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ABR Part III > Brachytherapy > Flashcards

Brachytherapy Flashcards

1
Q

What is the most common HDR source?

A

Ir192

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2
Q

What is the half-life of Ir192?

A

74 days

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3
Q

What is the gamma factor of Ir192?

A

4.69 R cm^2/(mCi h)

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4
Q

What is the decay energy of Ir192?

A

380 keV

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5
Q

What is the TVL of Ir192?

A

1.2 cm pb or 15 cm of concrete

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6
Q

What is the HVL of Ir192?

A

0.25 cm Pb

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7
Q

What is the general TG43 equation?

A

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

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8
Q

what is “r” in TG43

A

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

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9
Q

what is r0 in TG43?

A

reference distance = 1 cm

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10
Q

What is theta in TG43?

A

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

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11
Q

What is theta0 in TG43?

A

The reference angle = 90 degrees or pi/2 radians

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12
Q

What is SK in TG43?

A

Air Kerma Strength

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13
Q

How is SK found in TG43?

A

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

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14
Q

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

A

5 keV

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15
Q

What are the units of SK in TG43?

A

U = cGy*cm^2/h

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16
Q

What is Lambda in TG43?

A

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

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17
Q

What are the units of Lambda in TG43?

A

cGy/(h*U)

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18
Q

Where do you get SK for TG43?

A

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

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19
Q

Where do you get Lambda for TG43?

A

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

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20
Q

What is GL(r,theta) in TG43?

A

Geometry function, it’s your inverse square correction

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21
Q

What is gL(r) in TG43?

A

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

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22
Q

Where do you get GL in TG43?

A

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

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23
Q

Where do you get gL(r) in TG43?

A

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

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24
Q

What is F(r,theta) in TG43?

A

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

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25
Q

What is F(r,90) in TG43?

A

1

26
Q

What is the most common prostate seed source?

A

I-125

27
Q

What is the half-life of I-125?

A

60 days

28
Q

What is the decay energy of I-125?

A

28 keV

29
Q

What is the typical activity of an HDR source?

A

10 Ci

30
Q

~ how often is an Ir192 source changed?

A

quarterly (every 3 months)

31
Q

Why is Ir192 used for HDR?

A

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

32
Q

What is the clinical dose rate from Ir192?

A

700 cGy/min @ 1 cm

33
Q

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

A

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

34
Q

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

A

10% of the total

35
Q

Write up your radionuclide table thing

A
36
Q

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

A

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.

37
Q

What is the active-length limit of TG43?

A

sources up to 1 cm long

38
Q

What does TG43 calculate?

A

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

39
Q

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

A

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

40
Q

What does Sk * lambda get you in TG43?

A

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

41
Q

What is a standard prescription for a prostate seed treatment?

A

145 Gy

42
Q

What are required safety precautions for a brachytherapy implant?

A
  • Put up a radioactive materials sign
  • survey the room before
  • survey the room/occupants/items removed after
  • give patient release instructions/rad safety precautions
43
Q

Packaging and transportation of radioactive material

A

10 CFR 71

44
Q

How is air kerma strength measured at NIST?

A

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

45
Q

How is air kerma strength measured at an ADCL?

A

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

46
Q

How is air kerma strength measured clinically?

A

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

47
Q

What is a common dose-rate constant?

A

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

48
Q

What is a typical workload for brachytherapy?

A

20-30 cGy/wk

49
Q

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

A

20-50 mR/hr

50
Q

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

A

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

51
Q

Where is point A in the manchester system?

A

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

52
Q

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

A

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

53
Q

Where is point B in the manchester system?

A

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

54
Q

What does point B in the manchester system represent?

A

meant to represent dose to the pelvic lymph nodes

55
Q

What are the TG128-recommended tests?

A

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

56
Q

What are the definitions of HDR and LDR?

A

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

57
Q

What are the recommended QA tests for an afterloader from TG56

A

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

58
Q

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

A

1.5 mR/h

59
Q

What should you bring with you to a brachy implant?

A
  • radiation materials sign
  • Survey meter (geiger counter + ion chamber)
  • lead pig
  • tweezers
  • the source for the patient
60
Q

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

A

2 mR/h

ABR Part III (13 decks)

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