COMPARISON OF MEASURED AND MONTE CARLO CALCULATED … · NUCLEAR PHYSICS COMPARISON OF MEASURED AND MONTE CARLO CALCULATED DOSE DISTRIBUTIONS FOR THE NOVAC7® LINEAR ACCELERATOR D

NUCLEAR PHYSICS

COMPARISON OF MEASURED AND MONTE CARLOCALCULATED DOSE DISTRIBUTIONS

FOR THE NOVAC7® LINEAR ACCELERATOR

D. MIHAILESCU1, M. PIMPINELLA2, A.S. GUERRA2, R.F. LAITANO2

1 Faculty of Physics, University Al. I. Cuza, Iasi, Romania, [email protected] Istituto Nazionale di Metrologia delle Radiazioni Ionizzanti, ENEA, Centro Ricerche Casaccia,

Rome, Italy

Received October 10, 2005

Percentage depth doses (PDDs) and transverse dose profiles (TDPs) werecalculated by a Monte Carlo simulation of electron beams of different energies (3, 5,7 and 9 MeV) produced by a NOVAC7® accelerator. The NOVAC7® is a linearaccelerator designed for Intraoperative Radiation Therapy (IORT). The PDDs andTDPs were also experimentally determined. Both calculations and measurementswere done on the central axis of the beam for the PDDs and at 1 mm depth in waterfor TPDs. The electron beams obtained with and without IORT applicators (openbeams) have been modeled by the code BEAMnrc, a general purpose Monte Carlocode for simulating radiotherapy beams from linear accelerators or 60Co units. Therealistic electron beams were stored in phase-space files which have been used asinput into DOSXYZnrc code to calculate the dose distributions. The agreementbetween calculations and measurements was generally better than ±2% with andwithout IORT cylindrical applicator. This shows that the values adopted in this workfor the energy and spatial distribution of the initial electron beam as well as themodeling parameters are realistic. The accelerator modeling considered in this workgives accurate phase-space data files that can be used for determining in full thecharacteristics for all of the clinical beams produced by the NOVAC7®.

Key words: linear accelerator, Monte Carlo simulations, electron dosimetry,dose distributions, IORT applicators, BEAMnrc, DOSXYZnrc.

1. INTRODUCTION

The NOVAC7® [1] is a mobile electron accelerator especially designed forIntraoperative Radiation Therapy (IORT). IORT refers to the delivery of a singlehigh dose of radiation (usually more than 10 Gy) at the time of surgery when thetumor volume can be precisely defined and the adjacent normal tissue can beefficaciously spared. Some constructive characteristics of the NOVAC7® aregiven in the next section. The absorbed dose measurements in the IORT beams

Paper presented at the National Conference of Physics, 13–17 September 2005,Bucharest, Romania.

Rom. Journ. Phys., Vol. 51, Nos. 7–8 , P. 729–739, Bucharest, 2006

730 D. Mihailescu et al. 2

refer to conditions other than those relevant to most conventional radiotherapytreatments by electron beams. Moreover the specific design of the NOVAC7®accelerator can significantly affect the characteristics of the electron beams withrespect to the conventional electron beams. A simulation of these particularelectron beams by Monte Carlo calculation is then expected to be useful toimprove the accuracy of the beam dosimetry, because it makes possible todetermine a number of parameters needed for absorbed dose measurements(perturbation factors, detector response, stopping power ratios, etc) on the basisof the actual characteristics of the clinical beams.

The aim of this work was to benchmark these IORT electron beamsimulations by comparing calculated and measured dose distributions. Depthdose distributions and transverse dose profiles calculated for a representative setof the NOVAC7® beams were compared with those experimentally measured bya plane-parallel ionization chamber in water phantom.

2. MATERIALS AND METHODS

2.1. THE NOVAC7® IORT ACCELERATOR AND ITS MODELING

The NOVAC7® System (Hitesys S.r.l., Italy) is a mobile electron linac forIORT that produces pulsed electron beams (2÷9 cGy/pulse) with four differentnominal energies: 3, 5, 7 and 9 MeV (Fig. 1). It is equipped with a 3-D movablearm that can be pointed on the operating field. The basic system includes fourtypes of PMMA cylindrical applicators with inner diameters 4, 6, 8 and 10 cm,wall thickness 0.5 cm and lengths 69, 67, 67 and 87 cm, respectively. Theseapplicators are available in two different shapes: straight-ended (0°) and

Fig. 1 – The NOVAC7® IORT accelerator.

3 Dose distributions for the NOVAC7® linear accelerator 731

bevel-ended (15°, 22.5° and 45°), respectively. The source-surface distance(SSD) is 80 cm, excepting for the applicator with diameter of 10 cm for whichthe SSD is 100 cm.

The NOVAC7® accelerator was modeled as a series of simple BEAMnrc[2, 3] component modules with cylindrical symmetry centered on the Z-axis(Fig. 2). The accelerator head includes the exit window and a PVC structure thatacts as primary collimator. Inside of this structure there is a transmission monitorion chamber. The PMMA cylindrical applicators are connected to the primaryapplicator by PMMA adaptors.

Fig. 2 – BEAMnrc modeling used to simulate electron beams fromNOVAC7® accelerator and the water phantom (not to scale).

To characterize the clinical beams a scoring plane has been placed at thephantom surface, immediately after the IORT applicator. The completeinformation (energy, position, direction, charge, etc.) about every particle thatcrosses the scoring plane is stored in a phase-space data file. To determinespatial and energy distribution of the electron beam at the accelerator exitwindow (initial beam), open beams were simulated, i.e. the beams relevant to theclean accelerator head without any applicator. For these simulations slightly


modified accelerator geometry has been used in which the PMMA cylindricalapplicator is replaced with a layer of air and the scoring plane is placed at 100cm distance from the exit window (see Fig. 2).

2.2. MONTE CARLO SIMULATION

A simulation of the electron beams produced in different experimentalconditions by the NOVAC7® accelerator was made using the BEAMnrc MonteCarlo code. The simulation refers to electron beams obtained with straight-endedapplicators. Beveled applicators are also frequently used with NOVAC7® butthe Monte Carlo calculation referring to this different geometry conditions is notyet completed and will be described elsewhere. All the nominal energies havebeen considered for the largest applicator (10 cm diameter), while the 4, 6 and 8cm diameter applicators were investigated only for the higher nominal energies(7 MeV and, in particular, 9 MeV).

In the BEAMnrc simulations both photons and electrons were transporteddown to 10 keV kinetic energies (ECUT = 0.521 MeV, PCUT = 0.01 MeV). Aselectron transport algorithm and boundary crossing algorithm and PRESTA-IIand PRESTA-I were used, respectively. The cross section data were created usingPEGS4 [4] with AE = 0.521 MeV, AP = 0.01 MeV and the Sternheimer densityeffect corrections from ICRU 37 [5]. The number of histories was 10 × 106 for allthe energies. The CPU time/history was in the interval 1.59 × 10–3 ÷ 2.69 × 10–3 s,depending on the nominal energy (smaller for lower energies) and applicator size.

DOSXYZnrc [6] is an EGSnrc user code to calculate dose in Cartesianvoxels. The phase–space files obtained with BEAMnrc at phantom surface havebeen used as source inputs to the DOSXYZnrc to simulate dose distributions in awater phantom. Due to the cylindrical symmetry of the NOVAC7® acceleratorand the fact that the treatment field is centered on the beam axis, the phase-spaceparticles were redistributed into four symmetrical positions. For the percentagedepth doses (PDDs) calculations, the bin dimensions (crossplane x inplane xdepth) along the Z-axis were set to 1 cm × 1 cm × 0.1 cm. Taking into accountthat the ECUT value should be chosen so that the electron range at ECUT is lessthan about 1/3 of the smallest dimension in a dose scoring region, the energy cut-off for electron transport was set to 0.661 MeV (kinetic energy plus rest mass).The photons were followed up to 0.01 MeV (PCUT = 0.01 MeV). The numberof histories was N = 5 × 106 for the simulations regarding the clinical beams(obtained by the IORT applicators) and N = 10 × 106 for the simulationsregarding the open beams (obtained without IORT applicator, beam diameter atphantom surface 60 cm). For the simulations of the transverse dose profiles(TDPs), the bin dimensions were set to 0.5 cm × 0.5 cm × 0.2 cm,ECUT = 0.521 MeV, PCUT = 0.01 MeV, number of histories N = 20 × 106.


Detailed information (i.e. the position, direction, charge and energy) aboutall the particles leaving the accelerator was stored in a phase-space data file.From these data any beam characteristic (e.g. energy, spatial and angulardistributions of the particles in the beam) can be obtained and used to improvethe accuracy of clinical dosimetry.

The phase-space data depend on many different parameters used in thesimulation process. The most important of them are the properties of the initialelectron beam, generally not well known, and the configuration of the acceleratorcomponents. A necessary step in the beam simulation was to make the acceleratormodeling consistent with experimental depth-dose curves and dose profiles.Accordingly some parameters used in the beam simulation (i.e. the energy andthe spatial distribution of the initial electron beam) were iteratively changed untila good agreement between calculated and experimental data was obtained.

2.3. EXPERIMENTAL SET-UP

To compare the dose distributions obtained by the Monte Carlo simulations,PDD curves were obtained by a PTW 23343 “Markus” ion chamber and anautomatic water phantom. Percentage depth-ionization (PDI) curves and transverseprofiles at phantom surface were determined for each IORT beam that wassimulated. The source-surface distance (SSD) was 100 cm or 80 cm dependingon the PMMA applicator length. All measurements were done with a spatialresolution of 0.5 mm. Transverse profiles at the phantom surface were obtainedby positioning the ion chamber with its effective point of measurement at a depthof 1 mm in water. The PDI curves were converted in percentage depth-dose(PDD) curves using the water-air stopping power ratios according to the IAEATRS-398 protocol [7]. For all of the nominal energies (3, 5, 7 and 9 MeV) depthdose and transverse profiles were determined also in the open beams (i.e. withoutthe PMMA applicator attached to the accelerator head) at 100 cm SSD.

3. RESULTS AND DISCUSSION

In Fig. 3 the calculated and measured TDP curves obtained without IORTapplicator (open beam) at SSD = 100 cm are shown. The calculated data refer toan initial electron beam, from a point source situated on the accelerator Z-axis,with a radius of 0.1 cm at the exit window and a divergence of 1°. As shown inFig. 3, with this geometry a good match between calculated and experimentalTDPs for all of the nominal energies was obtained. Increasing the beamdivergence the simulated dose profiles become quickly too large and theagreement with experimental data is lost.


Fig. 3 – Comparison of MonteCarlo simulated (dashed lines)and measured (lines) transversedose profiles for NOVAC7®accelerator without the of IORTapplicator (open beam) fornominal energies 9, 7, 5 and 3 MeV and SSD = 100 cm.


In Fig. 4 the calculated and measured PDD curves obtained for the openbeams at SSD = 100 cm are shown. The values of the most probable energy andthe mean energy of the initial electron beams used as input into the Monte Carlosimulation are reported in Table 1. For all the nominal energies the calculatedPDDs agree within ±2% or ±1 mm with those measured.

Table 1

The parameters of initial electron beams used in Monte Carlo simulations ofNOVAC7® electron beams with nominal energies 3, 5, 7 and 9 MeV

Nominal energy [MeV] 9 7 5 3

Most probable energy [MeV] 8.3 7.3 6.3 5.1

Mean energy [MeV] 6.7 5.9 5.1 4.2

Fig. 4 – Comparison of Monte Carlo simulated (dashed lines) and measured (lines)depth-dose curves for NOVAC7® accelerator without the IORT applicator (open beam)

at nominal energies 9, 7, 5 and 3 MeV (SSD = 100 cm).

The results reported in Fig. 3 and 4, regarding the open beam conditions,are relevant to the optimization of the accelerator modeling. This wasaccomplished by tuning the Monte Carlo parameters in order to fit (within ± 2%)the experimental data as shown in the two figures. In general an agreementbetween experimental and calculated data is not immediate to achieve, as somesimulation parameters, e.g., the geometry and the spectral distribution of theinitial electron beam, are not known in detail. For most conventional linearaccelerators the initial beam is assumed to be mono-energetic and directed on-axis(with no angular distribution).


Generally, the value of the initial electron energy, that can be significantlydifferent from the nominal energy, is iteratively changed until a matching of thecalculated and measured values of R50 (i.e. the depth at which the dose falls to50% of its maximum) is obtained. The same approximation initially adopted tosimulate the NOVAC7® electron open beams was not adequate to provideconsistent results with measurements. Using the approximation of initial mono-energetic parallel beams was effective in matching the R50 values for all of thenominal energies but resulted in unacceptable deviations between experimentaland calculated depth–dose curves.

In particular the calculated absorbed dose values at the phantom surfaceresulted about 10% lower than experimental values and the dose gradient wassignificantly higher. Even changing the source geometry (i.e. radius of parallelbeam, point source with various divergences) the disagreement betweencalculated and experimental PDD curves did not disappear. It was then necessaryto assume a suitable energy distribution for the initial electron beams to obtainagreement between calculated and experimental PDD curves. In particular, usingan energy spectrum for the initial beam instead of a monoenergetic beam made itpossible to confirm by calculation the unusual high surface dose typical of theexperimental PDD curves for the NOVAC7® beams. The source geometry (i.e.radius and divergence of the initial electron beam), even if not influent on thePDD curves, was instead found effective in modifying the transverse doseprofiles (TDPs). The optimization of the accelerator modeling for the open beamssimulation was achieved with a “point source” giving rise to an initial electronbeam with 0.1 cm radius at the exit window and 1°divergence.

Fig. 5 – Comparison of Monte Carlo simulated (dashed lines) and measured (lines)depth-dose curves for NOVAC7® accelerator with 10 cm diameter IORT applicator

(collimated beam) at nominal energies 9, 7, 5 and 3 MeV and SSD = 100 cm.


Fig. 6 – Comparison of MonteCarlo simulated (dashed lines)and measured (lines) transversedose profiles for NOVAC7accelerator with 10 cm diameterIORT applicator for nominalenergies 9, 7, 5 and 3 MeV and SSD = 100 cm.


In Figs. 5 and 6 the calculated and measured PDDs and TDPs obtainedwith the IORT applicator of 10 cm diameter at SSD = 100 cm for all the nominalenergies are reported. As can be seen, the ± 2% agreement with the experimentaldata already found out in the open beams is confirmed. Similar results have beenobtained for the IORT cylindrical applicators with diameters of 8, 6 and 4 cm.

4. CONCLUSIONS

The NOVAC7® accelerator shows experimental PDD curves characterizedby higher dose at the phantom surface and a much a lower gradient with respectto the conventional electron beams. In this calculation it was necessary toassume for all the electron energies (3, 5, 7, 9 MeV) a spectral distribution in theinitial electron beam to obtain an acceptable agreement (better than ± 2%) withthe experimental data. The procedure to find out the characteristics finallyadopted for the initial electron beam consisted in the following steps. Firstly thesimulations were done for the open beams (i.e. without any applicator). Thetransverse dose distribution was calculated by modifying the source geometry toobtain the best agreement with experimental data. When an acceptable agreement(about ± 2%) with measured transverse dose distribution was found, the initialelectron spectra were changed until an agreement of about ± 2% was obtainedalso for the PDD curves. As second step, the source geometry and the electronspectra were used to simulate the actual clinical electron beams (with applicator).Additional changes of the source geometry were found unnecessary because the± 2% agreement with the experimental data already found in the open beams wasconfirmed.

The general agreement (within ± 2%) between calculated and measuredcollimated beams shows that the values adopted for the energy and spatialdistribution of the initial electron beam as well as the modeling parameters arerealistic. The accelerator modeling here considered gives accurate phase-spacedata files for all of the NOVAC7® beams considered.

The phase-space files obtained in this work, containing the fulldescription of the beam at the phantom surface, can be used for determiningthe characteristics of any clinical beam produced by the NOVAC7®. This canbe useful for determining a number of parameters needed for accurateabsorbed dose measurements as perturbation factors, detector response,stopping power ratios, etc.

Acknowledgements. One of the authors (D. Mihailescu) undertook this work with thesupport of the ICTP Programme for Training and Research in Italian Laboratories, Trieste, Italy.


REFERENCES

1. http://www.hitesys.it/Novac7.htm.2. D. W. O. Rogers, B. A. Faddegon, G. X. Ding, C.-M. Ma, J. Wei and T. R. Mackie, BEAM: a

Monte Carlo code to simulate radiotherapy treatment units, Med. Phys. 22, 503 (1995).3. D. W. O. Rogers, B. Walters and I. Kawrakow, BEAMnrc Users Manual, National Research

Council of Canada Report, PIRS –0509a, Ottawa: NRC (2005).4. I. Kawrakow and D. W. O. Rogers, The EGSnrc Code System: Monte Carlo Simulation of

Electron and Photon Transport, National Research Council of Canada Report, PIRS –701,Ottawa: NRC (2003).

5. International Commission on Radiation Units and Measurements (ICRU), Stopping Powers forElectrons and Positrons, ICRU Report 37, Bethesda, MD (1984).

6. B. Walters, I. Kawrakow and D. W. O. Rogers, DOSXYZnrc Users Manual, National ResearchCouncil of Canada Report, PIRS -794, revB, Ottawa: NRC (2004).

7. International Atomic Energy Agency (IAEA), Absorbed dose determination in external beamradiotherapy: An international code of practice for dosimetry based on standards ofabsorbed dose to water, Technical Reports Series, No. 398, IAEA, Vienna (2000).

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COMPARISON OF MEASURED AND MONTE CARLO CALCULATED … · NUCLEAR PHYSICS COMPARISON OF MEASURED AND MONTE CARLO CALCULATED DOSE DISTRIBUTIONS FOR THE NOVAC7® LINEAR ACCELERATOR D