Brachy equipment

Question 1

What is clinical brachy dosimetry performed with?

Answer 1

Ionisation chamber

Well chamber

Question 2

What is the source dosimetry based on?

Answer 2

TLD measurements to confirm MC models

Question 3

What is the dose rate of Ir-192?

Answer 3

~700cGy/min

Question 4

How is the source positioned in an HDR Microselectron?

Answer 4

Source diven along tubes using a drive cable which is ~1500mm
Driven at 500-1400mm/s
Forward or reverse stepping
Min step sizes 0.25-1cm
Thinner proximal section of the wire for greater flexibility
Source laser welded to drive cable

Question 5

Why is the mechanism end of the source cable coloured orange?

Answer 5

So it can be identified at source change

Question 6

What are the safety features of a HDR Microselectron?

Answer 6

Shielded source safe (1-6uSv/hr at surface)
Dummy source of separate drive cable which is slightly larger than source - driven past position to ensure no impedence
Battery backup
Optical detector of source leaving safe
Manual drive cable handles
Locking indexer ring/transfer tubes
Multiple Geiger-Muller tubes: 1 integrated, 1 in-room wall mounted, 1 independent wall mounted unit
Gamma ray illuminated sign
Interlocked shielded door with IR curtain
Emergency stops: on TCS, on microselectron, in room button
Audio/visual systems

Question 7

What are the main components of the Microselectron?

Answer 7

Tungsten source safe
Dummy source cable controller
Source cable controller
Emergency stop
Key enable/disable
Indexer ring
Transfer tubes
Manual cable drive controller
Gamma detector
Backup power supply

Question 8

What does the HDR CoP contain?

Answer 8

Based on NPL’s air KERMA primary standard facility
NPL supplied RAKR calibration coefficient NkR
Requires electrometer calibrated for current, I
Future move to absorbed dose to water factors Dw

Question 9

What is the NPL primary standard?

Answer 9

Spherical cavity ionisation chamber for direct measure of RAKR
Lead collimator designed to minimise penumbra and scatter contribution to primary beam from floor to walls

Question 10

What are the secondary standards for Brachytherapy?

Answer 10

Well Chambers - OTW3304, Nucletron SDS, Standard Imaging 1000 plus, Standard Imaging IVB1000

Question 11

What are the characteristics of a Standard Imaging IVB1000 well chamber?

Answer 11

Air vented well ionisation chamber
Sensitive volume 200-475cm^3
Polystyrene encased insert to minimise impact of 'hot' source on thermal equilibrium
Nominal potential = 300V
Ion recombination ~0.1%

Question 12

Why are well chambers used?

Answer 12

Simple geometry
4.pi geometry gives high SNR
Energy independent
Long term stability (+/-0.1%)
Simple setup compared to farmer jig, therefore smaller errors, less sensitive to positioning errors

Question 13

What is the equation for RAKR?

Answer 13

KR = M . ion recombination . source geometry factor . NPL calibration Coefficient (units Gy/s at 1m)

Question 14

How is RAKR determined?

Answer 14

Secondary source placed at the centre of the room (>1m from all walls) on a low scatter surface (plastic trolley) - measure in a well chamber
Initial assessment compares measured RAKR against manufacturer’s coefficient
Difference should be <3%, must be within 5%
Independent check should be within 1% of measured RAKR

Question 15

What are the issues around measuring RAKR?

Answer 15

Very ‘hot’ source may saturate chamber
NPL use specific RAL for a source for calibration - NPL now publish source specific factor
ksg may vary from unity but no data is available

Question 16

What QC testing is required for Brachy?

Answer 16

Source strength
Source positioning
All safety systems
Annual wipe test
Emergency training