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 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).

## what is isodose chart?

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**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 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 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.

### What is wedge factor?

I am a bit confused in definition of wedge factor. KHAN defines it as the ratio of the doses with and without the wedge, in a phantom at a suitable depth beyond the depth of maximum dose. Podgorsak defines the ratio usually at Dmax. You may also read,

### What is isodose curve?

ISODOSE CURVES DEFINITION: Isodose curves are the lines joining the points of equal Percentage Depth Dose (PDD). The curves are usually drawn at regular intervals of absorbed dose and expressed as a percentage of the dose at a reference point. ISODOSE CHARTS : It consists of a family of isodose curves. Check the answer of

### 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. Read:

### What is an Isocentric machine?

An isocentric technique is where all beams used in a radiation treatment have a common focus point, a.k.a. the isocenter. Isocentric techniques require less patient repositioning as multiple field arrangements can be delivered with gantry and collimator movements, reducing treatment times.

### What is PDD in radiotherapy?

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.

### What is source to axis distance?

The source-axis distance (SAD) is a treatment machine design parameter that affects integral dose, dose rate and patient clearance. For the calculations performed here it was assumed that dose equals terma.

### 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 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 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.