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Radiation Measurements 46 (2011) 1646e1649

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Radiation Measurements

journal homepage: www.elsevier .com/locate/radmeas

Monte Carlo modelling of a silicon strip detector for microbeam radiation therapy

Ashley Cullen*, Michael Lerch, Marco Petasecca, Anatoly RosenfeldCentre for Medical Radiation Physics (CMRP), University of Wollongong, Wollongong, Australia

a r t i c l e i n f o

Article history:Received 22 November 2010Received in revised form16 July 2011Accepted 17 July 2011

Keywords:DosimetrySilicon detectorMonte carlo modellingMicrobeam radiation therapy

* Corresponding author. Tel./fax: þ61 0 2 85397584E-mail address: ajc984@uow.edu.au (A. Cullen).

1350-4487/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.radmeas.2011.06.051

a b s t r a c t

Microbeam radiation therapy is an experimental technique utilising synchrotron X-rays collimated intoa planar array of microbeams. Due to the complex structure of the radiation field and high dose rate, thisintroduces dosimetric challenges. Current dosimetric methods are inadequate in that they lack eitherreal-time readout, or high spatial resolution. A detector system, consisting of the Silicon Multi-StripDetector and associated readout system was developed at the University of Wollongong. This systemperforms online, real-time dosimetry, and is designed for placement upstream of the patient asa transmission detector. The interaction of synchrotron radiation with this detector, both in terms ofperturbative effects on the beam and interactions within the detector are investigated with the MonteCarlo simulation toolkit, Geant4. The interaction percentage of the detector with the traversing beamwasdetermined as being (1.97 � 0.01) %. Energy deposition was found to be uniform across the depth of thedetector, negating the most proximal and distal 5-mm region, a phenomenon that may be attributed toa lack of charged particle equilibrium. The effect that the detector has on a traversing beam’s macro-scopic dose deposition was determined as being a mean reduction in dose of (1.44 � 0.15) % for depths inwater between 5 and 35 mm.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Microbeam radiation therapy (MRT) is an experimental radio-surgical method currently under investigation for the treatment ofotherwise inoperable neurological lesions. It involves the deliveryof a large, yet biologically tolerable dose to amacroscopic treatmentvolume via the use of a planar array of rectangular microscopicphoton microbeams (the peak region) separated by intermediateregions of minimal dose (the valley region), contributed to only byscattering and secondary particles (Slatkin et al., 1992; Blattmannet al., 2005). The radiobiological efficacy of the treatment relieson the delivery of sufficient dose to the peak region, whilst keepingthe valley region below healthy tissue tolerance thresholds. Addi-tionally, due to cardiosynchronousmotion of neural tissues, a singletreatment field must be delivered in far less than the period ofa single cardiac cycle. This strict criteria necessitates the use ofsynchrotron x-rays due to the very large dose rates achievable, andlow beam divergence (Braüer-Krisch et al., 2010a).

Current primary methods of dosimetry for MRT employ ionchamber measurements both in-phantom and transmissionmeasurements, in addition to radiochromic film measurements.

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(Braüer-Kirsch et al., 2010b) Ion chambers, whilst able to providereal-time dose rate information, are only able to provide thisinformation macroscopically, lacking detail about radiobiologicallyimportant parameters. Conversely, film is able to provide theinformation ion chambers lack, however only in an offline fashion.With such large dose rates and single-session treatment, onlinequality assurance is a must.

To address this inadequacy, two separate, but complementarysemiconductor-based detector system designs have been devel-oped at the University of Wollongong. The silicon multi-channelstrip detector (Fig. 1a and b) was developed to provide an onlinedosimetry service. This detector is positioned upstream of thepatient, and immediately downstream of the multislit collimator,which provides the spatial fractionation of the field. The detectorconsists of a 128-strip array, with a pitch of 200 mm, set such thatfor a microbeam array of pitch 400 mm, sensitive volume elements,when aligned provide both peak and valley fluence informationconcurrently.

The main purpose of this system is to provide onlinemeasurements of the treatment field, and to initiate an emergencybeam-dump should parameters exceed safety thresholds. Thesecond detector system, the Silicon Microstrip Detector providesoffline quality assurance services and is designed be placed ina solid phantom and scanned through the radiation field by meansof gantry translation. This paper solely investigates the Silicon

b

a

Fig. 1. (a) The Silicon Multi-Channel Strip Detector as mounted to electrical connectionboard, (b) cross-sectional schematic of detector construction.

Fig. 2. The photon input spectrum used for all simulations.

A. Cullen et al. / Radiation Measurements 46 (2011) 1646e1649 1647

Multi-Channel Strip Detector through the use of Monte Carlosimulations. The effect it has on the traversing beam and radiationinteractions within the detector are investigated.

a

b

2. Materials and methods

The Monte Carlo code Geant4 version 9.2 (Agostinelli et al.,2003) was used for all simulations discussed. Geant4, a particletransport toolkit was developed as part of an international collab-oration lead by CERN. Originally developed for high-energy physics,Geant4 has found significant use acrossmany physics disciplines, inparticular medical physics (Beaulieu et al., 2003). The Low EnergyElectromagnetic physics package was used for all simulations andwas selected for this paper as it provides tracking of photons tobelow 1 keV. The accuracy of the results derived from the MonteCarlo simulation studies performed in this work rely heavily on theaccuracy of the physics model employed; in particular, the LowEnergy Electromagnetic package, which has been the subject ofvalidation studies with good agreement displayedwith comparisonto NIST physical reference data (Amako et al., 2005). The photoninput spectrum (Fig. 2) used for all simulations was the measuredspectrum at the ESRF ID17 beamline. The spectrum peaks at 83 keV,and has a median energy of 107 keV.

Fig. 3. (a) Theoretical and Monte Carlo results for interaction spectra, (b) energydeposition spectrum within the confines of a single strip.

3. Results and discussion

3.1. Interaction and energy deposition spectra

A simulation was performed to determine the effect of themultistrip detector on the MRT field. A pencil beam was firedcentrally upon a single sensitive volume element, and upon theoccurrence of an interaction, the energy of the primary photonbinned in 1 keV increments. This was simulated for a total of 1010

primary photons, and the result compared to theoretical calcula-tions performed using photon cross-sections from the NIST PhotonXCOM database (Berger et al., 2010), as shown in Fig. 3a. Thecomputationally determined interaction fraction was (1.94 � 0.01)% (95% C.I.).

The effect the detector has on macroscopic dose deposition wasinvestigated through a simulation whereby a pencil beam againtraverses the detector, and dose was scored at various depthswithin a 10� 10� 10 cm3 water phantom. The simulationwas thenrepeated, but in the absence of the detector.

a

b

Fig. 4. (a) Dose deposition with depth and radial off-axis position within the stripdetector, (b) dose deposition within the strip detector with radial position.

Fig. 5. Dose deposited within sensitive sub-volume as a function of depth, with andwithout detector.

A. Cullen et al. / Radiation Measurements 46 (2011) 1646e16491648

Disparity was present in the interaction (Fig. 3a) and energydeposition (Fig. 3b) spectra in the form of impartial energy depo-sition across all interaction energies. This was due to themajority ofthe spectrum being within the dominant range of Compton scat-tering. Additionally, photons are of energy whereby forward scat-tering is preferential. This leads to the conditionwhere at all but thelowest primary photon energies; the Compton electron is respon-sible for the majority of energy deposition. This phenomenon alsoexplains the peak at lower energies in the energy depositionspectrum.

3.2. Energy deposition within detector

An infinitesimally thin pencil beam was fired centrally upona single strip. Energy deposition was recorded within the detectorand binned according to the spatial location in terms of depth andradial position from the beam axis. Radial symmetry was employedto improve statistics. The cumulative sum of all energy depositedwas determined and converted into a value of dose per 1010

primary photons. The total dose deposition as a function of radialposition (Fig. 4a) was determined by summation of all voxels ateach radial position.

As expected with such a low Z device with such thin thickness,dose was effectively deposited uniformly over the central axis dueto minimal attenuation of the primary photon beam. This wasobserved for all but the most proximal and distal 5-mm voxels,which can be attributed to a lack of charged particle equilibrium inthese regions.

The dose was calculated radially (Fig. 4b), with dose depositionat a distance of 20 mm approximately 1% of the central value. Ata 100 mm, a distance corresponding to half the inter-strip pitch, thedose was less than 0.01% of the central dose. This low lateral dosearising from lateral scattering is of importance, ensuring thatmeasurements of the valley region do not greatly incorporateinteractions occurring elsewhere in the detector.

3.3. Depth dose perturbation

A pencil beamwas fired centrally through a strip of the detectororthogonal to the substrate plane. After traversing the detector, thebeam was incident upon a 10 � 10 � 10 cm3 water phantom. Dosedeposited within water was determined at various depths within5 � 5 � 5 mm3 voxels. The simulation was performed with 109

primary photons, and repeated in the absence of the detector todetermine the perturbative effect on depth dose.

The results in Fig. 5 show a discernable and statistically signif-icant difference in dose observed consistently at all depths. Theaverage decrease was found to be (1.44 � 0.15) %, a relativelyminimal, yet measurable effect in the macroscopic depth dose.

4. Conclusion

The silicon multi-channel strip detector minimally perturbsa transmissive MRT beam in terms of both spectral changes andmacroscopic depth dose when compared to the absence of thedetector. Due to the minimal interaction with the beam, and lowenergy photon beam, energy deposition occurs relatively uniformlywith depth through the detector. The results enhance the case forthe use of the detector a transmission detector for microbeamradiation therapy, but an experimental investigation is required todetermine the accuracy of this study, and will be the subject ofanother paper validating these results and investigating the use ofthe multistrip detector in pre-clinical use.

Acknowledgements

The authors would like to thank the Australian Nuclear Scienceand Technology Organisation for the use of their computer clusterfacilities.

A. Cullen et al. / Radiation Measurements 46 (2011) 1646e1649 1649

References

Agostinelli, S., et al., Geant4 Collaboration, 2003. Geant4 e A simulation toolkit.Nucl. Instrum. Meth. A 506, 250e303.

Amako, K., Guatelli, S., Ivanchenko, V.N., Maire, M., Mascialino, B., Murakami, K.,Nieminen, P., Pandola, L., Parlati, S., Pia, M.G., Piergentili, M., Sasaki, T., Urban, L.,2005. Comparison of Geant4 electromagnetic physics models against the NISTreference data. IEEE Trans. Nucl. Sci. 52, 910e918.

Beaulieu, L., Carrier, J.F., Castrovillari, F., Chauvie, S., Foppiano, F., Ghiso, G.,Guatelli, S., Incerti, S., Lamanna, E., Larsson, S., Lopes, M.C., Peralta, L., Pia, M.G.,Rodrigues, P., Tremblay, V.H., Trindade, A., 2003. Overview of Geant4 applica-tions in medical physics. Nucl. Sci. Symp. Conf. Rec. 3, 1743e1745.

Berger, M.J., Hubbell, J.H., Seltzer, S.M., Coursey, J.S., Zucker, D.S., 2010. XCOM:Photon Cross Sections Database. Physics Laboratory, NIST.

Blattmann, H., Gebbers, J.O., Braüer-Krisch, E., Bravin, A., Le Duc, G., Burkard, W., DiMichiel, M., Djonov, V., Slatkin, D.N., Stepanek, J., Laissue, J.A., 2005. Applica-tions of synchrotron X-rays to radiotherapy. Nucl. Instrum. Meth. A 548,17e22.

Braüer-Krisch, E., Serduc, R., Siegbhan, E.A., Le Duc, G., Prezado, Y., Bravin, A.,Blattmann, H., Laissue, J.A., 2010a. Effects of pulsed, spatially fractionated,microscopic synchrotron X-ray beams on normal and tumoral brain tissue. Mut.Res. 704, 160e166.

Braüer-Krisch, E., Rosenfeld, A., Lerch, M., Petasecca, M., Akselrod, M., Sykora, J.,Bartz, J., Ptaszkiewicz, M., Olko, P., Berg, A., Wieland, M., Doran, S., Brochard, T.,Kamlowski, A., Cellere, G., Paccagnella, A., Siegbahn, E.A., Prezado, Y., Martinez-Rovira, I., Bravin, A., Dusseau, L., Berkvens, P., 2010b. Potential high resolutiondosimeters for MRT, vol. 1266. AIP Conf. Proc., p. 89.

Slatkin, D.N., Spanne, P., Dilmanian, F.A., Sandborg, M., 1992. Microbeam radiationtherapy. Med. Phys. 19, 395e1400.

A. Cullen holds a research Masters degree in the field of radiotherapy dosimetry, and iscurrently is a PhD candidate at the Centre for Medical Radiation Physics, University ofWollongong as part of a team researching novel dosimetry methods for microbeamradiation therapy.