Target volume localisation

Question 1

List the steps involved in the treatment planning process.

Answer 1

• Immobilisation (patient).
• Tumour localisation (patient).
• Virtual representation of patient (patient and data).
• Selection of treatment geometry (data).
• Optimisation of dose distribution (data).
• Calculation of dose and monitor units (data).
• Independent dosimetric verification (data).
• Documentation of treatment information (data).
• Verification of treatment set-up (patient and data).
• Treatment delivery (patient and data).

Question 2

What is the purpose of immobilisation?

Answer 2

• To achieve a reproducible patient position.

- To determine an appropriate position for site treatment and delivery geometry.

Question 3

What are the desirable characteristics for patient position during immobilisation?

Answer 3

• Ease of use.
• Ease of production.
• Patient comfort.
• Resistant to bending and stretching.
• Minimal attenuation of the radiation beam for treatment.
• Minimal imaging artefacts.

Question 4

List the types of immobilisation used.

Answer 4

• Support:
• e.g. footrest, head rest, full body vac bag etc.
• Restraint:
• e.g. face masks.

Question 5

What is the purpose of tumour localisation?

Answer 5

• To identify the gross tumour volume and clinical target volume.
• To identify any multi-focal disease and involved nodes.

Question 6

What are the methods of tumour localisation?

Answer 6

• Methods of localisation:
• Direct visualisation/palpation.
• Conventional simulator.
• CT.
• MRI.
• Ultrasound.
• PET-CT.

Question 7

Describe a conventional simulator for use in tumour localisation.

Answer 7

• Vertical position of X-ray tube and Image detector can be changed.
• Wires display rectangular field.
• Software overlays display MLC shaping.

Question 8

What are the Pros and cons of using CT for tumour localisation.

Answer 8

-Good for visualising some tumours
E.g. Lung.
-Insufficient soft tissue contrast for other tumours.

Question 9

How is MRI used in tumour localisation and what are the advantages?

Answer 9

• Image formation: Relaxation times of different tissues.
• Soft tissue definition better than CT.
• Ability to differentiate pathology of tissue.

Question 10

When can ultrasound be used in tumour localisation?

Answer 10

• BAT [B-mode and targeting] ultrasound of bladder and prostate.
• Ultrasound images are acquired each day before external radiation therapy, and the patient’s position is adjusted so the radiation beams are aimed accurately at the prostate.
• US used in brachytherapy to confirm correct applicator insertion and to plan treatment.

Question 11

State the advantages and disadvantages of using PET for tumour localisation.

Answer 11

• Advantages:
• Metabolic imaging.
• Can select different metabolic markers.
• Disadvantages:
• Intrinsic PET resolution low.
• Uptake distribution requires experienced interpretation.
• Thresholding remains a challenge.
• Requires nearby cyclotron.
• Poor anatomical information, unless combined with CT.

Question 12

What is the purpose of virtually representing the patient and what happens to any errors introduced at this stage?

Answer 12

• To provide a representation of the patient on which to base all subsequent stages of the planning process.
• Any errors or uncertainties introduced at this stage will be propagated throughout the process.
• Localisation and representation may contain common elements.

Question 13

What are common methods of virtual representation?

Answer 13

• Separation.
• Manually acquired contour.
• Single CT slice from simulator CT.
• Full volume CT dataset.
• Full volume MRI – primarily Brachytherapy at present.

Question 14

Give an example of a basic 1D representation of a patient.

Answer 14

• Assumes the patient is a box of water equivalent material.
• Only appropriate for a parallel-opposed pair.
• Separation measured along Central Axes of beams.

Question 15

Give two examples of a 2D representation of a patient.

Answer 15

• Manually acquired contour:
• Patient shape in a single plane represented:
• Allows for obliquity to be modelled.
• Patient modelled as water equivalent.
• Method:
• A wire or flexible Curve is used to transfer a single axial outline to paper where the couch level is also represented.
• Single CT slice:
• Patient shape in a single plane represented:
• Allows for obliquity to be modelled.
• Tissue densities modelled.
• Most modern planning systems require more than one slice.
• Method:
• Sim-CT fan beam single slice acquisition.

Question 16

Give an example of a 3D representation of a patient.

Answer 16

• CT volume:
• Full volume of patient represented:
• Allows for obliquity to be modelled.
• Tissue densities modelled.
• Basis of modern planning systems.
• Method:
• 3D acquisition in Planning CT:
• Patient in treatment position.
• Patient on flat couch.

Question 17

What are the benefits and downsides of using CT volume to virtually represent the patient?

Answer 17

• Benefits:
• Geometrically accurate.
• Hounsfield Units map to electron density/mass density.
• Good resolution.
• Good bony/soft tissue contrast.
• Disadvantages:
• Poor soft tissue contrast.

Question 18

What are the benefits of using CT in the planning process?

Answer 18

• Data transfer.
• Image manipulation.
• Windowing.
• Multi-planar reconstruction.
• Image fusion.
• Definition of volumes.
• Growing tools.
• Beam geometry.
• Optimisation of dose distribution.

Question 19

What is the method for data transfer of a CT volume?

Answer 19

• Image dataset must be transferred to the appropriate RT application:
• DICOM3 format.
• Magnification of image – correct physical size is required.
• DICOM RT objects:
• Structure Sets –contours.
• RTPlan – Treatment geometry.
• RTImage – DRRs.
• RTDose – dose distribution.

Question 20

What main things do we need to localise a tumour for RT treatment?

Answer 20

• Patient.
• Imaging equipment.
• Immobilisation.
• Method of patient marking to allow alignment of patient in treatment room.
• Software application to define treatment geometry.
• Method of dose calculation.
• etc.