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.

How does PDD change with field size?

Field size – for larger fields there is little change in central axis PDD with field size, but as field sizes get small enough to be affected by reduced lateral scatter (roughly when the dimensions are less than the practical range Rp), the PDD will start to pull towards the surface showing increased surface dose and a …

Why does PDD increase with field size?

As the field size increases the contribution of scattered radiation to the absorbed dose increases. The field size dependence of PDD is less pronounced for the higher energy beams than for the lower energy beams.

What is the build up region in radiotherapy?

The dose of build-up region consists of a primary component such as electrons, photons, and electron contamination from the treatment head and the air between the patient and treatment head.

What is isodose curve?

An isodose curve (or contour) is a line of constant absorbed dose. The line is in a plane and, for single radi- ation beams, its value is usually related by a simple percentage value (e.g., 90 percent, 80 percent, etc.) to the peak absorbed dose (or the surface absorbed dose, for x rays below 400 kV) on the beam axis.

What is skin sparing effect?

The so-called skin-sparing effect of higher energies of radiation is related to the electron build-up at depths under the skin surface varying from a few millimeters to several centimeters depending on the type of radiation.

How fast are photon beams?

Photons are massless, so they always move at the speed of light in vacuum, 299792458 m/s (or about 186,282 mi/s).

How does field size affect penumbra?

In this paper, we investigate changes of the physical penumbra widths according to various field sizes and depths, by using the three dimensional dosimetry system. As a result, we found that as field size and depth increase, the physical penumbra width also increases.

What does the 50% isodose line represent?

Currently, for vestibular schwannomas, the routine prescription is 12 to 14 Gy delivered to the 50% isodose line. The 50% isodose line shows where 50% of the prescribed dose lies. In the case of gamma knife treatments the dose is frequently prescribed to the 50% isodose line.

What is the bolus effect?

In medicine, a bolus (from Latin bolus, ball) is the administration of a discrete amount of medication, drug, or other compound within a specific time, generally 1–30 minutes, in order to raise its concentration in blood to an effective level.

What is the slowest light can travel?

The speed of light is normally about 186,000 miles per second, or fast enough to go around the world seven times in the wink of eye. Scientists succeeded in slowing it down to 38 mph. They did this by shooting a laser through extremely cold sodium atoms, which worked like “optical molasses” to slow the light down.

What are the photons in a 2pdd curve?

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 • Higher energy beams have more penetrating photons, and create more energetic electrons.

What is the PDD distribution of 180 photons?

180 referred to as the PDD distributions. The PDD is thus defined as follows: (6.34) where are the dose and dose rate, respectively, at point Q at depth zon the central axis of the phantom and are the dose and dose rate at point P at z maxon the central axis of the phantom.

What is the geometry of the PDD equation?

The geometry for PDD definition is shown in Fig. 6.9. 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.

How are protons lose energy in PDD PDD?

 pdd  lateralpenumbralateral penumbra interactions / energy loss Primarily protons lose energy in coulomb interactions with the outer-shell electrons of the target atoms. •excitation and ionization of atoms • loss per interaction small ‘continuously slowing down’