OBJECTIVE CHAPTERS 7 - 10

CHAPTER 7

1. State who introduced the concept of Tissue Air Ratios.

    ANS: Harold Johns in 1953

2. Define Isocentnc machines, TAR and SAD,

    ANS:

Isocentric Machines: a machine that rotates around a fixed isocenter, if the tumor is placed at the center of rotation it would always be in the beam as the machine rotated.

 

TAR: the ratio of the dose at a given depth and field size in a phantom to the dose in “air” at the same point and for the same field size.

 

SAD: SOURCE to axis distance, uses TAR , the output is defined at 80 or 100cm. From the source and in “air”

3. State the advantage of using TARs instead of PDDs to calculate a rotational "Beam On Time".

ANS: There is no correction factor needed because both readings are taken at the same distance

4. State the reason for this advantage.

ANS:Because the readings are taken at the same distance

5. State the equations for "Beam On Time" using PDD and TAR.

ANS: “Time On” = TD @ Depth/ (Output 10 x10)(Output Factor) (TAR @ depth)

6. State the reason for the difference Output readings defined For an a setup using PDD verses a setup using TARs.

ANS: They differ because distance of Dmax is different & there is no backscatter in a TAR setup SAD.

7. Calculate a "Beam On Time" problem using TARs.

ANS:

8. Define TMRs.

ANS:

TISSUE MAXIMUM RATIO: the ratio of the dose at a given depth and field size in a phantom to the dose at depth Dmax for the same field size and fixed distance from the source.

9. State the reason for its introduction.

ANS: When TAR’s were developed the energy of the equipment was considered high and the increase in energy made in “air” readings difficult because of the large build up cap that was needed so they came up with TMR’s.

10. Calculate a "Beam On Time" using TMRs.

ANS:

***11. State the relationship between TAR and BSF,

ANS: Backscatter Factor BSF is  equal to TAR at the depth of Dmax

12. State the relationship between TAR, TMR and BSF.

ANS: TAR = TMR x BSF

TMR = Ddepth            TAR =  Ddepth             BSF = Ddmax

           Ddmax                         D"air"                          D"air"

CHAPTER 8

1. Define CS, MDD, FSS and FSD.

ANS:

CS: The field size that a machine will give at its designated distance.

MDD: Machine Definition Distance, That is the designated distance of the equipment.

FSS: Field size on skin, this the true dimensions of field size as seen on skin.

FSD:   Field size at depth, this is true dimensions of the field size as seen at depth.

2. Define similar triangles.

ANS: When two angles of two triangles to be compared must be the same.

3. Diagram and label a normal SSD and SAD setup.

PG. 103

4. Diagram and label an extended distance setup.

PG. 103

5. State the relationship between CS, MDD, FSS and FSD.

  THE FORMULA    CS   =   FSS   = FSD

                             MDD     ODI      ODI + depth

6. Calculate the CS, MDD, FSS and FSD for a normal SSD or SAD setup.

ANS:

  7. Calculate the CS, MDD,FSS and FSD for an extended distance SSD or SAD setup.

ANS:

CHAPTER 9

1. State the inverse square equation.

ANS: I1  =  (D2)2

         I2      (D2)2

2. Diagram and label an inverse square problem for a PDD, TAR and TMR setups.

3. Calculate the output of the machine for a PDD, TAR and TMR setups at different distances.

4. Calculate the new "Beam On Time" at the different distances.

CHAPTER 10

1. State what Is moved out of the path of the electron beam as exits the waveguide.

ANS: Tungsten Target and Beam Flattening Filter

2. State what Is moved in the path of the electron beam as it exits the waveguide.

ANS: Beam Flattening Filter is replaced by scattering foil to spread out the electron beam.

3. State the comparison of the FDD of an electron beam with that of a photon beam.

ANS: It is not as penetrating as photons and electrons have a PDD is much shallower for an electron beam than for X or gamma rays of the same energy.

4. State the number and general arraignment of an electron beam setup.

ANS: 6 Mev to over 20 Mev

5. State the prescription level that is used in electron beams.

ANS: generally it is prescribed to the  80% level or higher, but commonly to the 90% level

6. State the most used electron energies.

ANS:6, 9, 12, and 16 Mev

7. Calculate the approximate depths of the 90%, 80% 10% and maximum range for different energy electrons,

ANS:

Depth of 90% = Mev/4 cm

Depth of 80% = Mev/3 cm

Depth of 10% = (Mev/2) - .5cm

8. State the number of calibration circuits used for the electron beam energies.

ANS: Each electron energy has its own electronic circuit for calibration.