problemWhen modelling a radiotherapy treatment from different

beam angles, what we want is something like this:

(Patient does not move. EPID moves. Linac moves.)

If the model EPID is simply combined with the patient’s CT

data, producing a single DOSXYZnrc phantom file, then

what we get is something like this:

(Patient does not move. EPID does not move. Linac moves.)

solutionThe methodology behind CTCombine is based on Chin et al's

TWIZ and GLU procedures [1] which simulate a θo gantry

rotation by rotating the CT volume by -θo, padding with it air

and then running the NRC CT conversion code CTCREATE

using the modified CT volume, before combining the

resulting text file with a model EPID [2].

1. Rotate CT data by -θo. Convert to EGSPHANT format.

2. Pad CT data with air. Add EPID.

3. Deliver treatment from 0o gantry angle.

4. This is equivalent to treating from θo gantry angle.

advantagesCTCombine simplifies and builds on previously reported

procedures by:

• combining all three steps (rotating the CT, converting CT

numbers to density and adding the detector) within one

simple program;

• introducing the ability to rotate around a user-defined

isocentre (not necessarily at the centre of the CT-volume);

• introducing the ability to interpolate between CT slices

when the desired Monte Carlo voxel size is smaller than the

slice width;

• using affine transformations to minimise distortion of the

geometry of the patient during rotation;

• introducing the ability to select subsets of the CT data for

inclusion in the final model (allowing couch removal); and

• adapting and re-implementing CTCREATE to allow more

complex CT-number-density relationships to be modelled.

validation against simulationReference simulations:

BEAMnrc was used to model the linac and a rotated solid water slab,

scoring phase space data upstream of the EPID. DOSXYZnrc was used to

model the EPID only.

Reference results:

CTCombine simulations:

BEAMnrc was used to model the linac only, scoring phase space data at the

linac’s exit plane. DOSXYZnrc was used to model both the rotated solid

water slab and the EPID. The DOSXYZnrc phantom used in these

simulations was generated using CT data with CTCombine.

CTCombine results:

Comparisons:

.Virtual EPID dose profiles

(normalised to maxima), showing

good agreement between doses

calculated using BEAMnrc model

and doses calculated using

CTCombine with DOSXYZnrc.

validation against experiment

Reference experiment:

A humanoid phantom, consisting of a plastic skull bone set in a jar of gel,

with an incised air cavity, was irradiated by a 6 MV photon beam from a

Varian Clinac 21iX linac equipped with a BrainLAB m3 micro-multileaf

collimator. The phantom was set up for CT scanning and irradiation using

a standard head board and immobilisation mask, and localised in the

treatment room using the ExacTrac system, according to local standard

procedures for stereotactic radiotherapy treatments. Four small square

fields and one set of four conformal fields were applied from a range of

gantry angles. Images were recorded using a Varian AS500 EPID.

Reference results:

CTCombine simulations:

BEAMnrc was used to model the linac only, scoring phase space data at

the linac’s exit plane. DOSXYZnrc was used to model both the rotated

humanoid phantom and the EPID. The DOSXYZnrc phantom used in

these simulations was generated using CT data with CTCombine.

CTCombine results:

Comparison:

There is qualitative agreement between the image of the conformal

treatment predicted by Monte Carlo, using DOSXYZnrc with CTCreate,

and the image of the conformal field obtained using the clinical EPID.

references and acknowledgements 1. P. W. Chin et al, Physics in Medicine and Biology 48: N231-N238 (2003). 2. T. Kairn et al, Physics in Medicine and Biology 53(14): 3903-3919 (2008).

Dr Kairn’s contribution to this work was funded by the NHMRC, through a project grant, and by the Wesley Research Institute, through grant number 2008/11. The authors wish to thank Darren Cassidy of the Royal Brisbane and Women’s Hospital, as well as

Steve Sylvander and Emmanuel Baveas from Mater Radiation Oncology, for assistance with CT scanning radiotherapy phantoms. The authors wish to thank John Kenny of Premion: first in cancer care, for designing and constructing the humanoid phantom, as

well as for assistance with CT scanning and irradiating that phantom. The authors are also grateful to Varian and Elekta for the provision of manufacturing specifications which permitted the detailed simulation of their linear accelerators and amorphous-

silicon electronic portal imaging devices. Computational resources and services used in this work were provided by the HPC and Research Support Unit, QUT, Brisbane, Australia.

conclusionsUsing CTCombine, it is possible to simulate the EPID imaging of complex radiotherapy treatments, delivered from non-zero gantry angles.

Sample results shown here illustrate the geometric accuracy of CTCombine’s rotation procedure as well as the dosimetric accuracy

obtainable using CTCombine’s CT-number to electron-density conversion procedure, in combination with an accurate model of the EPID.

Simple and complex fields are shown, irradiating planar and anthropomorphic phantoms, to indicate the broad applicability of this code.

The compatibility of the code with BEAMnrc and DOSXYZnrc means that CTCombine can be used to help model treatments delivered by

different types of linear accelerators and radiation sources (including stereotactic devices) and the code is also amenable to the inclusion of

couch supports as well as alternative EPIDs in the model.

CTCombine was primarily designed for use in Monte Carlo studies of EPID dosimetry, but this code might also be applied in Monte Carlo

studies of megavoltage and kilovoltage cone-beam CT imaging. CTCombine therefore has the potential to become a valuable tool for

radiotherapy treatment verification.

abstractEstablished Monte Carlo user codes BEAMnrc and DOSXYZnrc permit the accurate and straightforward simulation of radiotherapy

experiments and treatments delivered from multiple beam angles. However, when an electronic portal imaging detector (EPID) is included in

these simulations, treatment delivery from non-zero beam angles becomes problematic. This study introduces CTCombine, a purpose-built

code for rotating selected CT data volumes, converting CT numbers to mass densities, combining the results with model EPIDs and writing

output in a form which can easily be read and used by the dose calculation code DOSXYZnrc. The geometric and dosimetric accuracy of

CTCombine’s output has been assessed by simulating simple and complex treatments applied to a rotated planar phantom and a rotated

humanoid phantom and comparing the resulting virtual EPID images with the images acquired using experimental measurements and

independent simulations of equivalent phantoms. It is expected that CTCombine will be useful for Monte Carlo studies of EPID dosimetry as

well as other EPID imaging applications.

CTCombine: Rotating and combining CT data with an accurate detector model,

to simulate radiotherapy portal imaging at non-zero beam angles

T. Kairn1, M. Dwyer2, D. Warne2, T. S. Markwell1, J. V. Trapp1, A. L. Fielding1

1. School of Physical and Chemical Sciences, Queensland University of Technology, Brisbane

2. High Performance Computing, Queensland University of Technology, Brisbane

0o rotation 22.5o rotation 45o rotation 90o rotation 30o rotation around

shifted isocentre

1. 3. 4.

2.

0o rotation 22.5o rotation

45o rotation 90o rotation

30o rotation around

shifted isocentre

x-position at EPID

x-position at EPID

x-position at EPID

x-position at EPID

x-position at EPID

No

rma

lise

d d

ose

No

rma

lise

d d

ose

No

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lise

d d

ose

No

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lise

d d

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No

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lise

d d

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0o rotation 22.5o rotation 45o rotation 90o rotation 30o rotation around

shifted isocentre0o rotation 22.5o rotation 45o rotation 90o rotation 320o rotation

Experimental EPID image, conformal fields Virtual EPID image, conformal fields

0o rotation 22.5o rotation 45o rotation 90o rotation 320o rotation