Development and performance evaluation of a dynamic phantom for biological dosimetry of

moving targets

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2010 Phys. Med. Biol. 55 2997

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Phys. Med. Biol. 55 (2010) 2997–3009 doi:10.1088/0031-9155/55/11/001

Development and performance evaluation of adynamic phantom for biological dosimetry of movingtargets

A Gemmel1,2, C Bert1, N Saito1, C von Neubeck1, G Iancu1,W K-Weyrather1, M Durante1 and E Rietzel1,2

1 GSI Helmholtzzentrum fur Schwerionenforschung, Planckstr 1, 64291 Darmstadt, Germany2 Siemens AG, Healthcare Sector, Workflow & Solutions, Particle Therapy, Hofmannstr 26,91052 Erlangen, Germany

E-mail: alexander.ag.gemmel@siemens.com

Received 3 February 2010, in final form 28 March 2010Published 4 May 2010Online at stacks.iop.org/PMB/55/2997

AbstractA dynamic phantom has been developed to allow for measurement of 3Dcell survival distributions and the corresponding distributions of the RBE-weighted dose (RBED) in the presence of motion. The phantom consistsof two 96-microwell plates holding Chinese hamster ovary cells inside acontainer filled with culture medium and is placed on a movable stage. Basicbiological properties of the phantom were investigated without irradiation andafter irradiation with a carbon ion beam, using both a stationary (reference)exposure and exposure during motion of the phantom perpendicular to the beamwith beam tracking. There was no statistically significant difference betweenplating efficiency measured in the microwells with and without motion (0.75)and values reported in the literature. Mean differences between measured andcalculated cell survival for these two irradiation modes were within ±5% ofthe target dose of 6 Gy (RBE).

1. Introduction

The goal of radiation therapy is to achieve local tumour control whilst minimizing treatmentside effects (Evans 2008). Highly conformal dose distributions can be delivered by scannedion beam therapy (Haberer et al 1993, Lorin et al 2000, Pedroni et al 2004) whose potential isfully exploited with intensity modulated particle therapy (Gemmel et al 2008, Lomax 1999).However, intrafractional motion may compromise delivered dose distributions and thereforerequires thorough motion management (Langen and Jones 2001). Possibly, interactionsbetween beam delivery and organ motion lead to deterioration of delivered dose distributions.This effect, commonly known as interplay, has been investigated for intensity modulatedradiotherapy (IMRT) (Seco et al 2007) and scanned ion beam therapy (Bert et al 2008, Seiler

0031-9155/10/112997+13$30.00 © 2010 Institute of Physics and Engineering in Medicine Printed in the UK 2997

2998 A Gemmel et al

et al 2000). Techniques to mitigate motion effects especially in scanned ion beam therapyare rescanning (Phillips et al 1992), gating (Bert et al 2009) and beam tracking (Bert et al2007). To investigate and verify the effectiveness of these techniques, specific phantoms thatare suitable for measuring dose distributions in the presence of target motion are required (4Dtreatment plan verification).

For stationary targets, a variety of detector systems are used for dosimetry of the absorbeddose including films (Stern et al 1992, Wang et al 1996), gels (Lopatiuk-Tirpak et al 2008,Oldham et al 1998, Olsson et al 1990), gas electron multiplier detectors (Seravalli et al 2008),ion chamber or diode arrays (Bedford et al 2009, Karger et al 1999, Van et al 2007), plasticscintillators (Collomb-Patton et al 2008), or thermoluminescent chips (Ayyangar et al 1993).Recently, phantoms specifically dedicated for dosimetry of moving targets were developed,for example to simulate irradiations of lung tumours (Serban et al 2008), or to validate beamtracking based on dynamic multileaf collimators (Liu et al 2009). To measure absorbed dosesdelivered by beam tracking with scanned ion beams, a water phantom with a set of pinpointionization chambers was used (Bert et al 2007).

Due to their radiobiological properties, charged particles heavier than protons provide anincreased relative biological effectiveness (RBE) for cell inactivation towards the end of theparticle track. This leads to an enhanced therapeutic ratio when comparing the Bragg-peakregion to the entrance channel (Kraft 2000). Due to the superposition of many individualpencil beams and fragmentation of the primary beam into lighter particles (Golovchenkoet al 1999, Haettner et al 2006), the composition of the local particle and energy spectrumvaries within the irradiated volume and leads to spatially modulated RBE values (Kramerand Scholz 2000). Predominantly, the RBE varies as a function of the absorbed dose, linearenergy transfer, radiation quality, and tissue type (Kramer et al 2003b). These dependencesare included into our treatment planning system TRiP (treatment planning for particles) (Jakelet al 2001, Kramer et al 2000) by virtue of the local effect model (LEM) (Scholz et al 1997).Biological dosimetry of complex irradiation fields as they appear in typical patient treatmentplans was performed to validate the TRiP calculations. Calculations with TRiP were comparedto measured cell survival distributions (Gemmel et al 2008, Kramer et al 2003a, Mitaroffet al 1998).

Recently, TRiP was extended to 4D treatment planning capabilities (Bert and Rietzel2007). An algorithm to calculate the RBE-weighted dose in the presence of target motionis currently under development. Thus, a validation of the new 4D capabilities of TRiPfor moving targets has to include cell survival studies. Existing phantoms for biologicaldosimetry allow for measurement of 1D or 2D cell survival distributions and were testedwithout motion only. Therefore, a new phantom has been developed to allow for measurementof 3D cell survival distributions and the corresponding distributions of the RBE-weighted doseRBED in the presence of motion. An experimental evaluation of the phantom is presentedconsisting of two major steps: first, basic biological properties of the phantom are investigatedin performance tests without irradiation. Second, irradiations of the stationary phantom(reference) are compared to measurements with the phantom moving while tracking the targetwith the scanned ion beam (Bert et al 2007, Grozinger et al 2008, Saito et al 2009).

2. Materials and methods

2.1. The dynamic phantom

2.1.1. Biological target phantom. The most important requirements for a biological phantomin particle beam therapy are suitability for cell cultures and insensitivity of measured results

Development and performance evaluation of a dynamic phantom for biological dosimetry of moving targets 2999

Figure 1. The MicroWellTM plate in upright position (left) consists of 12 × 8 small wells thatallow for independent cell survival measurements. For irradiation, two plates can be inserted intoa specially designed container (right) to supply the cells with medium in a sterile environment.The camera perspective is comparable to the beam’s eye view for the experiments.

against mechanical phantom motion. Small lateral spacing between measurement positionsand high spatial resolution in radiological depth is desirable to allow for 3D cell survivalmeasurements. Handling of the phantom should be geared towards feasible cell processingprocedures. Materials in the irradiated field should ideally be water equivalent and shouldespecially not contain high-Z elements that would introduce additional scattering of the ionbeam.

Nunc F96 MicroWellTM plates fulfil the listed requirements, i.e. adequate cell cultureconditions and high spatial resolution. Figure 1 shows a photograph of a plate. The plates arecommercially available and consist of 96 wells arranged in an array of 12 × 8 positions withan equidistant spacing of 9 mm. Each well has an opening diameter of 7 mm and a diameterof 6.2 mm at the bottom. The capacity of a well is 400 μl. The bottom of the wells is coated toallow attachment of the cells. The side panel of each well prevents detachment of the cells dueto the culture medium’s flow during motion. Each well can be used to hold an independentcell sample that translates to one measurement position for irradiation experiments. The platesconsist of polysterol with a mass density slightly above the mass density of water. Ion rangemeasurements with a range telescope (Rietzel et al 2007, Schardt et al 2008) yielded a relativewater equivalent path length of 1.015 for the plate.

A container was designed to hold two MicroWell plates in culture medium in an uprightposition (figure 1). This allows acquisition of measurement points with a lateral distance of9 mm using the array of wells perpendicular to the beam’s direction. To provide a resolutionin depth, two MicroWell plates can be used with a minimum geometrical distance of 9 mm.The plates can be positioned aligned to each other or with a left–right offset in the beam’seye view of half of the distance of a well (4.5 mm). Moreover, the plates can be insertedwith the wells facing outside or inside. The inlets of the container provide sufficient space toinsert the MicroWell plates under small inclination to avoid generation of air bubbles withinthe wells. After insertion a set of spacers permits fixation of the plates at a defined positionthat prevents the plates from loosening during movement. Container and spacers are made ofpolymethylmethacrylat (PMMA) with a relative water equivalent path length of 1.165. Theycan be re-used but have to be treated with a disinfectant (MBT brand) prior to the measurementsto ensure sterile conditions.

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2.1.2. Movable stage. The platform of the dynamic phantom consists of a solid plate madeout of polyvinyl chloride (Trovidur R©). A second PMMA plate is used as a movable stage of47 × 47 cm2 with a thickness of 2 cm. It is mounted on wheels to allow for one-dimensionalmovement. Motion is introduced by a geared motor via an extender wheel. The peak-to-peakmotion can be adjusted in the range from 2 mm to 50 mm. The motion period is definedby the motor’s input voltage and was designed for motion periods between 2.5 s and 6.5 s.A commercially available laser triangulation system (OD100-35P840, Sick Vertriebs-GmbH,Dusseldorf, Germany) is integrated into the platform to monitor the motion of the stage. Thestage has been designed for mounting of different detector systems such as films, biologicalsamples or even heavy water phantoms of several kilograms weight.

2.2. Experimental evaluation

The goal of the measurements was to demonstrate the use of the phantom as a detector for cellsurvival of moving targets in scanned ion beam irradiations. In the first step we conducted cellsurvival measurements without carbon ion beam irradiations to test the basic properties of thedynamic phantom, especially the influence of motion on the measured cell survival. In thesecond step, the applicability of the phantom for biological dosimetry of scanned ion beamswas tested. The phantom was moving left–right in beam’s eye view and irradiated with lateralbeam tracking. With beam tracking, the expected dose distribution on the dynamic phantomis comparable to stationary reference measurements with the phantom at rest (Grozinger et al2008, Saito et al 2009). The phantom test can be considered successful if the difference incell survival between stationary reference and lateral beam tracking under motion was smallerthan the error of the measurements. Details of the measurement processes are given in thefollowing sections.

2.2.1. Performance tests and cell handling. In our experiments, Chinese hamster ovary cells(CHO-K1) were utilized to perform the cell survival studies. Their linear quadratic photonresponse curve, −ln(S) = αD + βD2, with cell survival S and dose D is characterized by theparameters α = 0.228 Gy−1 and β = 0.02 Gy−2 that were derived experimentally (Weyratheret al 1999). The cells were grown in HAM’S F12 medium with glutamine, 10% foetal calfserum (FCS) and 1% penicillin/streptomycin and incubated in an atmosphere at 37 ◦C, 5%CO2 and 95% humidity. 24 h prior to the experiments approximately 10 000 cells in 350 μlmedium per well were seeded and grown in the MicroWell plates under sterile conditions. Todetermine cell survival after the experiments, the cell samples underwent a cell processingprocedure that was based on the clonogenic assay by Puck and Marcus (1955): the mediumwas removed from the wells and the cells were covered with trypsin for 6 min outside theincubator. Then, the cells were carefully singularized, counted by a Coulter counter andre-seeded with respect to the expected survival level to achieve an appropriate number ofcolonies after 7 days’ incubation. Re-seeding was performed three times per well. After1 week, surviving cells have formed colonies that were stained and manually scored by use ofa microscope.

An important part of cell survival measurements is determination of the plating efficiency(PE). The PE is the fraction of cells that attach to the ground after insertion into a culture flask,starts proliferation and forms colonies. It can be obtained from non-irradiated cell samples,so called controls. In a repeated series the PE of CHO cells cultured in MicroWell plateswas determined from samples that served as controls in a stationary, not moving phantomsetup and samples that served as controls in a moving phantom setup. A PE comparable tothe literature is a necessary condition for using moving MicroWell plates. Additionally, thesemeasurements are used to estimate the uncertainties of the colony forming assay.

Development and performance evaluation of a dynamic phantom for biological dosimetry of moving targets 3001

In the following determination of the PE and its error is described in detail.

• From the number of colonies Ki1, Ki2, Ki3 of the threefold re-seeded cell sample themean number of colonies Ki and their standard deviation σi are determined, wherei ∈ {1, . . . ,MGroup} denotes the controls with the number of controls MGroup. The controlsare divided into groups containing only the controls that were moving, MGroup = MMov,that were not moving, MGroup = MStat, or all controls, MGroup = MTotal. The mean values

σ SeedGroup = 1/MGroup

∑MGroup

i=1 σi represent the mean error that is introduced by the re-seedingstep for each group.

• The surviving fraction, si , of the controls is determined by si = Ki

V ·C , where C is the cellconcentration measured by the Coulter counter and V the volume of cell suspension thatwas re-seeded for colony forming in each of the culture flasks. The plating efficiency PEis the mean value of the group of controls si : PEGroup = sGroup.

• The plating efficiencies PEGroup ∈ {PEStat, PEMov, PETotal} and their standard deviationsσGroup ∈ {σStat, σMov, σTotal} are calculated for the corresponding groups. The standarddeviations σMov, σStat and σTotal represent the error of the PE that is introduced by theuncertainties of all steps of the cell processing procedure.

• To estimate the relative error of all steps but re-seeding, σ RestGroup, Gaussian error propagation

is applied:

σ RestGroup =

√σ 2

Group − σ Seed 2Group

PEGroup. (1)

• From the number of colonies N1, N2, N3 of the threefold re-seeded and irradiated cellsample the mean number of colonies N and its standard deviation σ N are determined. σ N

accounts for the uncertainty introduced by the re-seeding procedure that is the last step ofthe cell processing procedure.

• The cell survival S of a cell sample is calculated by

S = N

C · V · PEGroup. (2)

To determine the performance of the implemented biological motion phantom we usedthree different methods.

(i) The resulting values for PEGroup, σGroup, σRestGroup were compared with each other and

compared to the literature values, where the cell samples were grown in standard cultureflasks. Applicability of the phantom to biological dosimetry in the presence of motionshould yield comparable results of the three groups (moving, stationary, all) with theliterature data.

(ii) The cell concentration within the medium of the container after removal of the MicroWellplates was checked. The concentration is a measure of the amount of cells that detachedduring the performance tests. The test is successful if the quantities of stationary phantomsand moving phantoms are comparable to the literature data.

(iii) With the cell-handling procedure described above, we measured a survival curve tox-rays (beam quality: 250 kV, 16 mA; filter: 7 mm beryllium, 1 mm aluminium and1 mm copper; model Isovolt DS1, Seifert & Co., Hamburg, Germany). We irradiated fivewells of a MicroWell plate for each of the eight dose points ranging from 1 to 10 Gy. PEwas measured in mock-irradiated control. The cell survival S of the exposed cell sampleswas calculated according to equation (2). The mean survival the standard deviation σ S

was calculated from the five samples per dose. A linear quadratic curve was fitted to the

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Figure 2. Scheme of the experimental setup. Table motion is detected by a laser triangulationsensor, processed in the therapy control system (TCS) and directly compensated for by the scannermagnets.

measured data points and the χ2fit-value was determined. We compared the best fit of our

experimental curve to the standard linear–quadratic survival for the same cell obtained inseveral previous experiments in our laboratory under standard conditions (Weyrather et al1999). Fisher’s F-test was used to compare the χ2

fit-values calculated using the publishedsurvival curve or the best fit of our experimental data.

2.2.2. Biological dosimetry for carbon beam irradiations. The purpose of the experimentswas to test the performance of the biological phantom with and without motion. To enablethe possibility of directly comparing experimental results, applied dose distributions with andwithout phantom motion should be identical. Therefore the GSI beam tracking system (Saitoet al 2009) was employed to achieve comparable dose distributions. Regular sinusoidal motiontrajectories were used. Additionally, the setup shown in figure 2 was chosen to avoid changesin radiological depth due to motion. This ensured that for example fragment spectra did notchange as a result of the phantom motion. Motion was mitigated by real-time adaptation ofthe carbon ion pencil beams during beam scanning with our beam tracking system. Measureddisplacements were directly transferred to the therapy control system to adapt the current beamposition with the scanning magnets (Bert et al 2007, Grozinger et al 2008, Saito et al 2009).Performance studies have shown that this can be achieved with a very small delay time of 1 ms(Saito et al 2009). Irradiations with a carbon beam under target motion therefore lead to a cellsurvival in the biological phantom that is comparable to irradiations of a stationary geometry.To ensure adequate beam tracking functionality the setup included two films that were placedon the movable stage. In addition, both measurement scenarios were compared to calculatedsurvival levels.

For treatment planning an artificial CT data set was constructed with TRiP’s CT generationfacility to resemble the geometry of the setup according to figure 2. The target within thecontainer was planned to be a cubical volume with the dimensions 28 × 45 × 23 mm3

(W × H × D). The treatment plan comprised a regular grid of raster points with a lateralgrid spacing of 2.5 mm and a step size in depth of 2 mm water-equivalent. The beam’s fullwidth at half maximum was 7.5 mm. The particle numbers were optimized to cover the cubicvolume homogeneously with a RBED of 6 Gy (RBE). Calculation of biological effects and

Development and performance evaluation of a dynamic phantom for biological dosimetry of moving targets 3003

the RBE-weighted dose were based on the local effect model (LEM) (Scholz et al 1997) in itslatest formulation (LEM III) (Elsasser et al 2008).

2.2.3. Experimental procedure. Four containers with two MicroWell plates each were used.One container was used for beam tracking (target motion: 40 mm peak-to-peak amplitude,6 s period) and one container was used in a stationary reference irradiation. Cell probes in thethird and forth container served as controls and were not irradiated but treated in the same way,i.e. they were delivered to the irradiation room and were placed on the table with and withoutmotion for a duration corresponding to the irradiation time. The containers were cooled duringdelivery to and from the irradiation facility. The irradiation procedure including positioningof the container took ∼15 min. After irradiation of a single container it was delivered back tothe biology laboratory, stored for about 1 h due to radiation protection and then processed asdescribed above.

2.2.4. Data analysis. Cell survival for the irradiated cell samples is calculated accordingto equation (2). Since each cell sample represents a single and independent measurement,its error σ S is estimated by the error of the cell processing procedure and calculated usingGaussian error propagation:

(σS/S)2 = (σPE/PE)2 + (σN/N)2 + σ Rest 2Group . (3)

Cell survival was determined along two perpendicular profiles through the target volume atselected positions that were chosen prior to irradiation. The cell survival S and its errorσ S were converted to the RBE-weighted dose D and corresponding the dose error σD . For

the reference irradiation and for lateral beam tracking the mean dose errors σ StatD and σ

CompD

were computed for the corresponding measurement positions. The two values indicate thedosimetric precision of the phantom.

The dose differences �D = DStat −DComp between irradiation of the stationary phantomand lateral beam tracking were calculated for each measurement position. From these valuesthe mean difference �D and its standard deviation σ�D were determined. σ�D describesthe variation of the difference of the two measurements. For comparison with σ�D , the total

experimental dose error of the difference σExpD = (

σ StatD

2+ σ

KompD

2)1/2was calculated.

Additionally, the measurements were compared to calculations of TRiP. Analogousto the comparison of the two experimental data sets described before the differences�DStat = DStat − DCalc and �DComp = DComp − DCalc were obtained for each measurementposition. Then, the corresponding mean values �DStat and �DComp as well as their standard

deviations σ Stat�d and σ

Comp�d were calculated.

The mean differences �D, �DStat and �DComp as well as their standard deviations σ�D ,

σ Stat�d and σ

Comp�d were calculated for the measurement positions of the proximal, the distal, and

for both plates taking either all points or only points inside the target volume into account.

3. Results

3.1. Performance test

The plating efficiency and its standard deviation were determined for stationary and movingcell samples that served as controls. The results are listed in table 1. The PE is ∼0.75 in bothcases as reported in the literature (Weyrather et al 1999). Standard deviations for the stationary

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Figure 3. Experimentally determined cell survival after photon irradiation (dots) and thecorresponding fit curve (dashed line) as well as the cell survival curve reported by Weyratheret al (1999) (solid line).

Table 1. Results of the plating efficiency (PE) analysis and its error for stationary and moving cellsamples in comparison to values in the literature.

Number ofGroup samples N PE σGroup σ Rest

Group (%)

Stationary 25 0.75 0.12 13.0Moving 46 0.74 0.12 13.6Total 71 0.75 0.12 13.5Weyrather et al (1999) – 0.75 0.11 –

σStat and moving σMov cell samples are identical and only slightly increased as compared tothe literature values. The relative errors σ Rest

Group that account for the uncertainties of all stepsof the cell processing procedure but the re-seeding are comparable for stationary and movingcell samples.

The concentration of cells in the medium of the container after removal of the MicroWellplates, i.e. the fraction of cells that detached during the performance tests, was below 5% ofthe cells that were seeded on the plates.

Figure 3 shows the results of the x-ray survival curve measurement in comparison to thesurvival curve parameterized by the α and β values reported by Weyrather et al (1999), as wellas the fitted survival curve. The statistical analysis yielded χ2

meas = 10.6 for the publishedcurve and χ2

fit = 8.6 for the best fit of our experimental data. In both cases the number ofdegrees of freedom is 6. According to the one-sided F-test at a significance level of α = 0.05,no statistically significant difference is detected between the fitted curve and the laboratorystandard survival curve.

3.2. Biological dosimetry in the presence of motion

Mean film response (1.407/1.408) and homogeneity, defined as one minus the ratio of thestandard deviation and the mean value (0.978/0.983), were comparable for irradiation of the

Development and performance evaluation of a dynamic phantom for biological dosimetry of moving targets 3005

Figure 4. Cell survival for a stationary setup and a moving setup irradiated with lateral beamtracking in comparison to calculations for the stationary, reference setup. Error bars correspond tothe value of σ S for each measurement position. In the picture at the top right, wells that were usedin the experiment are marked by blue dots.

stationary setup and the moving setup using beam tracking. Also, minimal values (1.32/1.28)and maximal values (1.45/1.43) were comparable. Therefore beam tracking was successful,and no impact on cell survival measurements due to the tracking performance is guaranteed.

Cell survival results for both stationary setup and moving setup are shown in figure 4 incomparison to the calculated cell survival. Again, the concentration of cells in the mediumwas below 5% for the stationary and moving setup. In total 15 and 19 cell samples wereprocessed for the stationary and the moving setup, respectively. For measurement pointsthat were positioned within the steep dose gradients at the target volume edges, analysis ofthe calculated dose distribution revealed that the finite size of the well can encompass dosegradients of up to 2 Gy (RBE).

Table 2 reports results for the three comparisons, namely stationary measurements versuscalculation, lateral beam tracking measurements versus calculation, and stationary versuslateral beam tracking measurements. To account for expected uncertainties caused by thecombination of finite well size and steep dose gradients, additional analysis for target pointsonly was performed.

The mean dose errors for stationary and moving setup, σ StatD and σ

CompD , were 522 mGy

(RBE) and 552 mGy (RBE) respectively. Normalized to the target dose of 6 Gy (RBE) thiscorresponds to a dosimetric precision of 8.7% and 9.2% respectively. The resulting totalexperimental dose error σ

ExpD is 760 mGy (RBE) (12.7%). This is comparable to the standard

deviation of the dose difference between stationary and moving setup σ�D which is 756 mGy(RBE) (12.6%) for all measurement positions.

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Table 2. Relative differences of the RBE-weighted doses and their standard deviations. All valuesare normalized to the target dose of 6 Gy (RBE) and given in percentage.

Comparison Proximal Distal Both

Stationary measurementversus calculation All positions 3.3 ± 8.0 −2.7 ± 6.0 0.3 ± 7.5

�DStat±σ Stat�d Target positions 3.6 ± 9.4 −1.1 ± 6.0 1.2 ± 7.8

Moving measurementversus calculation All positions −4.9 ± 9.9 −1.1 ± 10.9 −3.1 ± 10.0

�DComp±σComp�d Target positions 0.2 ± 4.5 3.4 ± 1.8 1.8 ± 3.6

Moving measurement versusstationary measurement All positions 8.7 ± 12.9 −0.8 ± 11.1 4.0 ± 12.6

�D±σ�D Target positions 3.9 ± 10.2 −2.6 ± 4.3 0.6 ± 8.0

4. Discussion

4.1. Performance test

The performance tests without carbon beam irradiation showed that despite the small volumesin the range of several 100 μl, moving MicroWell plates provide excellent cell cultureconditions for CHO cells that are comparable to standard flasks that have been used previouslyfor biological dosimetry. Plating efficiency, the number of detached cells detected in themedium as well as an x-ray survival curve yielded comparable results in all three groups, i.e.stationary conditions, moving phantom and the literature data of Weyrather et al (1999) fromstationary T75 culture flasks. Also the container and the handling procedure were found to beadequate.

Repeated studies of the deviation of the PE also showed no differences between movingand stationary cell samples. The uncertainty that estimates the error of the cell processingprocedure without re-seeding of the cells was determined to be 13.5%. This value representsan estimation of the error in cell survival measurements as denoted by equation (3).

4.2. Biological dosimetry

Cell survival measurements in the mixed particle fields of carbon beam irradiations in thepresence of target motion were performed with the developed phantom. The performance ofthe phantom was tested by comparing the results for irradiations of stationary setups to the onesincluding target motion and irradiated with lateral beam tracking as described by Grozingeret al (2008) as well as calculations with TRiP. Comparable to the reports of Mitaroff et aland Kramer et al for biological dosimetry of stationary targets, good qualitative overallagreement between measurements and calculations was determined (Kramer et al 2003a,Mitaroff et al 1998). Mean differences between measured and calculated cell survival werewithin ±5% of the target dose of 6 Gy (RBE) for both irradiation schemes. Taking only cellsamples into account that were located within the target volume and thus excluding wellspositioned in regions with large dose gradients, the mean differences between measurementsand calculations were found to be within 2%.

The biological dosimetry tests in the present study serve to verify the biological phantomwhich is a prerequisite for further experiments. The dosimetric precision of the phantom was

Development and performance evaluation of a dynamic phantom for biological dosimetry of moving targets 3007

estimated by the measurement errors that were evaluated for each cell sample and was around9% for all setups. This compares well to previously reported measurement errors for otherphantoms (Gemmel et al 2008). The precision of the RBED measurement is limited due to theinherent biological variability of cell systems and the cell processing procedure that requiresseveral manual interactions.

Cell survival after lateral beam tracking was comparable to the stationary reference,especially in the target area. The measurements show no significant influence of the motionof the phantom within its dosimetric precision. Mean differences within the target area werewithin ±5%. The variations of the differences up to 12.9% between the two irradiationschemes compare well to the total experimental error that was determined to be 12.7%.

The dynamic phantom is not designed to resemble realistic patient anatomy as in otherapproaches (Kashani et al 2007, Serban et al 2008). However, it complements an existing waterphantom for measurement of the absorbed dose with multiple pin-point ionization chambers ina water tank (Karger et al 1999) offering the possibility of measuring cell survival and RBEDfor moving targets in scanned particle therapy (Bert et al 2007). The minimal distance betweentwo measuring positions is 9 mm which is also the minimal distance between the two plates.This is comparable to the spacing of ionization chambers in the water phantom with distances of12 mm in a row and 10 mm between two rows. The finite size of the coated bottom area of thewells with a diameter of 6.2 mm is a compromise between spatial resolution and the numberof cells needed to measure cell survival in a range from 1% to 100%.

The experimental setup did not include explicit control of the oxygenation level. Inorder to avoid generation of hypoxic cells and to ensure comparable conditions, the Microwellplates were put in the container with fresh medium shortly before irradiation. In addition,the cells were cooled during transportation to slow down the cells’ metabolism includingthe uptake of oxygen. Also, the time for transportation and irradiation was kept below2.5 h.

The dynamic phantom will be used for commissioning and verification of an algorithmin treatment planning for calculation of the 4D RBE-weighted dose for moving targets thatis currently under development. Ideally, this will be done in experiments without motioncompensation, where interplay of target movement and beam motion leads to changes of theradiological depth and introduces changes of the fragmentation yield as well as the RBE.Moreover, further developments of biological modelling give rise to the need of a techniqueto verify the theoretical predictions for mixed irradiation fields. Also, new approaches intreatment planning optimization might be validated with biological dosimetry as described inGemmel et al (2008).

The phantom can also be used for different ion species or other beam qualities. Itis possible to apply the phantom in more complex setups with a stack of containers andabsorbers using possibly different cell lines in different containers to reflect different radiationsensitivities in the entrance channel and the high dose region. The phantom could be furtherextended by a robotic arm or several motion tables to generate differential motion patternsbetween the components of the setup.

5. Conclusion

A dynamic phantom for cell survival measurements in the presence of motion was developedand successfully tested using lateral beam tracking for scanned ion beams in comparison toa reference irradiation of a stationary setup. Cell survival distributions showed very goodagreement between beam tracking and reference irradiation. The dynamic phantom is capableto measure 3D survival distributions for moving targets.

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Acknowledgments

The authors appreciate Professor Dr G Kraft’s support. They are grateful to all colleagueswho helped in the laboratory during the cell processing procedure. This work was partiallyfunded by Siemens Healthcare, Particle Therapy.

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