Week 5 Board Review

The measured dose in air is 123 cGy. The dose in tissue is 97 cGy. What is the TAR?

.789 dose in tissue/dose in air

A patient is treated using two superiorly/inferiorly adjacent fields. The collimator setting for the superior field is 10 cm wide by 13 cm long. The collimator setting for the inferior field is 20 cm wide by 35 cm long. Both fields are treated at 100 cm SSD. Calculate the gap at the skin surface if the fields abut at a depth of 5 cm.

1.2

You are treating a patient at 100 cm SSD, and would like to block 3 cm on the patient's skin. What is the true block size if the source to tray distance is 40 cm?

1.2 cm

A film is taken at 140 TID and produces an image measuring 4cm. With a 100 TOD what is the magnification factor?

1.4

Electrons are chosen to treat a lymph node boost field considered to extend to 4 cm below the skin. The electron energy chosen to give effective treatment at this depth would most likely be:

12 Mev -Effective treatment depth is estimated at 1/3 of the beam energy in cm.

The field size at isocenter is 8 x 8 cm. What is the size on the skin, if the patient is treated 100 cm SAD with a tumor depth of 10 cm?

7.2 x 7.2 cm -This is a direct ratio: 8/100=x/90

As the depth increases, what happens to the PDD?

Decreases

To obtain a patient's thickness, which of the following can be used? I. ruler II. flouroscopy III. caliper

I, II or III

Methods of obtaining dose uniformity across field junctions include which of the following? I. Dosimetric isodose matching II. Separated fields III. Junction shift IV. Half beam blocking V. Geometric matching

I, III, IV, and V

As the SSD increases, the PDD does what?

Increases

Which of the following factors would be included in a hand calculation for mu with SSD and not SAD setups

PSF (peak scatter factor)

An important factor to consider when matching adjacent electron fields is:

lateral bulge effect of electron isodose curve

What is the Dmax depth of a 10 MeV photon beam?

2.5 cm

You need a block to be 5 cm wide at a depth of 11 cm in a patient treated 100 cm SSD. The source/tray distance is 65 cm. How wide is the block itself?

2.9 cm

Tissue air ratio decreases with decreasing: I. field size II. depth III. SSD

I only

A wedge filter is used to alter the isodose distribution on an anterior chest field. The monitor units setting must be ___________ to account for the wedge filter in the field.

increased

The central axis percentage depth dose for a 10 x 10 cm field at 80 cm from a 4 MV linear accelerator is 60.8% at 10 cm depth. Calculate the percentage depth dose for a 10 X 10 field at 100 cm SSD

-59.6 (101/110)Squared/(81/90) squared= 84/.81= 1.04 1.04 x .608= .633

A patient is treated with two adjacent 4 Mv fields 7 cm and 12 cm in length, respectively at 100 cm SSD. Calculate the gap on the skin necessary to give a uniform junction at a depth of 8 cm

.76 cm -gap = d(L1 + L2)/ 2SSD

A patient is treated on an 18X unit at a 5 cm depth. The original SSD=100 cm and the new SSD =95 cm. What is the Mayneord's "F" factor?

.9979 -(95 + 3.5/ 95 + 5)squared divided by (100 +3.5/100 + 5)squared

Find the degrees/MU for a 3 field 45 degree arc treatment that is treated with 180 MU's.

0.75 degrees/MU -divide dose into thirds due to 3 arcs then 45/60=.75

Calculate the inverse square used to correct dose rate for the following field: 10 MV photons, treat to a 7 cm depth. original SSD=100 cm, new SSD=115 cm

0.76

A patient's spine is being treated PA from T2-L3 and requires a gap from the above AP/PA lung field, matched on the PA field only. Calculate the gap needed if the cord is located at a depth of 5 cm, the lower PA field is 18x9 with a TSD of 100, and the upper AP/PA field is 14x9 with a TSD of 88 and treated midline.

0.85

A patient is being treated with an AP/PA mantle field with a 15 x 15 cm field size to the skin at 105 cm. The patient will be treated to a daily dose of 200 cGy at midplane. The patient measures 20 cm in diameter and will be treated utilizing 6 mV. What is the Mayneord's F factor.

1.008

What is the penumbra of a beam at 5 cm depth, with a source size of 2 cm, an SSD of 80 cm and an SDD of 50 cm?

1.4 cm -Penumbra=source size(SSD + depth-SDD)/SDD

Draw a rectangle with a width of 18 cm and height of 8 cm. On the left side of the rectangle a triangular area is blocked that extends from the upper left corner diagonally down to 2 cm right of the left edge. Another block is in the right lower corner and is 2 x 2 cm in size. What is the equivalent square of this rectangle?

11.5 -18 x8=144, 8 x2/2=4, 2 x2=4 144-4-2=132 square root of 132=11.5

Blocks and wedges should be placed at least ___ cm from the patient's skin.

15

If the measured surface exposure rate is 60 rads/min and the in air exposure rate is 50 rad/min, the percent backscatter is:

20% -60/50. The surface is getting 20% more than the in air exposure. You could also subtract the in air exposure rate from the surface and get 10. Divide that by the original to get the percentage of backscatter (10/50).

A patient is to be irradiated to a depth of 7 cm using a field size of 10 x 10 cm at 80 cm SSD. The PDD at the point is 65% and the dose rate at Dmax is 100 cGy/min. If 150 cGy is delivered to a depth of 7 cm, what is the dose at Dmax?

231 cGy

AT 120 SAD, 207 monitor units deliver 100 cGy. If the SAD is changed to 140 cm, what monitor units would be required to deliver 100 cGy?

282 -inverse square law affects dose intensity. You will need to increase MU by factor of (140/120)2 so multiply 207 by that factor

Find the skin gap necessary to match two symmetric ports at midline that are 21 and 37 cm in length. Assume the midline depth is 13 cm and both ports are treated at 100 cm SSD.

3.77

A field size setting of 20 x 20 cm at 80 cm SSD can be used at 120 cm SSD for a field size of:

30 x 30 -80/20=120/x

Calculate the wedge angle if the hinge angle equals 90 degrees.

45 degrees -90-(.5 x hinge angle)

Blocks are required to be made of at least ___ half value thickness.

5

The output at 80 cm is 75 rad/min. How long would the machine need to be on to deliver 250 rads to a point at 100 cm distance?

5.21 minutes -The inverse square law is needed to determine the new output which is then divided into the desired dose.

In using wedges to alter the isodose curve of a treatment beam, the wedge angle is defined as the angle the____% isodose curve makes with a line perpendicular to the central ray.

50

Calculate the PDD of depth (d) if the dose at Dmax is 180 cGy and the dose at d is 127 cGy.

70.6%

The percentage depth dose for 12 x 12 cm field with a 6 mV photon beam, at a depth of 5 cm and 80 cm SSD is equal to 82.8% Calculate the percentage depth dose for the same field size and depth for 100 cm SSD.

85/105 squared divided by 81.5/101.5 squared = 1.017 1.017 x .828= .842 or 84.2% Remember to use Maynard's F.

You need to treat a head and neck patient's lymph nodes in the cervical spine region with electrons. The separation of the neck is measured to be 12 cm. Which electron energy would you think is most appropriate?

9 MeV -Effective range is 3cm for nodes and beam is gone at 4.5 cm to spare cord

If a supine patient is being treated PA and the separation is 16 cm , what would the optical distance indicator read if the patient is setup with isocenter midplane?

92

The physician prescribes the AP/PA field to be weighted 3:1 for the posterior field. How much dose is coming from the AP and the PA if total dose prescribed is 200 cGy?

AP = 50 cGy, PA = 150 cGy

What is a TAR at the depth of Dmax used to correct for scatter of dose called?

Backscatter factor

Tissue-air ratio decreases with decreasing: I. field size II. depth III. SSD

I

Which of the following will increase TAR as it increases? I. energy II. field size III. SAD IV. depth in tissue

I & II only

Percentage depth dose increases with increasing: I. energy II. depth III. SSD

I and III only

You have to treat a patient at an extended distance in order to get a large enough field size. What calculations must be used to adjust for this change in distance? I. Direct ratio to determine the actual field size at the SSD from dial size which is set to isocenter II. Mayneord's F to adjust FDD or PDD III. Inverse square to adjust output IV. Inverse square to adjust FDD or PDD

I, II & III only

What factor(s) listed is/are needed for a rotational calculation? I. Arc degrees II. calibrated inverse square III. Mayneord's F factor IV. Percent depth dose V. output factor

I, II & V

An irregular body contour within a treatment field may require compensation by use of: I. bolus II. a wedge III. a custom compensator

I, II or III

Planned treatment volume is generally larger than clinical treatment volume because of which of the following factors? I. Mechanical uncertainties in the operation of the linacs and their readout devices II. Uncertainties in imaging and planning devices III. Patient movement IV. Positioning uncertainties

I, II, III & IV

What is true regarding the depth of a tumor? i. depth is the distance beneath the skin surface where the prescribed dose is to be delivered ii. for opposed fields, the patient's mid-plane is often used for the depth of calculation iii. most multiple field arrangements use the isocenter for the calculation depth iv. depth affects measurements of dose attenuation

I, II, III, IV

When MLC blocking is used in a treatment calculation, the area of the collimator is used in determining: I. IMR II. Scatter III. PDD

II

As a field size increases from the standard established, the output factor for a treatment machine will do what?

Increase

As the energy increases, the tissue-air ratio does which of the following?

Increases

As the energy increases, what happens to the PDD?

Increases

As the field size increases, the PDD does what?

Increases

As the field size increases, the tissue-air ratio does what?

Increases

Which is the advantage of using parallel opposed isocentric fields for treatment delivery (as compared with nonisocentric)?

Less opportunity for movement error

Which of the following does not apply to "absorbed dose?"

Measured in air

As the source-skin distance increases, what happens to the tissue-air ratio?

No effect

Absorbed dose at depth x 100% = absorbed dose at Dmax is the definition of which of the following?

PDD

Absorbed dose at a given depth divided by the dose in air dose at a fixed reference is the definition of which of the following?

TAR

Which of the following central axis depth dose quantities are most likely used to compute an accurate monitor unit setting on an 18-MV unit for an isocentric treatment?

Tissue-maximum ratio or tissue-phantom ratio

In comparing SSD and SAD treatments which of the following are true? SSD treatments are commonly used for a single field treatments SAD is commonly termed isocentric SAD treatments field size is defined at depth in patient SSD field size is determined at the surface

all of the above

What TAR would be used in mu calculations for a rotational or arc field?

average TAR of all angles treated

Which of the following devices can be used to account for varying surfaces across a radiation field while maintaining the skin sparing effect? Choose the best answer.

compensator

A wedge filter____the output of the beam and must be taken into account in the treatment calculation.

decreases