Oral Questions Flashcards
What is the energy threshold of photoneutron production and how does it vary for a 15 MV versus 18 MV photon beams?
a) Threshold is generally considered to be ~10 MV for shielding purposes, even though the cut off is 8 MV.
b) Photoneutron production as a function of energy varies by manufacturer (linac specific)
c) photoneutron production in varian linacs increases by ~60% when shifting from 15 MV to 18 MV beams.
What is the equation for P_gr and what values does it have?
P_gr=(M_raw (d_ref+0.5r_cav ))/(M_raw (d_ref))
- P_gr <1 for high energy but for low energy it could be over 1
- It depends on where d_ref is relative to d_max
https://www.aapm.org/meetings/02AM/pdf/8315-36141.pdf
Composition of radiographic film and function
- Consists of a silver halide (AgBr) emulsion coated on a transparent polyester base
- Within each crystal, there are impurities (e.g Iodine or Chlorine) and wen radiation is incident, freed electrons migrate to impurities.
- Electron induce a negative charge on the impurity which attracts Ag+ towards impurity.
- The Ag+ reacts with the electrons to form a very small amount of metallic silver. The grains that have reacted in this way form the latent image.
https://oncologymedicalphysics.com/?s=radiochromic+film
How does the electron gun in a linac vary the dose rate?
By controlling the frequency of electron bunches being injected into the waveguide (grid of a triode electron gun)
- The grid controls the electrons being injected, the grid can also perform a beam hold where the linac produces no beam while the components such as MLCs move into position
- It does this by moving the electron gun pulses out of phase with the microwave power in the accelerators so that no radiation is produced.
Why was the addendum to TG 51 created?
. Many new chamber designs became available and updated kQ tables were needed as a result.
- Provide guidance on what makes a suitable chamber for relative dose
- Provide guidance for implementation of TG due to new technologies (FFF beams).
- Discuss some of the details of the determination of N_D,w
- Discuss uncertainties present in TG 51 and how each part contributes to the final combined uncertainty (uncertainty budget)
Why is %DI equal to the PDD for photons but not for electrons?
In photon beams, due to the relatively constant collisional stopping power ratios, the %Di curve is approximately equal to PDD (within 0.1% beyond d_max).
In electron beams, the collisional stopping powers vary with depth, and therefore the PDD curve is equal to the %Di curves times the collisional stopping power ratio of water and air.
How is CT imaging used in the pencil beam algorithm?
- Used to obtain linear stopping power (range correction) and scattering power (scatter correction information).
- Used to define the external surface only to correct for air gap, non-conformal incidence, and irregular surface contour).
What is supralinear behavior as observed by a radiation detector? For what detector, and under what conditions, would this occur?
It occurs with OSLDs. At high dose rates, the detector loses its linear response between readout and delivered dose and goes supralinear. That is, the response increases faster than the ose (its no longer one-to-one). It also occurs with radiographic film.
What is the equation for kQ in TG-51 for electrions and how are the factors defined?
kQ= P_gr k’R50 kecal
- Pgr: used to correct the effective point of measurement being upstream from the center of the chamber. It, in effect, shifts your reading to the center of the chamber during output measurements (electrons stop quicker).
- k’R50: used to account for variations in readings due to your electron beam quality
- k_ecal: functions to take you chambers calibration factor which is photons and apply it to electrons.
Describe convolution and superposition and how they relate to dose calculations
- Convolution: a mathematical operation that shows the degree of overlap between two functions. It is represented by the symbol (arrow in). For dose calculations, we can use a convolution when we use a constant kernel that does not vary with density (no heterogeneity correction)
- Superposition: very similar to convolution except it uses density to scale the kernel prior to the convolution and therefore can account for heterogeneity inside of the model. Note that this causes the kernel to be spatially variant and patient dependent. Using this we can get to within a few percent of Monte Carlo.
What is IGRT and what is it used for?
Image guided radiation therapy.
Medical imaging for tumor localization during preparation phase, as well as in the treatment room for localization of the tumor and directing the beam.
- Used for precise adjustment of radiation beams targeting tumors that are located in the patient’s body
Extra: name some and how they work, advantage and disadvantages.
What are some limitations of MR linacs?
- Cost (10 M) rathen that 2-3M for conventional linac
- Allows for couch motion in 3D, but limited to bore size.
- Some patients are not eligible due to size or implanted devices.
- Increased staffing requirements
- Relatively new system with limited experience across institutions and physicists
- May require surface measurements to assess electrons return effect and changes in entrance/exit dose.
Does TG-51 address the use of sealed ion chambers and if so, what does it say?
- There is no a statement in TG-51 that precludes the use of a sealed ion chamber, however, the inclusion of the temperature and pressure correction factor in the protocol indicates that the use of unsealed chambers is expected.
- Sealed chambers for absolute calibration purposes are troublesome because a leaky chamber would produce incorrect results and may be difficult to detect.
Discuss motion artifacts associated with CT imaging, and what, if any, techniques can be used to minimize them.
- Motion artifacts are a result of patient motion during the CT acquisition.
- These artifacts are displayed through blurred or streaked images and as slice to aslice discontinuities such as organ elongation or shortening.
- Immobilization can minimize some patient motion.
- Minimization of scan times (reduce scan volume, increase slice thickness) where appropriate.
- Breath hold techniques for thoracic/abdominal mass
- 4DCT scan acquisition, either active or passive.
Describe CT imaging artifacts and partial volume effect.
- Partial volume effect (PVE) occur when a single voxel includes a tissue interface.
- With PVE you end with a single voxel representing a boundary with an average CT # for both tissues.
- The effects of PVE can be reduced by scanning with thinner slices, however, this results in larger scan times and increases motion artifacts.
