Stopping Power

Stopping power is defined as the rate of energy loss per unit path length as a charged particle travels through a material.

The total energy loss of the particle can be separated into collisional and radiative components:
S/dtot= (S/d) collision+(S/d) radiative
Where d is the density of the material transversed and S/d represents the mass stopping power.

Stopping power is greater for low Z material than for High Z because High Z materials have fewer electrons per gram and have more tightly bound inner electrons.

Mantle field

What are the factors involved in setting up Mantle and Inverted Y fields? (From Rosemarck)

Draw typical fields
Does matching the fields require a gap?
Are there multiple computer dosimetry points?
What concern do you have for the gonads? How can you protect the gonads? What can be done to protect the gonads of a young female? (Move them surgically)

Converting to depth dose

What are the factors required to convert electron depth ionization to depth dose?

Nomogram

Restricted mass stopping power

Define restricted mass stopping power.

Stereotactic question 1/10 07, 3d Conformal question

1/10 07 RADIOSURGERY
How do you setup a linac for radiosurgery?
How many beams do you need?
Which energy is best?
What relative dose can you get outside the target?
What was the original stereotactic unit and how many beams did it use?
Which medical conditions is it used for?

1/11/07 3d Conformal
What are the advantages of 3d Conformal therapy?
What changes in design made conformal therapy possible?
What considerations made conformal therapy necessary?
Have the clinical results been improved?

HVL's of radioisotopes

What are the HVL's of C0-60 and Cs-137 in lead?

Neutron detectors

What types of neutron detectors are available?

Followup:
Are they sensitive to spectra?
What is the energy of neutrons generated in a linear accelerator?

Shielding variables

Define the major variables used in a shielding calculation. What are typical values for each.

DRR writes, per NCRP reports No.49 and 51 we consider the workload (W), the use factor (U) the occupancy factor (T), the distance to the barrier (d) and the transmission factor of the shielding factor (B).

For a primary barrier this results in the following formula:
P=(W U T /distance squared)x B

Radiation detection for rad safety

Briefly explain the types of equipment your clinic uses for radiation safety and the physics behind their operation. (submitted 1/03/07)

WMK remarks. Pogorsak (chapter 4) covers these in some detail.

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
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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.

The circulator in a linac

What is the purpose of the circulator in a linac and where is it located?

Plane parallel vs thimble ionization chamber

Explain the main differences between plane parallel chambers and thimble ionization chambers.
Give an example of when you would use each.

DRR
The parallel plate chamber is one in which voltage is applied to two thin, closely separated parallel plates that collect ions. The measurement positon is defined as just inside of the front electrode. (ref Berman). We call this the "pancake chamber" in our clinic and it is typically used for monthly output. The chamber is recommended for measurement of electron beams with E<10 MeV. (So we should research why do we use this for higher energies on the Varian?)

The thimble ionization chamber is a coaxial chamber with one electrode that forms a thimble shaped shell around the collecting volume and the oterh electrode is a narrow central rod. In our clinic we commonly call this the "Farmer" chamber. It can be used to measure higher energy radiation beams. (Research again the reasoning why we use such a chamber for our annual)