What are photon beams?
A type of radiation therapy that uses x-rays or gamma rays that come from a special machine called a linear accelerator (linac). The radiation dose is delivered at the surface of the body and goes into the tumor and through the body. Photon beam radiation therapy is different from proton beam therapy.
What is DMAX in radiation therapy?
When a radiation beam interacts with tissue, a certain depth of tissue is required before we see the maximum amount of radiation (maximum dose or dmax). This region is called the build-up region and is responsible for the skin sparing effect.
Are photons radiation?
A photon is a tiny particle that comprises waves of electromagnetic radiation. Photons have no charge, no resting mass, and travel at the speed of light. Photons are emitted by the action of charged particles, although they can be emitted by other methods including radioactive decay.
What is photon attenuation?
Principles of photon attenuation Attenuation is the progressive loss of energy by a beam as it traverses matter. A photon beam may be attenuated by any of the processes described in the previous section. There are some more useful concepts when considering the attenuation of photon beams.
Are photons beams?
There are two origins of photon beams: g rays, which originate from radioactive nuclei, and X rays, which originate in a target bombarded with energetic electrons. The X rays from a target consist of bremsstrahlung photons and characteristic photons.
What is the difference between MeV and MV?
The energy of diagnostic and therapeutic X-rays is expressed in kV or MV. Whereas, the energy of therapeutic electrons is expressed in terms of MeV. In first case, this voltage is the maximum electric potential used by a linac to produce the photon beam. Thus, 1 MV beam will produce photons of no more than about 1 MV.
How is PDD measured?
PDDs were measured at 100 cm source-to-surface distance (SSD), for fields of size: 5 cm × 5 cm, 10 cm × 10 cm, 20 cm × 20 cm, 30 cm × 30 cm, and 40 cm × 40 cm. PDDs were measured at all available dose rates for 6 FFF and 10 FFF as PDD curves smoothing vary with dose rate.
What does DMAX stand for?
maximum of optical density
The D max (or maximum of optical density) refers to the deepest black which can be measured after printing. The D max on a given product can vary from many parameters : impression mode, type of printer, ink technology, etc. Back to FAQ.
Why does PDD increase with SSD?
The percentage depth dose (PDD) increases with SSD due to the effects of inverse square law. NO dependance on the SAD or SSD. and hν constant TMR decreases with increasing z.
Is there a maximum dose for MV photons?
MV photons do not reach maximum dose at the surface/skin but build-up to a maximum dose. This effect is referred to as skin sparing and is desirable for most photon treatments. The depth of maximum dose is closely connected to photon energy but also source-to-surface distance, field size and beam profile.
How are the dose distributions of megavoltage photons measured?
HVL is not a practical description of megavoltage photon beams because of the mass of material required. Megavoltage X-ray beam quality is usually measured by attenuation of water or synthetic water. One common approach uses the ratio of maximum dose to dose at a depth of 10cm.
How many photons are in a 10 cm field?
25 Co 4 6 10 18 Depth in water (cm) FIG. 6.10. 2PDD curves in water for a 10 × 10 cm field at an SSD of 100 cm for various megavoltage photon beams ranging from 60Co grays to 25 MV X rays. PDD Build-up to dmax
What is the PDD of a photon beam?
Point Q is an arbitrary point at depth zon the beam central axis; point P represents the specific dose reference point at z= z maxon the beam central axis. The PDD depends on four parameters: depth in a phantom z, field size A, SSD (often designated with f) and photon beam energy hn. The PDD ranges in value from 0 at zÆ • to 100 at z= z max.
