Brachytherapy

Question 1

What is the most common HDR source?

Answer 1

Ir192

Question 2

What is the half-life of Ir192?

Answer 2

74 days

Question 3

What is the gamma factor of Ir192?

Answer 3

4.69 R cm^2/(mCi h)

Question 4

What is the decay energy of Ir192?

Answer 4

380 keV

Question 5

What is the TVL of Ir192?

Answer 5

1.2 cm pb or 15 cm of concrete

Question 6

What is the HVL of Ir192?

Answer 6

0.25 cm Pb

Question 7

What is the general TG43 equation?

Answer 7

Dose rate at distance r and angle theta = SK * Lambda * GL(r,theta)/GL(r0,theta0) * gL(r) * F(r,theta)

Question 8

what is “r” in TG43

Answer 8

distance from the center of the source to the point of interest

Question 9

what is r0 in TG43?

Answer 9

reference distance = 1 cm

Question 10

What is theta in TG43?

Answer 10

The angle between the line connecting the center of the source to the point of interest and the longitudinal axis of the source.

Question 11

What is theta0 in TG43?

Answer 11

The reference angle = 90 degrees or pi/2 radians

Question 12

What is SK in TG43?

Answer 12

Air Kerma Strength

Question 13

How is SK found in TG43?

Answer 13

It’s the air Kerma rate at a distance of d in vacuo (so ignoring attenuation and scatter) due to photons above a cut-off energy (so excluding low-E contamination) TIMES d^2

Question 14

What is the delta cut-off typically for determining the air Kerma rate?

Answer 14

5 keV

Question 15

What are the units of SK in TG43?

Answer 15

U = cGy*cm^2/h

Question 16

What is Lambda in TG43?

Answer 16

Dose rate constant = dose rate at the reference location (r0,theta0)/SK

Question 17

What are the units of Lambda in TG43?

Answer 17

cGy/(h*U)

Question 18

Where do you get SK for TG43?

Answer 18

It’s what you should get with your source? From the company?

Question 19

Where do you get Lambda for TG43?

Answer 19

It’s published and you look it up based on the radionuclide and source model

Question 20

What is GL(r,theta) in TG43?

Answer 20

Geometry function, it’s your inverse square correction

Question 21

What is gL(r) in TG43?

Answer 21

Radial dose function, accounts for dose falloff due to scattering and attenuation and includes the effects of encapsulation, self-attenuation, etc. BUT is only along the transverse plane

Question 22

Where do you get GL in TG43?

Answer 22

Calculate it based on whether you’re using the point-source or line-source approximation.

Question 23

Where do you get gL(r) in TG43?

Answer 23

Either look it up on the table or use a 5th order polynomial fit

Question 24

What is F(r,theta) in TG43?

Answer 24

Anisotropy function - takes into account internal shielding of source that causes the emission to not be totally isotropic

Question 25

What is F(r,90) in TG43?

Answer 25

1

Question 26

What is the most common prostate seed source?

Answer 26

I-125

Question 27

What is the half-life of I-125?

Answer 27

60 days

Question 28

What is the decay energy of I-125?

Answer 28

28 keV

Question 29

What is the typical activity of an HDR source?

Answer 29

10 Ci

Question 30

~ how often is an Ir192 source changed?

Answer 30

quarterly (every 3 months)

Question 31

Why is Ir192 used for HDR?

Answer 31

because of its high specific activity, you can have a high dose rate from a small source (450 Ci/g)

Question 32

What is the clinical dose rate from Ir192?

Answer 32

700 cGy/min @ 1 cm

Question 33

How would you calculate a typical workload for an HDR room?

Answer 33

W = gamma factor (R/mCi/h @ 1m) * 0.97 cGy/R * A (mCi) * treatment time

Question 34

How many seeds do you need to assay for a brachytherapy seed treatment?

Answer 34

10% of the total

Question 35

Write up your radionuclide table thing

Question 36

Why does TG43 only go up to a 2D equation?

Answer 36

Because brachytherapy seeds are cylindrically symmetric, and so is the dose distribution around them. So you rotate that 2D dose around the longitudinal axis and get a 3D distribution.

Question 37

What is the active-length limit of TG43?

Answer 37

sources up to 1 cm long

Question 38

What does TG43 calculate?

Answer 38

Dose rate to water, in water, at a point (r, theta) from the center of the source.

Question 39

at what distance d is air kerma strength determined for TG43?

Answer 39

along the transverse plane at a distance far enough such that your source can be approximated by a point source typically done in air at a distance of 1 m

Question 40

What does Sk * lambda get you in TG43?

Answer 40

Takes you from kerma in air to dose to water in water at your reference location (1 cm, theta = 90)

Question 41

What is a standard prescription for a prostate seed treatment?

Answer 41

145 Gy

Question 42

What are required safety precautions for a brachytherapy implant?

Answer 42

- Put up a radioactive materials sign - survey the room before - survey the room/occupants/items removed after - give patient release instructions/rad safety precautions

Question 43

Packaging and transportation of radioactive material

Answer 43

10 CFR 71

Question 44

How is air kerma strength measured at NIST?

Answer 44

using a wide-angle free air chamber with an aluminum filter to filter out the low-energy photons originating in the source encapsulation

Question 45

How is air kerma strength measured at an ADCL?

Answer 45

using a calibrated well chamber, that's calibrated against a set of NIST-calibrated sources

Question 46

How is air kerma strength measured clinically?

Answer 46

using a well-type chamber that has been calibrated for a particular source design at an ADCL

Question 47

What is a common dose-rate constant?

Answer 47

~1 cGy/hr/U (1.1, etc.)

Question 48

What is a typical workload for brachytherapy?

Answer 48

20-30 cGy/wk

Question 49

What is your expected exposure @1 m from an LDR brachytherapy source?

Answer 49

20-50 mR/hr

Question 50

What mode is typically used for transrectal ultrasound for prostate seed implants?

Answer 50

B-mode (brightness mode) - producing a 2D image of the anatomy surrounding the transducer

Question 51

Where is point A in the manchester system?

Answer 51

2 cm up from the flange (cervical os) and 2 cm lateral from the tandem midline

Question 52

What does point A in the manchester system roughly correspond to?

Answer 52

Primary prescription point, approximately where the uterine vessels cross the ureter, the most dose-limiting structures in the area.

Question 53

Where is point B in the manchester system?

Answer 53

2 cm up from the flange (cervical os) and 5 cm lateral from the patient's midline

Question 54

What does point B in the manchester system represent?

Answer 54

meant to represent dose to the pelvic lymph nodes

Question 55

What are the TG128-recommended tests?

Answer 55

ReD GraDes Are VoluNTary - Resolution - Distance - Grayscale - Depth of penetration - Area measurement - Volume measurement - Needle/Template alignment - Treatment planning computer

Question 56

What are the definitions of HDR and LDR?

Answer 56

HDR > 12 Gy/hr at Rx point LDR > 2 Gy/hr at Rx point

Question 57

What are the recommended QA tests for an afterloader from TG56

Answer 57

SKA RAn Punk's DOME At Viking Speed - Survey meter - Key interlock - Activity - Radiation Alarm - Position accuracy - Dwell timer & Door interlock!!! - Obstruction/Misconnect - Emergency stop - Audio/Visual - Source alert

Question 58

What exposure rate do I expect at the surface of a patient who just got a brachytherapy prostate seed implant?

Answer 58

1.5 mR/h

Question 59

What should you bring with you to a brachy implant?

Answer 59

- radiation materials sign - Survey meter (geiger counter + ion chamber) - lead pig - tweezers - the source for the patient

Question 60

What line do you have to draw on the floor for an LDR treatment?

Answer 60

2 mR/h