Physics Flashcards
After performing a 4D CT study, what reconstructed CT image set best identifies the ITV of a hypodense lesion in the liver?
A. Average CT
B. 50% phase CT
C. Maximum Intensity Projection (MIP) CT
D. Minimum Intensity Projection (MinIP) CT
D. Minimum Intensity Projection (MinIP) CT
Because the lesion is hypodense within the liver, the MinIP will best represent the ITV of the lesion. The average CT will blur the edges of the lesion and will make it difficult to clearly delineate. The 50% phase will solely show the lesion in one phase of motion, and the MIP will erroneously show the lesion much smaller than it actually is.
When a therapeutic electron beam interacts with tissue, how do the electrons predominantly lose their energy?
A. Inelastic collisions with atomic electrons
B. Inelastic collisions with nuclei
C. Elastic collisions with atomic electrons
D. Elastic collisions with nuclei
A. Inelastic collisions with atomic electrons
In low atomic number media such as water or tissues, electrons lose energy predominantly through ionizing events (inelastic collisions) with atomic electrons.
Which ionization chamber minimizes cavity perturbation effects in a radiation field?
A. Parallel-plate chamber
B. Farmer chamber
C. Vented thimble chamber
D. Unvented thimble chamber
A. Parallel-plate chamber
The small electrode spacing in a parallel-plate chamber minimizes cavity perturbations in a radiation field. Farmer and thimble shaped chambers typically produce significant perturbations in the radiation field that require corrections.
What depth, in water, is the beam flatness typically specified for a clinical photon beam?
A. dmax
B. 5 cm
C. 10 cm
D. 15 cm
C. 10 cm
For a clinical photon beam, the beam flatness is typically specified at 10 cm depth in water for all photon beam energies. The depth is important as the flattening filter shape is designed to ensure a flat beam profile at 10 cm.
An Ir-192 HDR source is exchanged while a patient is undergoing a three-fraction vaginal cylinder course. If the first fraction was delivered in 375 seconds with a source strength of 5.8Ci, what is the expected treatment time for the second fraction if the new source strength is 9.9Ci?
A. 220 seconds
B. 290 seconds
C. 375 seconds
D. 450 seconds
A. 220 seconds
The treatment time is inversely proportional to the source strength if the dose is kept constant. If the source strength increased, the treatment time will decrease accordingly by the same proportion.
Generally, respiratory motion management techniques are recommended if the target motion, in any given direction, is larger than:
A. 3 mm.
B. 5 mm.
C. 10 mm.
D. 15 mm.
B. 5 mm
To ensure the dose is accurately delivered, AAPM Task Group 76 recommends that a 5mm motion limit criterion should be used as this level of motion can cause significant imaging artifacts and systematic errors during imaging procedures.
Compared to x-ray based IMRT/VMAT treatment plans, why is the high dose conformity of proton therapy to the target volume generally worse?
A. Need to use higher energy (100-200 MeV) protons
B. Enhanced biological effects of higher LET
C. Proton range uncertainty
D. Dosimetric contributions of neutrons
C. Proton range uncertainty
For the same target, the high dose conformity is generally worse for proton therapy delivery primarily due to the proton range uncertainty.
What is the activity on the 100th day for an Ir-192 source (T1/2 = 78.4 days) with an initial activity of 10 Ci?
A. 2.8 Ci
B. 4.1 Ci
C. 5.1 Ci
D. 7.8 Ci
B. 4.1 Ci
The equation to determine activity at a point in time is A(t) = A0*exp(-λt). The decay constant λ is related to the half-life by λ = ln(2)/T1/2.
What is the role of a magnetron or klystron in a linear accelerator during photon beam production?
A. Generates radiofrequency power for acceleration
B. Transfers radiofrequency power to an electron beam
C. Redirects an electron beam toward machine isocenter
D. Creates photons from an accelerated electron beam
A. Generates radiofrequency power for acceleration
The magnetron or klystron (depending on accelerator design) acts as a radiofrequency power source for a medical linear accelerator. Power from the magnetron or klystron is transferred to electrons within an accelerating waveguide. Bending and focusing magnets act on the accelerated electron beam to redirect it toward a tungsten target in line with machine isocenter. Interactions in the target convert the electron beam energy to Bremsstrahlung photons directed toward the patient.
Which electron beam energy is MOST appropriate when treating a target at 4 cm depth that is prescribed to the 80% isodose line?
A. 6 MeV
B. 9 MeV
C. 12 MeV
D. 15 MeV
C. 12 MeV
R80, the depth where dose from an electron beam falls off to 80% of the maximum, is approximated (in centimeters) by the nominal beam energy (in MeV) divided by 3. Therefore, to achieve prescription dose at 4 cm depth prescribed using the 80% isodose level, the appropriate beam energy is 4 cm * 3 = 12 MeV.
What dosimeter should be used when performing an absolute dose output calibration of a linear accelerator?
A. Geiger counter
B. Ionization chamber
C. Radiochromic film
D. Thermoluminescent dosimeter (TLD)
B. Ionization chamber
Ionization chambers are most appropriate as they can be calibrated accurately with methods traceable to primary standards laboratories to provide direct absolute dose measurements. Geiger counters are much less accurate and typically only calibrated for exposure. Film is a relative dosimeter, requiring a separate calibration to be used as an absolute dosimeter. Thermoluminescent dosimeters (TLDs) are capable of absolute dosimetry with careful pre- and post-processing, however this requires special equipment not commonly available in a clinic and still results in measurements with less accuracy and precision than an ionization chamber.
What does “UID” denote in the DICOM standard? A. Underlying dataset B. Uninterruptable data stream C. Unique identifier D. Universal indicator
C. Unique identifier
A DICOM unique identifier (UID) is structured yet unique number that allows an aspect of a DICOM- compliant dataset to be reliably referenced to other DICOM-compliant datasets. Some areas where UIDs are necessary include establishing a common UID among all slices in an image volume, matching a UID to a frame of reference for registered image volumes, and maintaining study/series and subject/object relationships, among others. Enforcing concordance of UIDs keeps relationships structured and predictable in the presence of large amounts of data.
Voxel values in a computed tomography (CT) image set are MOST closely related to which property of the object being imaged?
A. Metabolic activity
B. Nuclear spin
C. Photon attenuation
D. Physical density
C. Photon attenuation
Computed tomography (CT) images are formed by the mathematical reconstruction of the relative attenuation of a photon beam passing through the subject being imaged. It is possible to create a mapping between CT image values and physical density using specialized phantoms, but physical density is not the fundamental material property responsible for creating the image. Positron emission tomography (PET) images are most used to evaluate metabolic activity, and magnetic resonance (MR) images are formed using properties of nuclear spin.
What factor principally contributes to the reduced soft tissue contrast in MV portal images as compared to diagnostic radiographs?
A. Photon beam energy
B. Radiation field size
C. Source-to-detector distance
D. Source-to-subject distance
A. Photon beam energy
The higher beam energy in portal images leads to reduced contrast, as the dominant attenuation mechanism is the Compton interaction (independent of atomic number) for a megavoltage beam as opposed to the photoelectric effect (varies exponentially with atomic number) for a kilovoltage beam. While differences in acquisition geometry may affect the magnification or signal, detector response for a given acquisition is relatively uniform across the panel and therefore image contrast is independent of the geometry.
In general, which set of treatment techniques is correctly ordered from least MU to most MU when treating a given target to the same dose prescription?
A. 3D CRT, fixed-gantry IMRT, VMAT
B. 3D CRT, VMAT, fixed-gantry IMRT
C. VMAT, fixed-gantry IMRT, 3D CRT
D. VMAT, 3D CRT, fixed-gantry IMRT
B. 3D CRT, VMAT, fixed-gantry IMRT
While there are variations among treatment sites and planning approaches, estimates in literature suggest that fixed gantry IMRT treatment plans generally require 2-3 times more MU as compared to a 3D conformal technique to deliver equivalent dose to a given target. Arc-based IMRT is more time- and MU-efficient than fixed gantry IMRT, but as an intensity-modulated technique still requires more MU than for an equivalent 3D conformal plan.
There are two types of accelerating waveguides for medical linear accelerators: standing wave and traveling wave. Standing wave linac waveguides are _____ in length than traveling wave but both employ ______to increase the energy of electrons.
A. Shorter; magnetic fields
B. Shorter; microwaves
C. Longer; magnetic fields
D. Longer; microwaves
B. Shorter; microwaves
Linac standing wave waveguides are approximately half the length of traveling wave waveguides because they employ side cavity coupling. In both types of waveguide, the electrons are
accelerated by the electric field of the microwaves that are fed into the waveguide. A magnetic field cannot do work on charged particles and thus cannot increase their energy.
The AAPM Task Group 51 report for linac beam calibration requires that the absolute dose output calibration be performed at least ______with a dosimeter placed in a _________ phantom.
A. Monthly, water
B. Monthly, tissue equivalent solid
C. Annually, water
D. Annually, tissue equivalent solid
C. Annually, water
The TG51 report requires that calibration be performed at least annually in a water phantom. Solid phantoms are not permitted.
The absorbed dose, D1, in a linac beam is measured using an ion chamber placed at depth, d1, within a large water phantom. Water is added to the phantom without moving the ion chamber, and as a result, the ion chamber is now at a depth, d2. With no change in the linac configuration, what is the dose at d2?
A. D1 x TMR(d2)/TMR(d1)
B. D1 x TMR(d1)/TMR(d2)
C. D1 x PDD(d2)/PDD(d1)
D. D1 x PDD(d1)/PDD(d2)
A. D1 x TMR(d2)/TMR(d1)
This is based on the definition of TMR as the ratio of two doses measured at different depths but at the same distance from the source.
