Pre-imaging for radiotherapy Flashcards
Name the steps incorporated in the radiotherapy chain.
Consult → pre-treatment imaging → treatment planning → position verification → treatment delivery
What are:
- inter-fraction variations
- intra-fraction variations
- inter-fraction variations → variations in e.g. position of certain organs between fractions of radiotherapy.
- intra-fraction variations → variations in e.g. position of certain organs within/during a fraction of radiotherapy.
What is the difference between a random error and systematic error?
- Random error → cause one measurement to differ slightly from the next (due to unpredictable changes during an experiment)
- Systematic error → always affects measurements in the same amount or by the same proportion, provided that a measurement is taken the same way each time (i.e. predictable).
Note: random erros can’t be eliminated, systematic errors can be reduced
In regard to pre-treatment imaging, does the definition of the target volume take into account a systematic or random error?
It depends a bit how this question is perceived. During pre-treatment imaging, three main volumes are defined: the gross tumour volume (GTV), the clinical target volume (CTV) and the planning target volume (PTV).
- If this question refers to the total target volume, than yes pre-treatment imaging does take systematic and random errors into account.
- If this question refers to only the CTV, then no pre-treatment imaging does not take systematic and random erros into account. Since only definition of the PTV allows for uncertainties in planning or treatment delivery.
How does a CT scan work?
A CT scanner uses a motorized X-ray source that rotates around the patient. Narrow beams of X-rays are projected onto the patient, where digital X-ray detectors direcly opposite of the X-ray source collect the X-rays. The detector measures the amount of radiation (photons) that is absorbed throughout the body and generates a picture based on this.
With the use of a CT scanner, different tissues can be visualized. Describe and explain how different tissues can be visualized.
The density of the tissue is in proportion to the attenuation (/absorption) of the X-rays that pass through.
- Tissues with a lot of air and water have little attenuation and are displayed as low densitities with a dark(er) color on the CT scan.
- Tissues with a lot of density like bones have higher attenuation and are displayed as high densities with a lighter/brighter color on the CT scan.
Name advantages and disadvantages of the use of a CT scanner.
- Advantage → 3D model of anatomy and high density contrast
- Disadvantage → poor soft tissue contrast
What do dose calculations rely on?
On the electron density of the tissue
- What kind of radiation is used for CT imaging?
- What kind of radiation is used for radiotherapy?
- Does a CT account for a dose burden?
- Röntgen radiation → high energy protons
- Even higher energy protons
- Yes, it does (mSv)
- Why is target delineation so important in radiotherapy?
- How do we hadle delineation uncertainties?
- What kind of error is a delineation error?
- It needs to encompass the tumor (and spread)
- By applying margins
- Systematic and thus a large component in the margin.
Target delineation is crucial for radiotherapy. However, while CT has proper high-density contrast, it has poor soft-tissue contrast. What can we do to optimize delineation?
- Combine CT with other imaging modalities (e.g. PET or MRI)
- Enhance CT contrast
- Incorporate motion
- Personalize treatment
How can CT and PET be combined?
With CT, the different tissues can be depicted (to a certain extend). If PET is used in combination with CT, this means that e.g. the tumour can be visualized with the use of a radioactive substance for PET imaging.
If CT is not optimal for delineation strategies, why would we still use CT for pre-treatment imaging?
CT is still needed for accurate dose calculations for radiotherapy (since electron density correlates with the dose). For more accurate delineations, CT can then be combined with PET or MRI.
A contrast agent can also be used for CT imaging. Name three ways contrast agents (CA) can be used.
- Intravenous CA
- Gastro intestinal CA
- Vaginal CA
Lipiodol is a contrast agent that can be administered for CT imaging. For what is it mostly used?
For tumor demarcation
How can the filling of the bladder be used during a CT-scan?
Although the filling of the bladder cannot really be controlled, it is possible to tell the patient to drink or not drink before coming for a scan.
E.g. a patient can be asked to come to the first appointment with an empty bladder, where a CT scan will be made of the positions of organs. The second time a patient comes in, the patient can be asked to come with a full bladder, where again a CT scan is made for the organ positions. In this way, multiple possible treatment plans can be generated and the best fitting plan of the day can be used.
