Primary Radiation : photons that originate at the source and deposit energy at the point

Scatter Radiation : radiation that comes from the other parts of the irradiated phanthom and then deposit their energy at that point and every other point in the phanthom

An increase in field size will increase PDD, TAR and TMR because of an increase in scatter radiation (primary radiation stays the same with an increase in field size) also the inverse is true.

Penumbra – is the region at the edge of the radiation beam, over which the dose rate changes very rapidly with distance from the central axis (shadow of the beam)

Transmission Penumbra – is the variation in dose at the edge of the beam due to the collimator.   At the edge of the beam the dose will vary as you move laterally from the central axis (this is because of the different thickness of collimator block the beam as you move laterally)

Physical Penumbra – is the variation in dose at the edge of the beam that is measured, and is defined as the lateral distance between two specified isodose curves at a specified depth.   Physical pneumbra can be defined as the distance between the 90% and 20% isodose curves as measured at depth Dmax

Geometric Penumbra – this is the width of the shaded region of the diagram at any depth due strictly to the geometry of the setup

Increase in Source size will make penumbra larger

Decrease in Source size will make penumbra smaller

Increase in Source to Collimator Distance will decrease penumbra

Decrease in Source to Collimator Distance will increase penumbra

Increase in SSD will increase penumbra

Decrease in SSD will decrease penumbra

Field Size DOES NOT effect penumbra!!!

Dose Calculations for Implants:

For Temporary implants :

Dose = Activity X Exposure Rate Constant X time

For Permanent implants:

Mayneords Factor : is used to correct Perecentage Depth Dose for a change in treatment distance

• PDD increases with and increase in distance
• PDD decreases with a decrease in distance

tray factor – factor that takes into account the attenuation of the beam from the tray the block is mounted on. No matter what size the block is the same tray factor is always used

0 X 0 field size -   there is no scatter radiation and only the primary radiation contributes to the dose at that field size

For Irregular Field Sizes we use Clarkson Integration:

• PDD uses Scatter Function tables (SF)
• TAR uses Scatter Air Ratio tables (SAR)
• TMR uses Scatter Maximum Ratio tables (SMR)

Gap Calculations are used to: Accommodate for a field size that is to long to fit in 1 treatment field;   If a first field treatment is completed at an earlier time and a new field is added at a later date, or if Two different fields have a common border.

  Gap Calc Equation:

       +    

Dose Matrix - next beam profile data is aquired and then plotted for every point in a phanthom.   The dose matrix represents the dose each point will receive.

Isodose Curves – lines that are drawn to connect point with the same value within the Dose Matrix

Normalization   – is the comparison of the dose distribution to a particular point inside the patient (generally the point of intrest is the center of the tumor or at the isocenter.)

Wedges - is an absorber that is placed in the path of the beam that is thick at one end and thin at the other. (Usually made of steel.)   The general purpose of a wedge is to bend the radiation field to match the patient contour and/or tumor so as to achieve a uniform dose distribution.

The name of a wedge is determined by:

• The angle that the 50% isodse makes with a line perpendicular with the central axis (in low energy machines)
• The angle that the isodose curve make at a depth of 10 cm with a line perpendicular with the central axis (in high energy machines) (the 50% isodose curve in a high energy machine is too deep for most treatments)