Medical physics

Complex irradiation of all central nervous system and cerebrospinal fluid regions from behind the eye down to the midsacrum for treatment of medulloblastoma and other cerebrospinal fluid seeding tumors.

The dose equivalent weighted by the proportionate risk for various tissues. For example, the gonads are more sensitive to radiation than the fingers so there weighting of effective dose is higher. Thus for equal equivalent dose to the gonads and the fingers, the gonads would be more adversely affected.

An inverse treatment planning system for creating intensity modulate fields (IMRT). It was the first system used for IMRT in this department and has since been largely replaced by other TPS.

The region between the skin surface and the depth of Dmax. A buildup region is a characteristic of MV irradiation. In this region the dose increases with depth until it reaches a maximum at the depth of Dmax.

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.

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.

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.

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 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.

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 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).

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.

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.

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

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).

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.

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.

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).

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.

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