Medical physics Interview Questions And Answers

A system in Brachytherapy that allows the applicators to be placed at the treatment site, then loaded remotely without personnel in the room. This reduces dose to the staff and since done via computers, should ensure a more accurate placement provided the applicators are correctly placed. Usually associated with High Dose Rate Brachytherapy (HDR)

A sharp increase in the dose distribution curve of a charged particle at a particular depth. It is this physical phenomenon that is exploited in proton radiation treatments.

An arrangement of shielding material in the linear accelerator designed to define the dimension of the beam of radiation. The collimators are located in the treatment head and are usually made of tungsten alloy.

A geometrical principle describing two radiation fields configured in such a way that the beam edges lie in the same plane. An example of a non coplanar field would be delivering one beam at one couch angle, and then kicking (rotating) the couch and delivering a second beam.

The area in the 3D treatment planning system that dose is calculated. It is split into units called voxels (indicating a volume, hence 3D). The higher the resolution (smaller the voxel size) the more accurate the dose calculation, however, the trade off is a longer period of time to make the calculation as there are now more voxels to calculate.

A device for measuring dose. It makes use of the following phenomenon: when film is exposed to radiation and subsequently developed, the amount of blackening (AKA Optical Density) is proportional to the dose delivered to the film. By measuring this darkening, it is possible to determine the amount of dose the badge received. These are worn by all radiation workers to document the dose received during their time at work.

Brachytherapy The delivery of brachytherapy on an outpatient basis using HDR brachytherapy equipment. The actual treatment delivery last approximately 5-10 minutes in contrast to a hospital stay that might take several days for low-dose rate (LDR) brachytherapy. HDR is almost always done with remote afterloader devices due to the high exposures hospital personnel would receive if they stayed in the room with the patient during administration.

Dose is recorded by a material that is read by a LASER. The LASER stimulates the emission of photons relating to the total dose recorded. The benifit is that only part of the chip is read at a time, one can re-read the same chip many times.

A wave front is an imaginary surface joining all points in space that are reached at the same instant by a wave propagating through a medium.

Let's try some examples. When a rock is tossed into a calm lake, a surface disturbance radiates from the point where the rock broke the water. The leading edge of that entire wave forms a circle, and that circle is the wave front for that event. It is moving outward at a constant speed in all directions. Note that it's two-dimensional (2D). Want 3D? You got it.

In a burst of chemical energy, a star shell explodes at a fireworks display. The light moves away from the origin in all directions at the same speed - the speed of light. And the 3D surface of this wave front is a sphere, and it is expands around the origin at the speed of light. Pick an arbitrary distance, say, 1 kilometer. Anyone at a distance of 1 km from the event in any direction will find that the wave front reaches him at the same instant of time as anyone else in any direction who is that 1 km from the event. Even someone in an airplane that is 1 km away will be on the wave front for an instant - that same instant as any other observers 1 km away. Note that the sound will arrive later - but it, too, radiates forming a spherical wave front. Our observers at 1 km distance from the event all experience the arrival of the sound wave at the same time.

Doctors who oversee the care of each cancer patient undergoing radiation treatment. They develop and prescribe each cancer patient's treatment plan, they make sure that every treatment is accurately given, and they monitor the patient's progress and adjust treatment to make sure patients get quality care throughout treatment. Radiation oncologists also help identify and treat any side effects of radiation therapy and work closely with all members of the radiation oncology team. Radiation oncologists have completed four years of college, four years of medical school, one year of general medical internship, then four years of residency (specialty training in radiation oncology). They have extensive training in the safe use of radiation to treat disease. If they pass a special examination, they are certified by the American Board of Radiology. Patients should ask if their doctor is board certified.

A protein that protects the lung. A1AT deficiency puts a person at risk of developing emphysema or chronic obstructive pulmonary disease (COPD).

A cyclic particle accelerator in which the magnetic field (to turn the particles so they circulate) and the electric field (to accelerate the particles) are synchronized with the traveling particle beam. While the cyclotron uses a constant magnetic field and a constant frequency electric field, both are varied in the synchrotron. This allows for construction of large rings that can accelerate particles to much higher energies than a cyclotron which has a limited magnet size. The synchrotron uses multiple separate bending magnets and narrow bore tubes to connect them. It can be used to produce high energy protons and other particles such as carbon ions that are used to treat cancer. In addition the energy of the particles can be varied as needed which is very difficult in a cyclotron.

A measure of the energy deposited in a medium by ionizing radiation. It is equal to the energy deposited per unit mass of medium, and so has the unit J/kg, which is given the special name gray (Gy). You will also hear the term rad used. 1 Gy = 100 cGy = 100 rad

A beam spoiler is a piece of material, such as a 1-cm-thick lucite or polystyrene plate, placed into the path of the photon beam. As the primary photon beam passes through the plate, secondary electrons are generated. The beam which exits the spoiler is a combination of the spoiler-attenuated photons and the spoiler-generated electrons. The electron component of a spoiled photon beam alters the depth dose in the buildup region in a way that depends on the photon beam energy, the field size, and the distance of the spoiler from the treatment surface.

(BrainLAB) A treatment planning system for all patients on the Novalis Linear Accelerator. It can perform conventional, dynamic conformal arc, stereotactic cones, and IMRT plans.

A plot of target or normal structure volume as a function of dose. It is, in essence, a frequency distribution of the number of target or normal-structure voxels receiving a certain dose. In its most common form (the "cumulative" or "integral" DVH), it is a plot of volume versus the minimum dose absorbed within that volume.

An atom or atom group in a highly reactive transient state that is carrying an unpaired electron with no charge. These are often created by radiation and are one of the main mechanisms in which radiation damages chromosomal DNA, thus giving radiation its therapeutic value, making it carcinogenic and teratogenic.

A series of 0.3-1.0 cm wide metal leaves each robotically controlled to shape the radiation beam precisely. <2% leakage. The leaves are either singly focused, doubble focused, or rounded. Doubly focused MLC's are curved to match beam divergence.

An imaging test that uses a gamma camera and a computer to create three-dimensional (3-D) images of the distribution of a radiotracer in the body. SPECT is used to study blood flow through the heart muscle, and to study the brain, bones and to detect infection and certain types of tumors.

As a rough guide engine oil usually weighs about 0.87 kg/liter or about 7.2 pounds per US gallon.

Unfortunately a rough answer is the best that can be done as there are hundreds of different types of engine oil with slightly different densities. To get an exact density we would have to know the details of the oil you're using. For a really accurate answer we would also have to state the temperature as the density of oil as with most fluids varies with temperature.

Most oil manufacturers' provide data sheets for their products that should include density. If you search the manufacturer's web site you should be able to get an accurate figure for a particular grade.

Monitor chambers are located following the flattening filter. Used in several of the LINACs feedback systems to monitor and correct for dose rate, symmetry, and total dose delivered. The physicist is responsible for setting the dose calibration of the monitor chamber (ex. 1 monitor unit = 1cGy/MU at dmax on CAX 100SSD setup.

Equipment that converts kinetic energy to microwave energy in the linear accelerator. Klystrons are high-vacuum devices that use a well-focused pencil electron beam that directs the stream through a number of microwave cavities, which are tuned at or near operating frequency of the tube. Conversion takes place as a result of the amplified RF input signal, causing the electron beam to form "bunches". These "bunches" give up their energy to the high-level induced RF fields at the output cavity. The simplified signal is extracted from the output cavity through a vacuum window.

Administration of a local anesthetic into the subarachnoid space surrounding the spinal cord. Generally used to prevent pain and movement in areas below the chest and extending to the feet.

The technique in which the overall treatment time is shortened through the use of doses per fraction less than conventional doses two to three times a day

A special treatment procedure that utilizes the irradiation of a target with sealed radioactive sources placed at short distances from the target and inside the patient's body. Typically, the sources are implanted in the target tissue directly (interstitial brachytherapy) or are placed at distances of the order of a few mm from the target tissue, in body cavities such as the uterus, mouth, etc. (intracavitary brachytherapy)