Treatment Planning Flashcards
What is the GTV?
GTV is the Gross Tumour Volume: This is the gross demonstrable extent and location of the malignant growth (i.e., the visible disease)
What is the CTV?
CTV is the Clinical Target Volume: This contains the GTV + subclinical spread of the disease that must be eliminated
What is the ITV?
ITV is the Internal Target Volume: This contains the CTV + internal margin added for expected physiological movements and variations in size, shape and position of the CTV during therapy
What is the PTV?
The PTV is the Planning Target Volume: This contains the ITV with the set-up margin, which accounts for uncertainties in patient positioning and alignment of the beams during treatment planning and throughout all treatment sessions
What is the OAR - Planning Risk Volume?
The OAR-PRV are additional margins added to critical structures to account for organ position variations and patient positioning uncertainties
How is specifying and reporting the doses in radiation therapy for photon beams done in radiotherapy?
International Commission on Radiation Units and Measurements (ICRU) Report 50: Prescribing, Recording, and Reporting Photon Beam Therapy, specifies reporting the doses in radiation therapy, as well as the volumes in which they are prescribed.
ICRU Report 62, which is a supplement to ICRU 50, providing recommendations on the volumes and absorbed doses that are important in prescribing, recording and reporting photon beam therapy, utilizing the new and improved irradiation techniques
ICRU 83: Prescribing, Recording, and Reporting Intensity-Modulated Photon-Beam Therapy (IMRT).
- Relates previous the prescribing/recording/reporting of IMRT
- Gives recommendations on selection/delineation of the target volumes and OARs
What is the ICRU reference point?
As a general rule for reporting, it is recommended to identify a Reference Point where the dose can be determined accurately and can be considered to be representative of the dose distribution throughout the PTV.
The dose at the point (the ICRU Reference Point) is selected firstly at the centre, or in the central part, of the PTV, and secondly on, or near, the central axes of the beams.
Often known as the prescription point. Dose to the PTV should be uniform and close to prescription.
Choice of reference point is important
- Should represent the PTV
- No boundaries (changes in density) and not in air
Aim for homogeneity across PTV, where variation is between 95-107%
Point-doses: D_{ICRU point}, D_{min} ,D_{max}
ICRU 83 - QC Recommendations
- Machine Testing
Specific for new machinery to ensure IMRT fields can be accurately delivered (procedures) - Patient Specific Tests
One or more of the following:- Measurement of intensity pattern from individual beams for a specific patient
- Measurement of absorbed dose in a phantom of the beam intensity pattern planned for a specific
patient - Independent but equivalent absorbed dose calculations for the patient specific beam intensity pattern
How is IMRT: Patient Specific QA Performed?
Patient specific QA is required to ensure IMRT/VMAT plans are deliverable
- i.e., ensures the plan respects the physical limitations of the system.
Delta-4 phantom used for pre-treatment verification
Gamma analysis is used to compare the: 1. Predicted dose (from the planning system) and 2., the measured dose (from the delta-4 array)
What is IMRT?
IMRT = Intensity Modulated Radiation Therapy
MLCs are moved to achieve INTENSITY MODULATION
- Practical example: Sliding window technique.
Can create MORE conformal dose distributions
MULTIPLE PTVs possible within ONE PLAN
What is VMAT?
VMAT = Volumetric Modulated Arc Radiotherapy
VMAT is another technique of IMRT
- Radiotherapy machine ROTATES around patient DURING IRRADIATION (moving gantry)
- Machine CONTINUOUSLY RESHAPES and changes the intensity of the radiation beam as it MOVES AROUND THE BODY (dynamic MLC)
Gantry moves at maximum speed as much as possible
- Dose rate variation less demanding
SHORTER treatment than static IMRT
Discuss briefly the benefits of PET images for treatment planning
PET provides FUNCTIONAL imaging and provides useful planning information
PET reveals targets that are not well visualised by CT/MRI structural imaging
Can reveal targets which are REMOTE from the primary tumour
Discuss briefly the benefits of CT images for treatment planning
These are taken in the TREATMENT position, where reference markers are used for alignment at treatment
CT provides an ELECTRON DENSITY MAP, which is necessary for DOSE CALCULATION in the planning system
Discuss briefly the benefits of MRI images for treatment planning
MRI has the advantage over other modalities by providing SUPERIOR soft tissue contrast compared to CT
Additional advantage is that NO ADDITIONAL DOSE is given to the patient in the process
Disadvantages include specific distortions in the MRI images caused by both the system and the patient
A CT planning scan may still be needed to allow electron density calculation for dose calculations
What is the reasoning behind the use of positioning aids and patient preparation
Reproducibility is KEY
- Maintain ONE POSITION throughout treatment
- Image as patient will be treated
There are a variety of immobilisation options
Patient preparation can include:
1. Specific procedures
Anaesthesia
2. Clinical restrictions
Colostomy
3. Bladder/rectal filling protocols
Describe the basic elements of CONFORMAL therapy planning, with advantages/disadvantages of the technique
Conformal planning requires:
1. Static fields
2. Jaws and MLCs are fitted to the PTV
3. OARs are blocked out
4. Custom blocks can also be used
Advantages: It is a proven technique and is comparatively simple
Disadvantages: It is limited in terms of the dose distribution that can be generated
Summarise the process of FORWARD Planning
- START: Set field angles
- Fit fields to structures
- Adjust field weights
- Apply field modifiers
- FINNISH: Evaluate Dose to target and OAR
Summarise the process of INVERSE Planning
Makes use of COST FUNCTIONS
- START: Define required dose to target and AOR
- Set constraints: Technique/ no. of beams/ dose rate etc
- FINISH: Computer calculates solution to problem
- Field weight/MLC motion/ dose rate
Why is inverse planning required for VMAT?
Too many solutions for a human to explore, too little time
Little chance of arriving at optimal solution by trial and error
If an acceptable solution is found, there is no guarantee it is the best
Define cost functions and dose objectives with reference to inverse treatment planning
The cost function is a measure of the distance between the prescribed dose and the obtained one. Cost function includes the clinical objectives of the treatment plan, and represents a critical point of the optimization process
Dose objectives:
- Tend to use TIGHTER OBJECTIVES in optimiser than we want to achieve; as more likely to meet clinical objectives and generate optimal plan
- Cost -> 0 as you get closer to achieving objective, but may be possible to do better than cost = 0.
Priorities and compromises:
- Sometimes clinical objectives are clinically exclusive
- Need to tell the planning system our priorities based on the clinical situation and experience
- Use of optimization structures is common to create NON-CONTRADICTING objectives
Described the Dose Volume Histogram (DVH)
In its simplest form, the dose volume histogram is a FREQUENCY DISTRIBUTION of dose values within a defined volume such as the PTV or OAR
It is displayed as PERCENT OF TOTAL VOLUME
What are the main options for delivering IMRT Fields?
- Static Gantry
a. Sequence of static fields (SMLC) - Step and Shoot (3-9 beams of fixed angle), MLCs form STATIC shapes
during delivery
b. 3-9 beams of fixed angle, static gantry but MLCs form DYNAMIC shapes with CONTINUOUS delivery at
each angle - ARC Techniques (moving gantry, VMAT)
What QA specific tests are required for dynamic MLC treatments?
Must Check:
1. Leaf positions
2. Accuracy and reproducibility of leaf gaps
3. Stability of leaf spread
4. Acceleration/deceleration of CARDINAL ANGLES
Tests:
1. Picket fence pattern (quick visual check)
2. Leaf gap test
3. Leaf speed test
What are the major differences in dose distribution when comparing VMAT to conformal?
INCREASED VOLUME of normal tissue receiving LOW DOSE
- Relatively new technique, so the long term effects are not well studied
DECREASED VOLUME of normal tissue receiving HIGH DOSE
- Lower absorbed toxicity
- Allows higher doses to be delivered to targets
How are clinical goals set for class solutions in Radiotherapy?
The clinical goals will have come from the site-specific planning and treatment protocol in QPulse, which are developed based on trial data and literature
e.g., for protsate it’s the CHHiP trial (Conventional or hypo-fractionated high dose intensity modulated radiotherapy for prostate cancer)
Define ‘Class-solution’
A class solution for radiotherapy planning can be defined as a set of dosimetric objectives and geometric beam arrangements that are sufficiently robust to produce a clinically acceptable dose distribution regardless of the patient anatomy, target volume or organs at risk (OAR).
Under what situations would you want to use an extended SSD?
In certain situations, anatomic constraints may dictate the use of an extended SSD (for example, 110 cm SSD). In the treatment of the lateral neck region, for instance, a patient’s shoulder may obstruct the positioning of the electron applicator
Why might electrons be chosen over photons in treatment planning?
Unlike that of photons, the electron skin dose is high, about 80%-95% of the maximum dose. A rule of thumb regarding the depth of penetration of electrons is that 80% of the dose is delivered at a depth (in cm) corresponding to one-third of the electron energy (in MeV).
Why are Sr-90 Sources used for constancy checks?
Beta-emitting 90Sr check sources have historically been used to monitor the stability of ion chambers as originally recommended by IEC 60
They are long lived (28.1 year half life)
546 KeV energy of beta
