Ch2 Clinical Dosimetry in Radiation Therapy Flashcards
What is the fundamental quantity to consider when measuring dose in clinical dosimetry?
The measurement of dose in water
What tasks are involved in radiation therapy related to dosimetry?
Assessment of beam quality, measurement of dose distributions in a phantom, measurement of kerma strength, determination of the relationship between monitor units and dose applied to a phantom/patient
What is the purpose of a treatment planning system (TPS) in radiation therapy?
To calculate dose distributions within a patient using a beam model based on basic dosimetric beam data
Why is it important to ensure that dose calculation algorithms of a treatment planning system are accurate?
To ensure correct dose delivery to the patient, requiring validation and verification through measurements
What accuracy requirement did Brahme propose for the dose in radiation therapy?
5% accuracy in dose
What is the total uncertainty in radiation therapy according to Hassenstein and Nüsslin?
Uncertaintiy in treatm. planning 5.3%
Uncertainty in dose delivery 4%
Patient positioning 2%
Calibration of detectos 2.1%
Dose at reference point 3%
Total 7.8%
What device is commonly used for dose measurement in clinical routine?
Ionization chamber
What is the principle behind the operation of an ionization chamber?
Charges are collected due to an applied electric voltage, and the resulting electric current is measured by an electrometer and related to a dose rate
What is the typical accuracy of doses measured in radiation therapy?
About 1 mGy
What is the relationship between the calibration factor and the dose in water for a clinical ionization chamber?
Dose in water (DW,Q) is equal to the measurement signal (M) multiplied by the calibration factor (NW,Q)
What is the purpose of the correction factor kQ in dosimetry?
To convert the calibration from the 60Co beam quality to the user’s beam quality
What factors contribute to the correction factor kQ for ionization chambers?
Energy dependence of stopping-power ratios, beam quality, and other correction factors
What is the significance of the correction factor Pion in ionization chamber measurements?
It accounts for the recombination of charges and ensures accurate dose measurements
Why is the correction factor Pwall needed in dosimetry?
To account for differences in wall materials of the ionization chamber compared to the medium where the dose is measured
How is the effective point of measurement adjusted in photon beam dosimetry for cylindrical ionization chambers?
By displacing the detector by 0.6 times the radius of the cavity (rcav) towards the beam
