BEAM DATA FOR LINAC'S

When taking Beam Data for LINAC's 2 chambers must be used:

• The chamber inside the phanthom
• And a chamber (monitor chamber) is placed somewhere outside the chamber in the path of the beam

The Chamber (monitor chamber) is placed outside the phanthom because we need to stabalize the readings due to changes in the output from the LINAC (the monitor chamber remains stationary and can be placed anywhere in the beam)

The 2 readings are taken and then normalized to each other as shown in the example below:

these readings are normalized and then the new values are used to plot the data.

WEDGES:

Hinge Angle – angle where 2 fields meet of a wedged pair according to the wedge angle that is used.

Hinge(angle) = 180 – 2 x wedge(angle)

Example : using a 30 degree wedge

H(angle) = 180 – (2 x 30)

H(angle) = 180 – 60

H(angle) = 120   (so there would be 120 degrees in between the wedged fields)

CORRECTION FOR COUNTOUR IRREGULARITIES:

3 Methods can be used to correct for irregulararities in a contour:

1. Effective SSD
2. TAR Method
3. Isodose Shift Method

Isodose Shift Method:

We will use shift factors to determine at what level the reading are taken at:

this data would be plotted as above and used to determine treatment fields and dose inside the patient.

PARALLEL OPPOSED FIELD DATA:

Readings are taken at both sides of the phantom:

• There will be a Dmax for the Anterior Beam and the Posterior Beam

The data is plotted for the Anterior Beam and for the Posterior Beam and then the readings are combined by adding the doses to the same part of the phanthom on the

Now the Isodose Curves are Drawn for the Combined Beams:

Now we will Normalize the Plan to 100% to the Isocenter:

To find the new values we will use the ratio:

Solving for the new value:

And for the Rest of the Values:

To find the new values we will use the ratio:

Solving for the new value:

As seen above this Dose Matrix, the Isodose Curves have been normalized to 100%, but in real life we like to use the following Isodose Curves:

Some things to Note from the Graph above:

• Point of normalization is the center of the phanthom and is 100%
• Maximum dose in the phanthom (114%)
• Depth of maximum dose (.5cm)

If the tumor is at the center of the phanthom then:

In the example below:

• The tumor is receiving a minimum dose of 95% of the prescription dose
• The treatment volume will extend from the Anterior Dmax point to the Posterior Dmax point

The Lateral Width of the Treatment Volume will be determined by the field size and the Doctors definition of what level is included in the treatment volume:

The MD could prescribe to the 95% line or to the 90% line:

FOR LINAC's THERE WILL BE A BETTER DOSE DISTRIBUTION:

• Better can mean a lot of things but in this case we are saying the Dose Distibution is more even with higher energy machines
• The maximum dose in a patient with a Co60 machines is 114% but in a 6MV LINAC is is 106%