IGRT Flashcards
What is the main purpose of Electronic Portal Imaging Devices (EPIDs) in clinical routine?
EPIDs are primarily used to verify patient positioning and field shape, and can also be used for daily image-guidance and dosimetric verification.
What technology are modern EPIDs based on?
Modern EPIDs are based on amorphous silicon technology.
Why is it challenging to detect MV photons in EPIDs?
Photon detection of MV photons is challenging because the cross sections are low.
How is inter-fraction motion of the prostate typically managed in radiation therapy?
Inter-fraction motion is managed by implanting gold markers in the prostate and adjusting patient position daily by comparing the current marker positions with a reference.
What is cone beam computer tomography (CBCT) and how is it used in IGRT?
CBCT is a technique that acquires 3D data sets directly in the treatment room to improve patient setup accuracy in IGRT.
What is the purpose of gating in radiotherapy?
Gating in radiotherapy is used to deliver MV photons only when the tumor is in the correct position, based on a patient’s breathing curve.
What is the duty cycle in the context of gating in radiotherapy?
The duty cycle is the ratio of the gating interval to the respiratory period, typically ranging from 20-50%, and it characterizes the prolongation of treatment time.
What is tracking radiotherapy and how does it differ from gating?
Tracking radiotherapy involves following the tumor with the beam continuously, using dynamic MLCs or couch motion, unlike gating which only delivers the beam during specific intervals.
What are the main uncertainties considered when determining margins around target volumes in radiation therapy?
Systematic and random uncertainties are the main considerations, where systematic uncertainties cause shifts in the cumulative dose distribution and random uncertainties cause blurring.
How can systematic and random uncertainties affect dose distribution differently?
Systematic uncertainties cause a shift in the cumulative dose distribution, while random uncertainties result in a blurring of the dose distribution.
What formula is commonly used to calculate the margin (M) in radiation therapy, according to van Herk’s model?
The margin (M) is calculated using the formula: M = 2.5S + 0.7s, where S is the systematic uncertainty and s is the random uncertainty.
