# What is penumbra in radiation therapy?

In radiotherapy, a percentage depth dose curve (PDD) (sometimes percent depth dose curve) relates the absorbed dose deposited by a radiation beam into a medium as it varies with depth along the axis of the beam.

Additionally, what is geometric penumbra? Geometric Penumbra: Transmission Penumbra: variable transmission of beam through non divergent collimator angle. Physical Penumbra: the lateral distance between two specified isodose curves at a specified depth. ( lateral distance between 90%?10% or 80?20%)

Regarding this, what is beam profile in radiotherapy?

le is The variation of dose occurring on a line perpendicular to the central beam axis at a certain depth is known as the beam pro?le. It represents how dose is altered at points away from the central beam axis.

What is isodose chart?

ISODOSE CHART. An isodose chart for a given beam consists of a family of isodose curves usually drawn at equal increments of percent depth dose, representing the variation in dose as a function of depth and transverse distance from the central axis.

### What is beam flatness?

Flatness is usually defined at 10 cm depth, and in order to achieve this, the beam must exhibit horns near the surface. Horns are required due to differential hardening in the flattening filter. In the center, the beam is hardened and therefore penetrates more resulting in more fluence at 10 cm depth.

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). The dose at dmax is defined as 100% and then the radiation dose decreases as the depth increases, the energy being absorbed within the tissue.

### What is Kerma in radiation?

Acronym for kinetic energy released in materials. Kerma is the sum of the initial kinetic energies of all the charged particles liberated by uncharged ionizing radiation (neutrons and photons) in a sample of matter, divided by the mass of the sample.

### What is SSD in radiation therapy?

Modern photon beam radiotherapy is carried out with a variety of beam energies and field sizes under one of two set-up conventions: a constant source to surface distance (SSD) for all beams or an isocentric set-up with a constant source to axis distance (SAD).

The source-axis distance (SAD) is a treatment machine design parameter that affects integral dose, dose rate and patient clearance. The sensitivity of SAD to beam energy, PTV size, body size and PTV position were determined for conformal arc therapy.

### What is wedge in radiotherapy?

In radiation therapy, wedge filters are commonly used to improve dose uniformity toward the target volume [2]. A physical wedge is usually constructed from a high-density material, such as lead or steel, which attenuates the beam progressively across the entire field.

### Why does PDD increase with SSD?

Because this increase in scattered dose is greater at larger depths than at the depth of Dmax, the percent depth dose increases with increasing field size. Percent depth dose increases with SSD because of the effects of the inverse square law.

### What is fluence rate?

The Fluence rate is the number of particles crossing per unit time (which is numerically equal to the product of number of particles and their average speed).

### What is beam softening?

Beam softening is depending on the material atomic number and photon beam energy for photon beam filtration; the filtration quality is essential to producing a clinical beam in external radiotherapy. Beam softening analysis is done using Monte Carlo method.

### What is photon fluence?

photon fluence, Hp,o, Fp,o Amount of photons (quanta of radiation) incident on a small sphere from all directions, divided by the cross-sectional area of that sphere and integrated over time. Photons per surface area (quanta m−2). SI unit is m−2.

### What is energy fluence?

Energy fluence (concentration) is the amount of radiation energy delivered to a unit area. For a specific photon energy, fluence is proportional to exposure.

### What is tissue maximum ratio?

Tissue Maximum Ratio (TMR) The TMR is a special case of the TPR, where the depth y used for the comparison is at the depth of maximum dose zmax (depth z).

### Why does beam divergence affect penumbra?

it is reduced dose at the periphery of the radiation field due to partial transmission of the beam through the edge of the collimator or shielding block. the closer the collimators are to the source the greater the penumbra therefore if the source to collimator distance is increased then penumbra will be smaller.