Wk09 Brachytherapy Practce And Planning

Question 1

What is the most common source for HDR brachytherapy?

Answer 1

Ir192

Question 2

What are the common dose rate and activity for HDR brachytherapy?

Answer 2

Dose rate: >12Gy/hr
Activity: 4 to 12 Ci

Question 3

What is the half life of iridium 192

Answer 3

74days
Need replace the source every 4-6 months

Question 4

What is the decay chain of iridium?

Answer 4

Beta decay to excited state of Pt-192
Then electron capture to Os-192

Question 5

What is the half value layer of iridium?

Answer 5

HVL 2.5mm Pb and 44.5mm concrete

Question 6

Name 4 common Ir192 afterloaders

Answer 6

MicroSelectron
Flexitron
BEBIG
Varisource

Question 7

What are the treatment parameters for HDR brachytherapy

Answer 7

1. Dwell positions
2. Dwell times

Question 8

What is the shielded safe made of?

Answer 8

Tungsten or depleted uranium

Question 9

How much air kerma rate is reduced using the shielded safe?

Answer 9

The shielded safe reduce the air kerma rate to 1-4uGy/h

Question 10

What is the check cable drive consist of?

Answer 10

1. Check cable stepper motor
2. Shaft encoder
3. Cable guide tube

Question 11

What is the source cable drive consist of?

Answer 11

1. Source stepper motor
2. Shaft encode
3. Wire-in switch
4. Emergency stoop motor
5. Cable guide tube

Question 12

What is the indexer consist of?

Answer 12

1. Indexer stepper motor
2. Indexes disc
3. Indexer channel encoder
4. Treatment tube
5. Locking ring
6. Channel selection tube

Question 13

What are the safety features in a brachytherapy suite?

Answer 13

1. Emergency switches
2. Emergency crank
3. Door interlock
4. Audio / visual system
5. Radiation monitor / treatment on indicator
6. Emergency service instruments
7. Back-up battery

Question 14

What are the 3 major categories of brachytherapy

Answer 14

1. Interstitial
2. Intracavitary
3. Superficial

Question 15

What is the common dose scheme for combined MRI/CT guided BT for CA cervix?

Answer 15

4 frs of BT with 7 Gy, in 2 applications n consecutive weeks.

Question 16

Which guideline guides brachytherapy dose calculation geometry ?

Answer 16

AAPM TG-43

Question 17

What are the limitations of TG-43 guideline?

Answer 17

• homogeneous water medium is assumed
• full scatter condition assumed
• no electrons is taken into account
• full 3D source geometry is not taken into account

Question 18

What is the use of shielded applicators? (Made by tungsten)

Answer 18

Significantly reduce dose to OARs (rectum / bladder)

Question 19

How can we evaluate a brachytherapy treatment plan?

Answer 19

• dose points
• dose distribution
• dose volume histogram parameters

Question 20

What is IPSA and HIPO?

Answer 20

IPSA = inverse planning by simulated annealing (dwell positions and dwell time varies dramatically)

HIPO = hybrid inverse planning optimisation
(more user friendly and realistic / applicable)

Question 21

What components do TG-43 2D formalism consist?

Answer 21

Source strength
Dose rate constant
Geometry function
Radial dose function
Anisotropic function

Question 22

Where is the reference point of TG-43 2D formalism ?

Answer 22

1cm from the source at 90degrees

Question 23

A typical 10 Ci Iridium192 = _______________ U

Answer 23

40000

Question 24

What is the unit of source strength ?

Answer 24

1 ugym2/h

Question 25

What is the dose rate constant influenced by?

Answer 25

Influenced by the radionuclide and the design of source model

Question 26

What do the geometry function takes into account?

Answer 26

It accounts the inverse square law, neglecting scatter and attenuation

Question 27

What do the radial dose function takes into account?

Answer 27

It accounts for dose fall off in the transverse plane due to photon scattering and attenuation, does not include inverse square fall off which is handled by the geometry function

Question 28

Anisotropy function is always _________ for a perfect point source?

Answer 28

Unity