Collimator Scatter vs Phantom Scatter

Describe the difference between collimator scatter and phantom scatter. Tell how you would measure each clinically.

DRR
PROCEDURE FOR DETERMINING Sc
Total collimator scatter

This setup is in air and the chamber is placed on the ring stand 300 cm away from the treatment head. We have buildup cap over the chamber in this case to account for the needed buildup for the appropriate energy in air. Ex 6MV has its own buildup, 18MV has another.

The same field size as the phantom scatter are utilized.

PROCEDURE FOR DETERMINING St
TOTAL PHANTOM SCATTER

Total phantom scatter is determined for the annual calibration of each linear accelerator.
The setup requires the large water tank to develop full scatter conditions.

1) The chamber is placed on a ring stand at Dmax (the easiest way to do this is to tape the side of the box and measure carefully).
2) First the chamber is place even with the surface of the water.
3) Then this line is marked on tape and we measure the distance of Dmax (for SL-75 and 6 MV this is 1.5 cm) and mark this carefully on the tape.
4) Next we lower the chamber to this point and add an amount of water corresponding to Dmax
5) Now that the chamber is at dmax we plug in the electrometer and measure at various field sizes as defined in the worksheet.

SEE ATTACHED PHOTO OF SETUP

SEE SAMPLE SPREADSHEET
__________________________________________________________________

Phantom scatter is then obtained from knowing that St=Sc X Sp

NOTE THERE IS SOME THOUGHT THAT THE CHAMBER SHOULD BE SET AT 5 CM PER SOME READING WMK DID. THIS SEEMS TO MAKE MORE SENSE ANYWAYS AS WE WOULD BE PAST THE BUILDUP REGION.