Brachytherapy

Question 1

Give some examples of interlocks that need to be checked in an HDR suite

Answer 1

Guide tube length
ALMO radiation monitor
Interrupt buttons
Emergency stop
Door interlock
Battery backup

Question 2

What does RAKR stand for

Answer 2

Reference Air Kerma Rate

Question 3

Define RAKR

Answer 3

Reference Air Kerma Rate

The kerma rate to air, in air, at a reference distance of 1m, corrected for air attenuation and scattering

Question 4

Where is RAKR defined in the literature?

Answer 4

IPEM COP for HDR BT

Question 5

Why is using a well chamber better than using a thimble chamber for calibration?

Answer 5

Less sensitive to source positioning errors due to 4pi geometry.
Much smaller uncertainty
Hard to get reproducible positions when using a thimble
Much quicker to take measurements with a well chamber
Transfer uncertainty between well chambers smaller than between Farmer chambers

Question 6

What is the primary standard for RAKR at the NPL?

Answer 6

Graphite-walled cavity ionisation chamber

Question 7

What are the two steps for the calibration of secondary standard chambers?

Answer 7

RAKR of Ir-192 source determined using NPL primary standard

Calibrated source used to calibrated SS instrument

Question 8

What is the source calibration set-up used at NPL?

Answer 8

Source inside a lead collimator with aperture such that the direction from the source centre to the reference point is at right angles to the long axis of the source.

Collimated beam directed towards primary standard

Centre-to-centre source-to-chamber distance 1433mm

Question 9

Why are the source-to-chamber distance and aperture sizes chosen as they are at the NPL calibration?

Answer 9

To give a uniform field over the whole of the ionisation chamber

Question 10

What correction factors are applied to determine the RAKR

Answer 10

Temperature and pressure
Normalised to 1m
Air attenuation
Scatter correction factor
Primary standard sensitivity

Reference time and date

Question 11

What is the secondary standard calibration set-up that should be used?

Answer 11

Well chamber 1m from any wall and 1m above floor level on a low scatter surface.

Time for chamber to reach thermal equilibrium.

Bias voltage s.t. central collecting electrode is positive wrt outer electrode.

Question 12

What is the position of the source that is used in the chamber for calibration?

Answer 12

The “sweet spot”, or position of maximum response .

Question 13

What is the equation for RAKR in terms of Gy/s at 1m?

Answer 13

Kdot_R = M x k_ion x k_sg x N_Kdot_R

Question 14

What correction factors are applied to the instrument response?

Answer 14

Electrometer correction factor
Decay correction factor to correct the measured current to a reference time
Temperature and pressure

Question 15

At what threshold should discrepancies between the manufacturer’s source certificate and the measurement be investigated?

Answer 15

3%

Question 16

At what threshold discrepancy between calculation and certificate should a source not be used clinically?

Answer 16

5%

Question 17

How can the ion recombination be measured for a well chamber?

Answer 17

kion = (4/3 - (I300V/3xI150V))^-1

Two polarising voltages used

It is the reciprocal of the ion collection efficiency

Question 18

How is the HDR source position checked?

Answer 18

Source step viewer - can see using the CCTV camera and where the source wire lines up.

Use Gafchromic film, tape the applicator to it at known positions and then expose each region a known distance apart.

Question 19

What is transit time?

Answer 19

The time for the source to reach the dwell position; the time it is moving along the guide tube

Question 20

What is the tolerance on the dwell position time?

Answer 20

0.5s

Question 21

What is the tolerance on the difference between measured time and actual time - the transit time?

Answer 21

<2s.

Question 22

How is linearity measured for HDR BT?

Answer 22

Charge vs actual dwell time is plotted and should be linear

Question 23

What is the dose rate range for HDR?

Answer 23

12Gy/h or more

Question 24

What is the dose rate range for LDR?

Answer 24

0.4 to 2 Gy/h

Question 25

What is the definition for PDR?

Answer 25

Dose "pulses" given for about 30mins separated by gaps in the order of hours.

Question 26

What is the level 1 standard for reporting BT plans?

Answer 26

The minimum of data required to perform BT in an efficient and safe way. Total Reference Air Kerma Point A dose Recto-vaginal reference point dose D0.1cc, D2cc for bladder, rectum

Question 27

What is level 2 reporting for BT?

Answer 27

``` Must contain all the information of L1 plus information to perform a state of the art tx Dose reporting for defined volumes: D98, D90, D50 for CTV_HR D98 for GTV_res D98, D90 for CTV_IR if used for px ``` Dose reporting OARs

Question 28

What is the definition for the TRAK?

Answer 28

Total Reference Air Kerma The sum of the products of the RAKR and the irradiation time for each source.

Question 29

What are the units for U?

Answer 29

uGym^2/h | or cGycm^2/h

Question 30

What is the equation for total air kerma strength?

Answer 30

S_k = A x N x exp((-ln(2)/59.4) x t)

Question 31

What is the equation to convert total air kerma strength to dose?

Answer 31

D = S_k x T(x) x (100/d)^2 x f x h x 1/1000 ``` T(x) = tissue transmission factor d = distance from point of interest to centre of implant f = conversion factor from air kerma to dose in tissue h = time in hours of the surgery ```

Question 32

What are the two types of calculations for sources given in TG43?

Answer 32

Line and point sources

Question 33

What are some limitations of TG43?

Answer 33

``` Assumes full scatter conditions Assumes homogeneous water Source to source shielding not accounted for Applicator shielding not accounted for Doses close to sources to accurate ```

Question 34

What does the dose rate constant, Lambda, define, and what are it effects in the TG43 2D formalism?

Answer 34

The dose rate to water at 1cm per unit air kerma strength. Includes effects of: source geometry, activity distribution, encapsulation, self-filtration, scattering in water, changes in S_k.

Question 35

What does the geometry factor (G(r,\theta)) account for in the TG43 2D formalism?

Answer 35

Variation of relative dose due only to the spatial distribution of activity within the source. Different value for point source or line source approximation. Purpose for interpolation between tabulated data points so the simplistic forms still yield accurate results.

Question 36

What does the radial dose function, g(r), account for in the TG43 2D formalism?

Answer 36

The effects of absorption and scatter in the medium along the transverse axis of the source.

Question 37

What does the anisotropy function, F(r,theta), account for in the TG43 2D formalism?

Answer 37

Anisotropy of the dose distribution around the source, including the effects of absorption and scatter in the medium. Includes effects of: self-filtration, encapsulation, scattering in medium. It is unity for a point source.

Question 38

What are some advantages of the TG43 model?

Answer 38

Specific data for each source model RAKR/Sk traceable to primary standards Rigorous process to get the parameters Fast computation

Question 39

What regulations regarding security do HDR users need to be aware of?

Answer 39

EPR 2016 HASS regulations Transport of Dangerous Goods

Question 40

What brachytherapy TPS QA needs to be done?

Answer 40

Compare TPS calculated dose to manually calculated at a number of points Check source decay Recalculation of standard source arrangement to check consistency of calculated source dwell times.

Question 41

How would you go about verifying dose calculation of a TPS?

Answer 41

Scan phantom of well-defined geometry Position source at px points inside phantom Compare TPS calculated dose rates/dwell times with independently calculated values