Treatment Planning Final

Calculate the entrance dose from a single portal. A patient will be treated isocentrically to mid with 200cGy using bilateral fields on a 100cm TAD machine. the PDD at mid is 81.9%.

(Direct proportion) 122cGy

Calculate the dose rate for 100cm. The machine is calibrated to 80cm for 1cGy/mu.

(Inverse square) .64 cGy/mu

If you are unhappy with distribution of an AP/PA or single field plan. What are 4 things you can do to improve it?

- Beam weighting -Change the energy -Normalize; make 95% line the 100% line - Add additional fields

Name 3 advantages of POP

- Deliver a fairly uniform dose -Simple to set up -Less risk of geometric miss

Name 5 disadvantages or POP

- High dose to normal tissue -Depends on patient thickness (an arm is easier than a pelvis- cause hot spots) - Depends on beam energy - Decrease energy increase dose at DMax - This is called Tissue Lateral Effect

Give three examples of when you might need a wedge in a treatment plan.

1. Hinge angle less than 180 degrees 2. Sloping surface (isodose tilts from normal position) 3. 3 or more treatment fields

The dose in free space in 100cGy. The BSF is 1.02. The dose at dmax is?

102cGy

Standard isodoses: (5)

10x10 FS Tissue Equivalent Material Flat Phantom DMax Depth Isocentric Distance

What is the minimum distance that a physical wedge should be away from the patient and why?

15cm for skin sparing

Indicate the dmax depths: 4mv 6mv 10mv 18mv

1cm 1.5cm 2.5cm 3.5cm

If the dose to the patient is 90cGy with a 45 degree wedge in place, what dose was delivered if the wedge is omitted and the wedge factor for this wedge is .417?

216

If the WTF is .728 and the MU setting for an open beam is 200MU, what are the MU's for the same treatment field with this wedge in place?

275MU

What wedge angle would you use if the AP ganrty angle is 180 and the RPO angle in 300?

30

How do you calculate a wedge angle?

90- 1/2 hinge angle

Describe the advantages and disadvantages between hand molded contours and image-related contours

A CT is the most commonly used image-related contour and solder wire is the most common hand molded contour. An advantage of CT is that you can see internal structures as well as the external contour. Solder wire could be used for a larger patient that the CT may cut some of the external contour off of.

What is an isodose chart?

A family of isodose curves usually drawn at equal increments of PDD

What is an isodose curve?

A single isodose curve connecting points of equal dose expressed as percentage

Coronal contour

A slice cutting a patient from front to back

Sagittal contour?

A slice cutting a patient into left and right

Which plane of the anatomy is viewed on a transverse contour?

A slice cutting a patient into top and bottom

Other names for given dose:

Applied dose Entrance dose Peak absorbed dose DMax dose

What is the definition of a wedges in a RX plan?

Beam modifying device

If the WTF increases, what happens to the MU's if all else stays the same?

Decrease

As the wedge angle increases, what happens to the WTF if the energy stays the same.

Decreases

What happens to output when distance is changed from 100cm to 110cm.

Decreases

Describe the effect on output factors as FS increases from 10x10 to 15x15.

FS increases, Scatter increases, Output increases

T or F: An increase in SSD has a large effect on PDD

False

T or F: FS is always determined at depth of tumor?

False

T or F: FS is always determined at skin surface?

False

FS definition: Geometric Dosimetric

Geometric- Defined by light localizer Dosimetric- Field equal to 50% isodose line

Name at least four different types of contouring materials or methods. State your preference and give reasons why.

Hand molded: Solder, plaster, aquaplast Mechanical: pantograph Image related: CT, MRI, Ultrasound I would use a CT fused with an MRI for best visualization of contour and hard/soft tissues.

Name the tree different types of wedges with a brief explanation of each.

Hard Wedge: Individual- Specific to machine Universal - 60 degree built in machine head Dynamic Wedge

What happens to the WTF if the energy of the beam increases.

Increase

In AP/PA treatments higher dose is seen above and below the tumor than at midline. This is called ?

Lateral tissue effect

Why do we take all of our measurements at isocenter?

No divergence at isocenter!

What is the formula for calculating penumbra?

P=S(SSD+d-SDD)/SDD

For fixed SSD, _______ is used

PDD

Backscatter=

TAR at DMax

What is the definition of a patient contour?

The reproduction of the external shape of a person.

What is the purpose of the patient contour?

To help the dosimetrist determine dose distribution to isocenter based on body shape

Why do we need to identify where the internal critical structures?

To make sure the dose given to the treatment area does not exceed the maximum dose to a critical structure.

T of F: Back scatter factor (BSF) is the ratio of dose at dmax in phantom to dose at dmax in air with all other factors the same?

True

T or F: PDD is used primarily for SSD set ups?

True

What is demagnification? And why do we need to do it?

Use of math or a ruler to account for the distance of the source from the film. We need to do it to correct for divergence. The source will appear bigger on a film than the actual size. If we planned to the original film the dose would be incorrect.

Why would we want to take an off axis contour? Specify 1 area of the body that usually needs it.

You would have to take an off-axis contour where the body does not lie flat on the table top. An example would be the neck because of the use of a head holder. Taking the contour off axis would allow you to get more of an average of the dose that needs to be delivered.

When using superflab bolus as a compensator ____________ skin sparing is to be expected

a decrease in

A treatment calculation was make for 100cm SSD. The patient was mistakenly treated at 95cm SSD. What is the error in dose delivered? a. 11% overdose b. 11% underdose c. 90% overdose d. 90% underdose

a. 11% overdose

What best describes transmission penumbra? a. Is caused by variable transmission of beam through a nondivergent collimator edge. b. Is defined as the lateral distance between two specific isodose curves at a specific depth. c. Is caused by variable transmission of beam through a divergent collimator edge

a. Is caused by variable transmission of beam through a nondivergen collimator edge

Mayneord's F factors is used when there is a change in: a. SSD b. FS c. Energy d. PDD

a. SSD

Absorbed dose at a given depth divided by the dose in air dose at a fixed reference is the definition of: a. TAR b. TMR c. TPR d. PDD

a. TAR

Physical penumbra is the sum of: a. Transmission penumbra b. Biological penumbra c. Geometric penumbra d. Superficial penumbra

a. Transmission penumbra, c. Geometric penumbra

As a field size increases from the standard established, the output factor for a treatment machine will: a. increase b. decrease c. stay the same

a. increase

As FS increases, the TAR: a. increases b. decreases c. stays the same

a. increases

As the FS increases, PDD: a. increases b. decreases c. stays the same

a. increases

As the SSD increases, the PDD: a. increases b. decreases c. stays the same

a. increases

As the energy increases, the PDD: a. increases b. decreases c. stays the same

a. increases

As the energy increases, the TAR: a. increases b. decreases c. stays the same

a. increases

What kind of treatments do we use single beam? a. superficial lesions b. electrons c. PA spine d. Midline lesions

a., b., c.

PDD =

absorbed dose at depth/absorbed dose at DMax

1 Gy is equial to: a. 1000rads b. 1 J/kg c. 100 erg/g d. 10 rads

b. 1 J/kg Also: -100 rads

A tray holding custom blocks is measured as 97 cGy and the dose without he tray is 100 cGy. What is the tray transmission factor? a. 103 b. 97 c. 100 d. not enough information to determine

b. 97

What best describes geometric penumbra? a. Is caused by variable transmission of beam through a nondivergent collimator edge. b. Is due to the finite dimension of the source (or focal spot). Its width is proportional to the source diameter. It increases with increase in SSD and depth but decreases with increase in SDD. c. Is caused by variable transmission of beam through a divergen collimator edge

b. Is due to the finited dimension oft the source (or focal spot). Its width is proportional to the source diameter. It increases with increase in SSD and depth but decreases with increase in SDD.

The advantages of dynamic wedges over physical wedges include all of the following except: a. Less possibility of injury to pt or staff b. Limited number of options in wedge angles c. For breast tangents, less dose to the contra lateral breast d. There is no hardening of the beam e. The field width is usually not limited

b. Limited number of options in wedge angles

As the depth increases, the PDD: a. increases b. decreases c. stays the same

b. decreases

Select all the apply to geometric penumbra a. increase with decrease in source size b. decreases with decrease in source size c. increases with increase in source size d. decreases with increase in source size

b. decreases with decrease in source size, c. increase with increase in source size

Dmax is defined as: a. build-up region b. depth of maximum equilibrium of occurs for photon beam c. depth of dose deposited per unit of time d. dose of scatter to surface

b. depth of maximum

A TLD can measure the radiation absorbed when it is: a. exposed to light b. heated c. chemically processed d. scanned with a digital reader

b. heated

Select all that apply to geometric penumbra a. increases with decreased in SSD b. increases with increase in SSD c. increases with increase in SDD d. increases with decrease in SDD

b. increases with increase in SSD, d. increases with decrease in SDD

Half value thickness is a. used to describe the energy of megavoltage machines b. used to describe low energy machines c. increases the intensity by 1/2 d. changes based on the divergence of the beam

b. used to describe low energy machines

What best describes physical penumbra? a. Is cause by a variable transmission of beam through a nondivergent collimator edge. b. is due to the finite dimension of the source( or focal spot). Its width is proportional to source diameter. it increases in SSD and depth and decrease with increase in SDD. c. Is defined as the lateral distance between two specific isodose curves at a specific depth. (The lateral distance between 90% and 20% isodose lines at the depth of DMax)

c.

Convert 2 Gy into rad a. 2 rad b. 20 rad c. 200 rad d. 2000 rad

c. 200 rad

You are measuring a patients diameter and there is an air gap, the caliper measurement must be read from: a. Table top to anterior skin point b. Table top to posterior skin point c. Anterior skin point to posterior skin point d. Anterior skin point to midline

c. Anterior skin point to posterior skin point

A TAR at the depth of DMax used to correct for scatter of dose is called the: a. scatter-air ratio b. tissue-phantom ratio c. backscatter factor d. Mayneord's F Factor

c. backscatter factor

What is the definition of Beam quality? a. beam scatter b. beam divergence c. beam energy d. beam attenuation

c. beam energy

What is the definition of Penumbra? a. is a dose transition region near the center of the field b. is a dose transition region near the isocenter of the field c. is a dose transition region near the borders of the field d. is a dose transition region near the 30% isodose line of the field

c. is a dose transition region near the borders of the field

Thermoluminscent dosimeter is made of what material: a. calcium carbonate b. silver halide c. lithium fluoride d. selenium

c. lithium fluoride

All of the following apply to "absorbed dose" except: a. energy deposits at a point b. measured in Gy c. measured in air d. can measure absorbed dose using ionization chambers

c. measured in air Other methods of dose determination: -TLD -Photographic film

Which isodose line is used to define the field light? a. 80% b. 70% c. 60% d. 50%

d. 50%

Orthogonal radiographs provide valuable contour information by locating structures in the follow perspectives: i. Length ii. Width iii. Depth a. i and ii b. i and iii c. ii and iii d. All of the above

d. All of the above

Absorbed dose at depth x 100% = absorbed dose at Dmax is the definition of: a. TAR b. TMR c. TPR d. PDD

d. PDD

Mayneord F Factor is used in: a. isocentric setups when distance is increased b. isocentric setups when field size changes c. SSD setups when field size changes d. SSD setups when distance increases

d. SSD setups when distance increase

Ways to get the penumbra smaller a. Decrease source size- you will decrease the geometric penumbra b. Increase depth will increase penumbra c. decrease SDD will decrease penumbra d. increase SDD will decrease penumbra

d. increase SDD will decrease penumbra

As the SSD increases, the TAR: a. increases b. decreases c. stays the same d. shows no effect

d. shows no effect

TAR=

dose in tissue/dose in air independent of distance

A prescription is not a single sheet but includes: a. Computerized isodose treatment plan b. SIM films c. Clear instructions d. All parts of chart match e. All of the above

e. All of the above

When taking a contour, which of the following are important points of information that must be known before beginning the procedure? a. Patient ap/pa separation b. Patient lateral separation c. Measurement from table top to the posterior surface at the central axis d. Anterior and lateral RF's e. All of the above

e. All of the above

Dose delivered to patient depends on a. Depth of the calculation point b. Beam energy c. Distance from the source d. Type of tissue e. Field size f. All of the above

f. All of the above All of these change intensity of radiation

T or F: As a general rule, one would use a high energy photon beam on a superficially located tumor?

false

T or F: As patient separation increase we must use lower energy beam.

false

T or F: One method of decreasing the geometric penumbra is to extend the SSD.

false

One method of reducing subcutaneous dose from parallel opposed treatments is to (increase/decrease) the number of treatment fields.

increase

As field size increases, output (increases/decreases)

increases

T or F: FS- The field defining light is made to coincide with the 50% isodose line of the radiation beam.

true

T or F: In AP/PA treatments, the dose to subcutaneous tissue increases with a decrease in energy.

true

T or F: One method of decreasing the geometric penumbra is to extend the SDD.

true

T or F: Since SSD is a nonisocentric treatment calculation the FS is defined on the skin?

true

T or F: Source size, affects the shape of isodose curves by virtue of geometric penumbra.

true

T or F: TAR and TMR are isocentric techniques?

true

T or F: The flattening filter which is used for megavoltage x-tray beams. With out this filter the isodose curves would be conical in shape. Showing increase x-ray intensity along the central axis and a rapid reduction transversely.

true

T or F: The term collimation refers to blocks that give shape and size to the beam

true