Water Equivalence Of Life Assessment Answer

Assignment 2

• This lecture describes Assignment 2.

• Assignment 2 involves modelling a Lithium Fluoride thermoluminescent dosimeter (TLD).

• Ultimately, we are never interested in the dose to a dosimeter. Rather, we are interested in the dose to the medium at the spatial location occupied by the dosimeter.

• We frequently deal with dose to water in radiotherapy. Ideally, the radiological properties of a dosimeter placed in water will match those of water. However, all detectors exhibit some level of energy dependence, particularly in energy regimes where interaction processes are highly Z dependent (such as the photoelectric and pair production regimes).

Assignment 2

• In practice, a TLD used in a radiotherapy context would be calibrated at reference conditions (e.g. 10 cm depth, 90 cm SSD and a 10×10 cm2 field).

• However, under different conditions, spectra will be different (for instance, different depths, different field sizes, off-axis locations, irradiation with different sources, such as brachytherapy or kV sources).

• The question becomes – how great is the influence on the dosimeter for different energies/spectra?

• In this project, we will try to evaluate this for LiF type dosimeters. For simplicity we will ignore the radiological effects of dopants, whose influence we anticipate to be slight (Taylor 2011 Nucl. Instr. Meth. B 269:770-773).

Assignment 2

• We will make a further assumption in discussing energy response: that the light output by the TLD is directly proportional to the absorbed dose in the TL material.

• In this project, the response to monoenergetic photons will be modelled, and an assessment of water equivalence will be undertaken.

Using multiple CPU cores on Jack

• Assignment 2 requires more histories to be run than previous tasks

–This means more processing power is needed to complete the simulations in a

reasonable amount of time.

• Romeo has 24 CPUs

–Romeo is a virtual machine (VM) –Very small for a HPC cluster

–NCI’s ‘Raijin’ at The Australian National University has 84,656 cores –More than enough for our purposes –During class don’t run batch mode with n > 2

–~15 people all wanting 16 cores at the same time will lead to lengthy waits in

the queue –Outside class n = 4 – 8 should be fine, just check the load at the time (qstat –q)

Using multiple CPU cores on Jack

• When running large simulations you should not run on the login node

–You should run them in “batch” mode even if you only want a single processor –E.g. exb dosrznrc dosrznrc_template 521icru batch=pbs batch p=1

Execute in batch mode

Usercode Input file PEGS4 data file

Queuing system

Queue name

Number of CPUs to use

Assignment 2: Considerations for input

• Media (defining materials to be used)

• Geometry definition (physical dimensions)

• Assignment of media to regions

• Scoring system (what results do we want?)

• Source definition – geometry and energy

• Cutoff energies

• Go to your egsnrc/dosrznrc directory

• Copy the template file

dosrznrc_template.egsinp

to a new file with an appropriate and descriptive name

• In the new file change the first line TITLE to, again, something appropriate and descriptive

– Use a meaningful description – this text appears in output files, so its well worth

while changing it for each project and variation

• We are going to model a LiF TLD chip of dimensions:

–Diameter: 4 mm, Thickness: 2 mm

• The TLD will be submerged in water at a depth of 5 cm.

• Model the water phantom such that there is 5 cm of water both in front and behind the TLD, with a total diameter of 10 cm.

• See the diagram on the following slide.

Assignment 2: Geometry

R=5

R=0.2 IRL=1

R=0 R=0.2

R=5

Z=0 Z=5 Z=5.2 10.2

IRL=5, Water

IRL=6, Water

IRL=7, Water

IRL=2, Water IRL=3, LiF IRL=4, Water

IRL=5, Water

Assignment 2: Equivalent geometry for comparison with water

• Ultimately, we want to be able to compare the dose scored in the LiF TLD and compare it to the dose to water for equivalent geometrical arrangements.

• This allows us to evaluate its ‘water-equivalence’. Water equivalence plays a strong role in dosimetry, because of the advantages of having media-matched dosimeters (refer to any radiation dosimetry textbook for a detailed explanation).

• Therefore, in this project it is necessary to construct an entirely equivalent geometry, with no TLD (i.e. water only).

• Thus, you will have 2 input files for each arrangement – one with a TLD, and one without.

–Best to make these two versions after editing all the rest of the inputs

Assignment 2: Defining LiF in PEGS4

• You may need to create PEGS4 data for LiF (lif521.pegs4dat)

• Copy the compound template pegs4 input file

Copy command File to copy Where you want to copy to

– cp $HEN_HOUSE/pegs4/inputs/p4icomp.pegs4inp $EGS_HOME/pegs4/inputs/lif521.pegs4inp

“HEN_HOUSE” env variable /opt/egs

“EGS_HOME” env variable /home/<username>/egsnrc

Assignment 2: Calling pegs4.exe to create the LiF pegs4 data file

• Go to your egsnrc/pegs4 directory

• Run ‘p4 –i lif521’, this will create a new datafile in the egsnrc/pegs4/data directory lif521.pegs4dat as well as diagnostics files in your egsnrc/pegs4 directory pegs4.log.

• Use the command ‘wc – l lif521.pegs4dat’ to check the length of the lif521.pegs4dat, zero length means a failure (should be about 684 lines long). –For more information on the command wc query the Linux manual by typing 

  man wc Note: ‘man p4’ will yield nothing as it is an alias to pegs4.exe file, and pegs4 is not a Linux command. To get help, run pegs4.exe --help.

• Failure usually means you made a typo check the diagnostics file data/pegs4.log and the input file.

• In order to get a file containing both water and LiF, you can generate it by adding H2O entries to your pegs4 input file, or simply concatenate your LiF .pegsdat file with the regular PEGS4 data file:

from in your pegs4/data directory use the command:

• cat lif521.pegs4dat $HEN_HOUSE/pegs4/data/521icru.pegs4dat > lif.pegs4dat

• Go back to your egsnrc/dosrznrc directory and edit the MEDIA variable in the dosrznrc_lab.egsinp file to:

MEDIA= LIF,H2O521ICRU

–Note: This ignores density effect correction

Assignment 2: Assign media to regions

• In the input file assign the media to the regions

DESCRIPTION BY= regions MEDNUM= 1,2,2 START REGION= 3, 1,4 STOP REGION= 3, 2,7

DESCRIPTION BY= regions MEDNUM= 2,2,2 START REGION= 3, 1, 4 STOP REGION= 3, 2, 7

LiF + water water

Assignment 2: Source definition

• Use a parallel photon beam, of diameter 3 cm.

– Which source number is this? (see Table 1 in section 2.7 of the RZ manual) – What source options need to be specified?

• In this study, we aim to construct a plot of dose to the TLD as a function of energy.

• Note we also want the dose to water at the equivalent location for the same energies.

• As such, you will need to run numerous simulations, each with a different monoenergetic source.

• Use the following energies:

• 0.1, 0.2, 0.3, 0.4, 0.5, 1.0, 2.0, 5.0, 10.0, and 20.0 MeV photons

• In the previous example cross sections have been defined for energies down to 10 keV photons and electrons with 10 keV kinetic energy (0.521 MeV total energy), so ECUT=0.521 PCUT=0.010

• Consider your modelled arrangement. The volume occupied by the TLD is very small relative to the entire geometry. Thus, the number of incident particles actually depositing energy in the TLD is small. What effect will this have on the statistical uncertainty of your calculations? Make sure you run sufficient histories to have meaningful results and be able to undertake a sensible comparison between TLD dose and water dose.

• Also consider the energy cutoffs. Are they sufficiently low considering the size of the ROI?

• The scoring system has a number of parts.

–The program needs to be told the basis of scoring. –The program needs to be told regions to be scored.

• Dose scoring is the default for DOSRZ.

IFULL= dose and stoppers

Note this is in the uppermost section of the file

• Score doses in the LiF region (you can score dose in others, too, but you must score it in the region of interest)

• (recall that Region 1 is outside the geometry... before the first plane, after the last plane, and outside the last cylinder)

DOSE ZBOUND MIN= 1 #Min plane number defining dose region DOSE ZBOUND MAX= 4 #Max plane number defining dose region DOSE RBOUND MIN= 0 #Min cylinder defining dose region

#(could also start at 1) DOSE RBOUND MAX= 2 #Max cylinder defining dose region

Assignment 2: Output files

• dosrznrc_lab.egsinp

• dosrznrc_lab_dd.plotdat Dose Depth

• dosrznrc_lab_rad.plotdat Radial Dose

• dosrznrc_lab.egsdat

• dosrznrc_lab.egslst

• dosrznrc_lab.errors

• You can use the WinSCP/FileZilla to transfer the files to your local computer and you can open them with excel / word, Matlab etc.

• We’re only really interested in dose deposited in one or a few regions, so plotting isn’t really necessary, but it is one way of viewing output data

– please yourself

What regions? Off or on

Assignment 2: The influence of energy on water equivalence

• You can use the EXAMIN usercode to investigate the interaction data for photons in the media (H2

  O and LiF) as a function of energy. – i.e. to generate plots of the cross sections, mean free path, attenuation coeff’s, etc in the pegs4 data.

>examin –p pegsdatfile which prompts for some options... – First specify a material in the pegsdata by its name (e.g. “PB521ICRU”) – Provide a title if you want – Specify which cross sections you want and where to output them

• There are entries for EXAMIN in the EGSnrc manual (PIRS-701)

• Transfer the output file to your local computer and graph the relative interaction probabilities as a function of energy in Excel.

• Does this data help you explain the variation of water equivalence with photon energy?

• You need to construct a report of your results, similar to that for the previous project (i.e. the style of a scientific publication).

• You should report on the response of TLDs across the energy range.

• You also need to indicate the water equivalence of the LiF TLD (do this in a readily interpretable fashion).

• Data should be presented graphically where possible and explained in the main body of text. You should present a detailed discussion.

• Discuss the implications of your results. Compare with published data.