Treatment Planning and Verification Flashcards
What is the purpose of patient immobilisation?
- Achieves a reproducible patient position
- Stops patient movement
What are desirable characteristics for an immobilisation device?
- Easy to use
- Easy to produce
- Patient comfort
- Resistance to deformation
- Minimal attenuation of the radiation beam for treatment
- Minimal imaging artefacts
What is the purpose of tumour localisation?
- To identify the gross tumour volume and clinical target volume
- Identify any multi-focal disease and involved nodes
What is the gross tumour volume?
Gross palpable or visible/demonstrate extent and location of the malignant tumour.
What is the clinical target volume?
- Includes the GTV and/or subclinical microscopic malignant disease.
- The CTV must be adequately treated in to ensure the intended outcome of the radiotherapy.
What does the delineation of the CTV depend on?
Tumour characteristics
Probable invasive capacity
Likelihood of microscopic occult disease.
Anatomical/topographic and biological considerations
When might a CTV be defined without a GTV?
At sites with suspected malignant cells (e.g. regional lymph nodes)
What is the internal target volume?
CTV plus an internal margin.
What does the internal margin depend on?
- Variation in shape, position, and size of organs adjacent to CTV
- Change in size shape and position of CTV
What is the planning target volume?
Internal target volume plus a setting-up margin.
What does the setting-up margin depend on?
- Uncertainties in patient positioning
- Alignment of radiation beams.
What is the treated volume?
The tissue volume that is planned to receive at least the dose selected as appropriate to achieve the purpose of the treatment.
What is the irradiated volume?
The tissue volume that receives a dose that is considered significant in relation to normal tissue tolerance.
What is an Organ at Risk (OAR)?
Normal tissues whose radiation sensitivity may significantly influence treatment planning and/or prescribed dose.
What is the planning organ at risk volume?
Margin around an organ at risk with the same function as internal and set-up margins
What criteria define a good treatment plan?
- 100% dose at isocentre
- PTV enclosed in 95% iso-dose line
- Maximum dose in PTV no higher than 107%
- Rapid dose fall-off outside PTV
- No significant dose to critical organs
What dose specifications must be reported in line with ICRU 50?
- ICRU reference point
- Max dose in PTV
- Min dose in PTV
What criteria should guide the placement of the ICRu reference point?
- Dose at point should be clinically relevant
- Dose at point is representative of the dose throughout the PTV
- Easy to define in a clear and unambiguous way
- Selected where the dose may be accurately determined
- In a region with no steep dose gradient
What is the maximum planning target dose?
The maximum dose delivered to a clinically meaningful volume (sphere 15mm of diameter for large organs)
What is the minimum planning target dose?
Smallest dose in the PTV (no volume constraint)
What is a hot-spot?
A clinically meaningful volume outside the PTC which recieved a dose larger than 100% of the specified PTV dose
