Difficult Medical physics Interview Preparation Guide

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Based on acquired CT information, these are images that render a beam's eye view display of the treatment field anatomy and areas of treatment interest. These images resemble conventional radiographs and can be used in conjunction with patient Port Films to check patient treatment setup prior to irradiation.

A cylindrical phantom, 30 cm in diameter and 18 cm long, made of Solid Water. For patient QA, it separates into two hemispheres to place film and can have an ion chamber within 5 mm or film.

A boost technique in which a single dose of 10-20 Gy is delivered directly to the tumor bed with electrons or photons. The tumor bed has been surgically exposed, allowing critical normal structures to be shielded or displaced out of the radiation beam.

Therapy that, with the use of 3D treatment planning, allows the delivery of higher tumor doses to selected target volume without increasing treatment morbidity. Requires a CT Simulator, 3D Treatment Planning System (TPS) and usually Multi-Leaf Collimators (MLCs) on the linear accelerator.

A system producing near real-time portal images on a computer screen for evaluation. This is attached to the linear accelerator and is composed of amorphous Silicon (aSi) crystals. They receive MV x-rays that images the patients setup and is used to compare to the DRR's from the Treatment Planning System (TPS). These are done almost universally on all patients now.

Photon radiation that is emitted in discrete energies when an outer shell electron fills an inner shell position after the original inner shell electron is ionized. The binding energy difference between residing in the outer shell compared to the inner shell is the energy of the photon emitted.

A standard for handling, storing, printing, and transmitting information in medical imaging. This protocol was formed to provide communication standards for sharing image information regardless of manufacturer and has included radiation therapy treatment information. This facilitates the use of picture archival and communications systems (PACS) and allows diagnostic images to be widely distributed.

The body radiates energy through thermal conduction through the skin to the air, clothes, etc, around the body. A small amount of energy would also be radiated as electromagnetic radiation with a peak wavelength directly related to the Kelvin temperature of the skin.

Also very small amounts of other radiation are emitted due to naturally occurring radioactive isotopes in the body (i.e. Carbon-14).

Therapy that delivers non-uniform exposure across the beam's eye view (BEV) rather than an open field. This type of treatment can be delivered with a machined compensator (rare outside of community clinics), linear accelerator Multi-Leaf Collimator (MLC) leaves or Tomotherapy binary leaves. The IMRT plan is usually (but not alwaysi.e. breast tangents with control points) developed with an inverse treatment planning system. While it can deliver a more conformal plan to the patient, extra care and quality checks must be done to ensure proper delivery due to its increased complexity..

The visible (imaged) or palpable tumor plus any margin of subclinical disease that needs to be eliminated through the treatment planning and delivery process.

Used in the gantry head of high-energy linear accelerators to bend the electrons that will either be used directly for electron radiation treatments or will hit a target and produce photons for photon radiation treatment. Bending magnets are used along with an energy slit that physically limits the energy of the electron beam to within a desired range. Two models are in primary use 270degree (Varian), slolom method (Seimens) 3 magnets used to do slights bends.

A genetic disorder caused by low levels of alpha-1 antitrypsin, a protein that protects the lungs. Alpha-1 antitrypsin deficiency (A1AD) puts a person at risk of developing emphysema or chronic obstructive pulmonary disease (COPD).

The amount of energy required to remove an electron from its orbit in an atom. This amount is subtracted from the initial energy acquired by the photon or electron once it leaves the proximity of the atom.

The equivalent rectangular field dimensions of the open treated area. The blocked field size is the actual area treated. Therefore the blocked field size is normally smaller than the collimator field size. Blocked field size dimensions are used when determining TMRs and PDDs. Collimator field size dimensions are used to determine Output Factors.

Particulate radiation, positively charged, which consists of two protons and two neutrons (effectively a He nucleus). It has a high Radiobiological Effectiveness (RBE) compared to photons, and thus will cause significant damage to tissue, but it has a short track length (a piece of paper will shield most of it).

An early version of IMRT that is still used today in centers without Multi-Leaf Collimators (MLCs). It is a beam modifier that allows known non-uniformities within the irradiated field to better conform to the target volume. It is made of tungsten and is machined

Product of the absorbed dose and a quality factor (QF), which takes into account the biologic effects of different types of radiation on humans; units are the rem (1 rem = 1 rad x QF = 1 cGy x QF = 1 cSv). Photons and electrons have a QF of 1 for instance where as protons have a QF of 3 and neutron 5-20.

Electromagnetic radiation emitted from decaying isotopes and used for external beam and brachytherapy treatments. Since it is electromagnetic radiation it is a photon and has no mass and no charge. Gamma Rays typically have similar energies to X-rays but are distinguished from these radiations by the origin of emanation. Gamma Rays are from the nucleus of the atom whereas X-Rays come from the electron shells outside of the nucleus.

Buoyancy is the net upward force experienced by an object submersed in a fluid. Pascal's principle dictates that fluid pressure on an object increases with depth, so there is greater pressure on the bottom of the object than the top, resulting in a net upward force. When an object's buoyancy is greater than its weight, the object will float.

An object with neutral buoyancy has a density such that when it is fully submerged the upward force due to buoyancy is exactly equal to the downward force due to the weight of the object.

In pure water this is a density of 1000 kg/m3 (1kg/litre)
In seawater, this is a density of about 1025 kg/m3 (1.025kg/litre)

A radioactive isotope with a half life of 30.2 years and an average energy of 662 keV. It is a pure beta emitter and is used for Low Dose Rate (LDR) Brachytherapy especially for use in tandem and ovoid treatments for gynecological cancers.

A type of simulation that operates along with a 3D geometric planning computer. The extension of a CT system allows the single acquisition of many thin slices over a required treatment area. After scanning, radiation beams, isocenter, and blocks can be visualized and placed on the three dimension CT images set. CT simulation also allows for the construction of a Digitally Reconstructed Radiograph (DRR) that is helpful for verifying patient setup when compared to port films with a film cassette or Electronic Portal Image Device (EPID).

The section of the target at which radiation is produced. The smaller the focal spot, the sharper the field produced (very defined penumbra), however the smaller the focal spot also means the hotter the target material gets and can cause damage. Therefore, a compromise must be struck between sharper fields and adequate and reasonable heat loading of the target.

A mathematical relationship that describes the change in beam intensity as the distance from the source changes. The change in intensity is primarily caused by the divergence of the beam. The mathematical formula states that the intensity decreases inversely proportional to the square of the distance. (Intensity = 1/r2). Example: Triple the Distance (r), the Intensity drops by a factor of 9.

Approaches that guide a patient to a state of relaxation by focusing attention on pleasant thoughts. Guidance is provided by specially trained radiology or other medical personnel. This condition may be achieved via distraction techniques or self-hypnotic relaxation.

The technique in which there are more treatment days than accelerated fractionation. The total dose (cGy) of primary radiation is more than conventional fractionation, hyperfractionation, or accelerated fractionation.