List the steps involved in the treatment planning process.
- 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).
What is the purpose of immobilisation?
- To achieve a reproducible patient position.
- To determine an appropriate position for site treatment and delivery geometry.
What are the desirable characteristics for patient position during immobilisation?
- Ease of use.
- Ease of production.
- Patient comfort.
- Resistant to bending and stretching.
- Minimal attenuation of the radiation beam for treatment.
- Minimal imaging artefacts.
List the types of immobilisation used.
- Support:
- e.g. footrest, head rest, full body vac bag etc.
- Restraint:
- e.g. face masks.
What is the purpose of tumour localisation?
- To identify the gross tumour volume and clinical target volume.
- To identify any multi-focal disease and involved nodes.
What are the methods of tumour localisation?
- Methods of localisation:
- Direct visualisation/palpation.
- Conventional simulator.
- CT.
- MRI.
- Ultrasound.
- PET-CT.
Describe a conventional simulator for use in tumour localisation.
- Vertical position of X-ray tube and Image detector can be changed.
- Wires display rectangular field.
- Software overlays display MLC shaping.
What are the Pros and cons of using CT for tumour localisation.
-Good for visualising some tumours
E.g. Lung.
-Insufficient soft tissue contrast for other tumours.
How is MRI used in tumour localisation and what are the advantages?
- Image formation: Relaxation times of different tissues.
- Soft tissue definition better than CT.
- Ability to differentiate pathology of tissue.
When can ultrasound be used in tumour localisation?
- 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.
State the advantages and disadvantages of using PET for tumour localisation.
- 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.
What is the purpose of virtually representing the patient and what happens to any errors introduced at this stage?
- 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.
What are common methods of virtual representation?
- Separation.
- Manually acquired contour.
- Single CT slice from simulator CT.
- Full volume CT dataset.
- Full volume MRI – primarily Brachytherapy at present.
Give an example of a basic 1D representation of a patient.
- Assumes the patient is a box of water equivalent material.
- Only appropriate for a parallel-opposed pair.
- Separation measured along Central Axes of beams.
Give two examples of a 2D representation of a patient.
- 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.
Give an example of a 3D representation of a patient.
- 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.
What are the benefits and downsides of using CT volume to virtually represent the patient?
- Benefits:
- Geometrically accurate.
- Hounsfield Units map to electron density/mass density.
- Good resolution.
- Good bony/soft tissue contrast.
- Disadvantages:
- Poor soft tissue contrast.
What are the benefits of using CT in the planning process?
- Data transfer.
- Image manipulation.
- Windowing.
- Multi-planar reconstruction.
- Image fusion.
- Definition of volumes.
- Growing tools.
- Beam geometry.
- Optimisation of dose distribution.
What is the method for data transfer of a CT volume?
- 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.
What main things do we need to localise a tumour for RT treatment?
- 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.
