
• Any hotspots.
• Max dose to clinically significant volume (min dimension greater than 15 mm.
• Min dose delivered to PTV (of any size).

Question 5

What is a hotspot? (2 marks)

Answer 5

• Any clinically significant volume outside the PTV which receives a significant dose ( > 100% of the prescribed dose)

Question 6

What types of OAR are there and how are DVHs used to assess doses to OARs? (5 marks)

Answer 6

OARS:
- serial OAR such as the spine which has a set dose limit for any point in the organ. Any point in the spine should not exceed e.g. 40 Gy otherwise patient may be paralysed.

• parallel OAR, such as the lung, where there are set dose limits to particular volumes of the OAR, such as V20 < 30 % (V20 means volume of the lung receiving 20 Gy.

Can use DVHs either with a cumulative or frequency plot to determine whether or not the OARs have met their dose limits.

Question 7

Give two limitations of DVHs? (2 marks)

Answer 7

• No positional information can be obtained in terms of what area is being irradiated.
• The shape of them may vary depending on size of voxels chosen