Dosimetry Midterm

Front 1

Factors that Affect Monitor Units

Back 1

-beam energy
-calculation depth
-prescribed dose
-tissue density
-source to skin distance
-source to axis distance
-collimator field size
-treatment field size
-beam modifiers

Front 2

Dmax

Back 2

-depth where electronic equilibrium occurs for photon beams
-depth of maximum dose or given dose
-dependent on photon energy
-provides skin sparing effect, which is diminished by any sort of bolus
-point where %DD=100
-%DD falls less rapidly as the photon energy increases

Front 3

Dmax of Co-60 beam

Back 3

0.5 cm

Front 4

Dmax of 4 MV beam

Back 4

1.0 cm

Front 5

Dmax of 6 MV beam

Back 5

1.5 cm

Front 6

Dmax of 10 MV beam

Back 6

2.5 cm

Front 7

Dmax of 15 MV beam

Back 7

3.0 cm

Front 8

Dmax of 18 MV beam

Back 8

3.5 cm

Front 9

Dmax of 24 MV beam

Back 9

4.0 cm

Front 10

Half-Value Thickness

Back 10

-thickness of a material that will reduce the beam's intensity or penetration to half it's original value
-used to describe orthovoltage beams
-usually aluminum, copper, or tin
-high energy beams are described in MV or the accelerating voltage

Front 11

Dose Rate

Back 11

-measurement of dose of a treatment machine
-describes the amount of exposure produced and measured at a specific distance for a reference field size of 10 x 10

Front 12

Dose Rate & Field Size

Back 12

-as field size increases, dose rate increases
-due to increased scatter output
-not recommended to use a field size less than 4 x 4

Front 13

Dose Rate & Distance

Back 13

-inverse square law represents the change in beam intensity caused by divergence of the beam
-as the beam diverges, the beam intensity decreases

Front 14

Inverse Square Factor

Back 14

(SSD1/SSD2)2

Front 15

Distance Factor Correction

Back 15

-used for isocentric calculations
-(Dmax depth of beam + 100/100)2

Front 16

Percent Depth Dose
%DD
PDD

Back 16

-ratio expressed as a percentage of the absorbed dose at a given depth to the absorbed dose at a fixed depth, usually Dmax
-determined using the field size treating the patient
-used for SSD calculations
-also called Depth Dose Ratio, or DDR

Front 17

Factors That Affect PDD

Back 17

-energy
-SSD or depth
-field size
-tissue type
-distance from source

Front 18

PDD & Energy

Back 18

-PDD increases as energy increases
-higher energies are more penetrating, so a greater percentage is available

Front 19

PDD & SSD

Back 19

-PDD increases as SSD increases due to the Mayneord F factor
-due to the inverse square law and increased scatter production

Front 20

Mayneord F factor

Back 20

-inverse square correction for the PDD

-F Factor=
(SSD2 + Dmax)2 (SSD1 + depth)2
----------- x ------------ x
(SSD1 + Dmax) (SSD2 + depth)
PDD old

or

PDD of SSD2=F x PDD of SSD 1

Front 21

PDD & Field Size

Back 21

PDD increases as field size increases due to increased scatter production

Front 22

PDD & Depth

Back 22

-PDD decreases as depth increases
-dose is deposited in tissue as it traverses, thus a smaller percentage is available at greater depths

Front 23

PDD & Medium

Back 23

PDD varies depending on the type of tissue, as some tissue attenuates more of the beam than others do

Front 24

Tissue Air Ratio
TAR

Back 24

-ratio of absorbed dose at a given depth in a phantom to the absorbed dose at the same point in free space
-same characteristics as PDD except TAR is independent of SSD

Front 25

Characteristics of TAR

Back 25

-TAR increases as energy increases
-TAR increases as field size increases
-TAR decreases as depth increases
-TAR is independent of SSD

Front 26

Tissue Phantom Ratio
TPR

Tissue Maximum Ratio
TMR

Back 26

-ratio of the absorbed dose at a given depth in a phantom to the same dose at the same point at a reference depth in a phantom
-TMR when the reference depth is Dmax
-determined using the field size treating the patient
-used for isocentric calculations

Front 27

Characteristics of TMR

Back 27

-the same as TAR
-TMR increases as energy increases
-TMR increases as field size increases
-TMR decreases as depth increases
-TMR is independent of SSD

Front 28

SSD

Back 28

-SSD setups are when the isocentric machine distance is established on the patient's skin
-PDD is used for calculations

Front 29

SAD

Back 29

-SAD setups are when the isocentric machine distance is established within the patient
-TMR is used for calculations

Front 30

Isodose Distribution

Back 30

-dose distributions are usually measured in a phantom for different field sizes and then plotted in terms of isodose curves

Front 31

Geometric Field Size

Back 31

-usually defined at the intersection of the 50% PDD isodose line and the surface on the isodose curves

Front 32

Penumbra

Back 32

region near the edge of the margin between the 90% and 20% lines

Front 33

Factors Influencing Penumbra

Back 33

-size of radiation source, with lower energy having greater penumbra
-distance from the source to the distal part of the collimator
-SSD

Front 34

Flattening Filter

Back 34

flattens the beam by reducing the dose along the CAX, producing a flat beam at a specified depth, usually 10 cm

Front 35

Dose Normalization

Back 35

isodose distribution concept used when doing external beam planning

Front 36

Dose Profile

Back 36

used for beam flatness and symmetry measurements, shows the variation of dose across the field at selected depths

Front 37

Collimator Scatter Factor
Sc

Back 37

-determined by field size as defined by collimator opening
-also known as output factor
-ratio of the beam output in air to a reference field size of 10 x 10 cm

Front 38

Sc & Field Size

Back 38

-as field size increases, the beam output increases due to the increase in scatter
-collimator scatter is added to the primary beam

Front 39

Scatter Phantom
Sp

Back 39

-the ratio of a dose in a phantom for a given field at Dmax to the dose at the same point for a 10 x 10 cm field, with the same collimator opening
-determined by field size
-takes all blocking into account

Front 40

Tray Factors

Back 40

-trays are made with acrylic, either solid or slotted, and may vary in thickness
-useful for supporting cerrobend blocks, compensators, or total body irradiation screens
-set factors in calculation book based on beam energy

Front 41

Wedges

Back 41

-beam modifying devices that are inserted into the pathway of the beam which attenuate the beam
-attenuation is greater at the thicker end (heel) than at the thinner end (toe) of the wedge, resulting in a tilt of the isodose curve (if the beam profile is for treating a flat surface)

Front 42

Wedge Angle

Back 42

-the angle through which an isodose line is tilted at the CAX of the beam and the 50% isodose curve
-not the actual angle of the wedge

Front 43

Wedge Factor

Back 43

-included in the dose calculation to compensate for the dose at CAX
-set values in the calculation book

Front 44

Universal Wedge System

Back 44

-system that is fixed in the beam for all field sizes
-center of the wedge is aligned to the CAX of the beam

Front 45

Dynamic Wedge System

Back 45

-dynamic motion of independent jaws moving within the treatment beam
-dynamic MLCs are in use with IMRT, replacing the need for wedges

Front 46

Monitor Unit

Back 46

number set on the console to deliver a prescribed dose of radiation

Front 47

Equivalent Square Equation

Back 47

Equivalent Square= (area/perimeter) x 4

Front 48

Tolerance for Kidney

Back 48

23 Gy to 1 kidney

Front 49

Tolerance for Intestine

Back 49

40 Gy

Front 50

Tolerance for Lens of the Eye

Back 50

10 Gy

Front 51

Tolerance for Liver

Back 51

30 Gy

Front 52

Tolerance for Spinal Cord

Back 52

45 Gy

Front 53

Given Dose

Back 53

-point where PDD is 100%
-applied dose
-entrance dose
-peak dose

Front 54

Calculations for SSD Treatments

Back 54

dose at Dmax, Dmax is given dose

Front 55

Calculations for Points Other Than Dmax

Back 55

given dose (Dmax) will be greater than prescribed dose, as in the case of a direct field

Front 56

Calculating a Dose Delivered to a Depth Different than Dmax

Back 56

Direct Proportion
Tumor Dose Will Be Lower Than Given Dose

Front 57

Exit Dose

Back 57

-dose absorbed by a point located at the depth of Dmax at the exit of the beam
-used when calculating parallel opposed SSD fields
-must know Dmax of the beam and patient separation through region of interest

Front 58

How to Determine total Dmax Dose

Back 58

-calculate anterior dose contribution to Dmax (entrance dose)
-calculate posterior dose contribution to Dmax (exit dose)
-add anterior and posterior dose contributions for total Dmax dose

Front 59

Half-Life of Co-60

Back 59

5.3 years

Front 60

Treatment Delivery on a Co-60 machine

Back 60

timer is used instead of MU

Front 61

Equation for Cobalt Treatment Adjustments

Back 61

new minute setting=old minute setting x 1.01

Front 62

Extended Distance Calculations

Back 62

-allow larger field areas to be treated by extending the distance from the source to the patient
-due to divergence
-always set up SSD
-PDD used for calculation

Front 63

SSD for Co-60 Machines

Back 63

usually 80 cm

Front 64

Monitor Unit Equation

Back 64

MU=Dose/ [Sc x Sp x DF x TF x OAF x NF x WF x (PDD {also called DDR} or TMR)]

Front 65

Calculating Monitor Units for SSD Treatments

Back 65

-no distance factor is used
-PDD used

Front 66

Calculating Monitor Units for Isocentric Treatments

Back 66

-distance factor correction
-TMR used

Front 67

Determining Sp for SSD Treatments

Back 67

defined at patient's skin

Front 68

Determining Sp for Isocentric Treatments

Back 68

defined at isocenter

Front 69

Distance Factor
DF

Back 69

-determined using the inverse square law
-in unity when SSD=100 cm
-distance factor listed on bottom of linac calculation sheets

Front 70

Calculating Distance Factor for Extended SSD Treatments

Back 70

-DDR is used
-DF=(100 + d/SSD + d)2 where d is treatment depth
-distance factor also includes Mayneard's F factor

Front 71

Calculating Distance Factor Using TMR When Calculation Point is Not At the Isocenter

Back 71

-DF=(100 + Dmax/SSD + d)2 or DF=(Dmax + 100/100)2

Front 72

Off Axis Factor
OAF

Back 72

-factor used to correct for dose differences between the central axis and the calculation point
-in unity when and only when the dose calculations are performed on the central axis

Front 73

Normalization Factor
NF

Back 73

corrects for any normalization that occurs in a patient plan

Front 74

Reasons SAD Treatments are Used Preferentially To SSD Treatments

Back 74

-usually no reason to move patient after isocenter has been established
-reduces risk of errors due to positioning variations that occur during patient movement

Front 75

Calculating Dose To a Point Other Than Isocenter Using SAD Technique

Back 75

-field size defined at isocenter, not skin surface
-inverse square correction or distance factor correction needed

Front 76

Standard Factors Used To Calculate SAD Techniques

Back 76

-TAR
-TMR
-TPR
-factor used determined when initial beam outputs are measured on the machine

Front 77

When TAR Is Used

Back 77

usually for low energy beams like 4 MV

Front 78

Energies For Which TMR and TPR Are Used

Back 78

usually for high energy beams like 10 MV

Front 79

Substitution For TMR Values

Back 79

can be broken down into Collimator Scatter Factor (Sc) and Phantom Scatter Factor (Sp) multiplied together to give one output factor

Front 80

Source to Calculation Point Distance
SCPD

Back 80

prescribed dose for the field at some point

Front 81

Determining Total Dmax Dose for Single Field, SAD setup

Back 81

-must know SCPD for each point, TMR at each SCPD, and equivalent square at each SCPD
-Dose A=
Dose B x (SCPD@B)2 x TMR@A
------ --------
TMR@B (SCPD@A)

Front 82

Field Weighting

Back 82

-weighting sum is sum of the ratio (3:1 ratio has weighting sum of 4)
-divide total dose by weighting sum
-multiply by any part of the ratio greater than 1
-divide if multiple fields share any part of the ratio

Front 83

November 8, 1895

Back 83

William Conrad Roentgen discovers the first x-ray

Front 84

January 29, 1896

Back 84

first therapeutic x-ray used to treat breast cancer

Front 85

1908

Back 85

-radiation therapy first used in the United States
-used by surgeons and dermatologists
-dosed by erythema dose
-x-rays in kilovoltage range until 1950s

Front 86

Forms of Radiation Under 300 kVp

Back 86

-Grenz Rays: less than 20 kV
-Contact Therapy: 40-50 kV
-Superficial Therapy: 50-150 kV
-Orthovoltage Therapy: 200-300 kV

Front 87

1928

Back 87

Roentgen is introduced as unit of measurement for x-rays and gamma rays

Front 88

1934

Back 88

combination external and internal 'brachytherapy' given

Front 89

1953

Back 89

-Rad is introduced as unit of measurement for absorbed dose
-recommended by the IRCU

Front 90

1980

Back 90

-Gray replaces the Rad

Front 91

Converting Gray to Rads

Back 91

1 Gy=100 rads
1 cGy=1 rad

Front 92

1940-1960's

Back 92

-supervoltage machines used with energies greater than 1 MV
-Co-60 treatment machines used
-treatment planning computers introduced

Front 93

1960-1990

Back 93

major advancements in technology benefit treatment machines and planning computers

Front 94

Treatment SSDs

Back 94

4 MV with 80 SSD
6 MV with 100 SSD

Front 95

1990's to present

Back 95

-IMRT
-IGRT
-CBCT
-Gamma Knife
-Cyberknife
-Tomotherapy
-more focus on restricting beam to spare tissue

Front 96

Varian Trilogy Accelerator Component

Back 96

-all capabilities of 21 EX or 23 EX
-dose rate of up to 1000 MU/min
-tighter radius isocenter for gantry and collimator axes
-smaller couch rotation motions of under 2 cm and under 2"

Front 97

IMRT

Back 97

-delivery of radiation fields designed to reproduce the shape of the tumor precisely
-computer planning allows radiation to be delivered heterogeneously within a treatment field, fields to conform to complicated shapes, and concave structures to be treated

Front 98

IGRT

Back 98

-delivery of radiation using on-board imaging to localize treatment volumes daily at the time of treatment
-3-D scans allow us to minimize dose to healthy tissue and accurately treat areas of the body that are subject to organ motion

Front 99

CBCT

Back 99

on-board imaging that uses a cone-shaped x-ray beam (as opposed to a fan-shaped beam) to create a three-dimensional image of the patient just before they are treated, allowing us to verify patient alignment and dose during daily treatments

Front 100

Gamma Knife

Back 100

-uses gamma radiation from Co-60 sources to irradiate tumors, predominantly in the brain
-houses source in a heavily shielded assembly, retracting shielding from the appropriate sources to expose the target volume through stereotactic radiosurgery, using radiation from many paths to deliver high doses of radiation to the tumor at one time without causing excessive damage to the normal tissues

Front 101

Cyber Knife

Back 101

-uses x-rays to irradiate through radiosurgery
-images region of interest during treatment
-can track movement and treat the patient while tissue is moving

Front 102

Tomotherapy

Back 102

-delivery of radiation using spiral CT to verify the location of the treatment volume prior to daily treatments
-uses IMRT and daily verification to acheive greater precision when irradiating structures that have the potential to move
-capable of treating using stereotactic radiosurgery and stereotactic body radiotherapy

Front 103

Problem Solving MUs

Back 103

-find equivalent square of the collimator setting
-find equivalent square of the blocked field size
-look up dose rate factors
-determine the prescribed dose

Front 104

Usual Dose Rate Factors

Back 104

-Sc or Sp
-TMR or PDD

Front 105

Problem Solving for Extended Distance

Back 105

-find equivalent square of the collimator setting to be used for output factor
-find equivalent square of the blocked field size to be used for the PDD
-look up the dose rate factors
-determine the prescribed dose
-calculate the PDDs using Mayneord's factor
-determine the time setting using the appropriate equation

Front 106

Isocentric Calculation Process

Back 106

-find the equivalent square of the collimator setting to be used for output factor
-find the equivalent square for the blocked field size to be used for TAR, TMR, or TPR
-look up the dose rate factors
-determine the prescribed dose
-use the appropriate equation to determine the time setting

Front 107

Geometric Matching

Back 107

Gap=
(Length1 x depth of abutment)
-------- ----------------- +
(2 SSD1)
(Length2 x depth of abutment)
-------- ------------------
(2 SSD2)