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British Journal of Radiology (2005) 78, 899-905
© 2005 British Institute of Radiology
doi: 10.1259/bjr/57562962
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Full Paper

Technical note: evaluation of dosimetric performance in a commercial 3D treatment planning system

P Sandilos, PhD S Seferlis, MSc C Antypas, MSc P Karaiskos, PhD C Dardoufas, MD and L Vlahos, MD

Department of Radiology, Medical School, Areteion Hospital, University of Athens, 76 Vas. Sophia's Avenue, 11528 Athens, Greece


   Abstract
Top
 Abstract
Introduction
 Materials and methods
 Results
 Discussion
 Conclusion
 References
 
The aim of this work was to evaluate the dosimetric performance of a commercial treatment planning system (TPS) which employs a three-dimensional calculation algorithm (Nucletron Plato version 2.2.3), following the guidelines of the AAPM Task Group 23 (TG23). Seven test cases were used to test the TPS dosimetric performance in homogeneous water. These cases involved absolute dose measurements on central as well as off-axis points situated at various depths, using simple field arrangements, and comparison with corresponding TPS calculations. This comparison yielded differences within ±2% at all points, for all test cases. To test the ability of the TPS to account for tissue inhomogeneities, corresponding comparisons were performed with the presence of a low-density material in the beam to resemble an air inhomogeneity. Absolute dose measurements and corresponding TPS calculations showed a mean deviation of the order of ±3.5%, reaching a maximum of 11.5% for small field sizes (5 cm x 5 cm). In summary, observed deviations are well within the set tolerance levels while comparison with previous TPS versions showed that Plato version 2.2.3 is significantly improved, especially in dose calculations in the presence of low density inhomogeneities.


   Introduction
Top
 Abstract
 Introduction
Materials and methods
 Results
 Discussion
 Conclusion
 References
 
Radiotherapy aims to cure, or locally control disease, while concurrently minimizing complications in normal tissue. The International Commission on Radiation Units and Measurements (ICRU) has recommended that radiation dose must be delivered to within 5% of the prescribed dose [1, 2], while Minjeer et al [3] analysed clinical dose–effect curves and concluded that an overall accuracy of ±3.5% (1 standard deviation (SD)) of dose delivered to the ICRU reference point is required.

While treatment planning is one of the main steps in radiotherapy, dosimetry also includes calibration of the dosimetry equipment, determination of absorbed dose under reference conditions, phantom measurements under non-reference conditions, calculation of dose distribution in the patient and, finally, treatment delivery via monitor units or treatment time calculations. Consideration of the uncertainties associated with each of the above steps and their propagation increases the demand for accuracy in the dose calculation algorithm employed in treatment planning. Therefore, quality assurance of dose calculation algorithms, including extensive and detailed checks, is necessary in the commissioning stage of treatment planning systems (TPS) prior to their use in clinical practice. Additionally, the evolution of dose calculation algorithms from two to three dimensions has increased the number and the complexity of the required checks.

In general, two data sets are used for quality assurance purposes: basic beam data and test data. The basic beam set comprises relative dosimetry data measured by the user in a homogeneous water phantom and fed into the TPS for beam modelling purposes. These include percentage depth dose values (PDDs) or tissue phantom ratios (TPRs), beam profiles and output factors for field sizes and depths of clinical interest. The influence of beam modifying devices is taken into account with additional measurement of wedge beam profiles, wedge transmission factors and tray transmission factors. The test data set is used to check the performance of the dose calculation algorithm in clinical situations and includes absolute dose values measured at specific points under specified and representative beam geometries for a given number of monitor units or treatment machine times.

Commissioning of the dose calculation algorithm is performed by entering the basic beam data into the system and comparing dose calculation results with corresponding measurements of the test data set.

Evaluation of dose calculation algorithms, especially for TPS intercomparison purposes, relies on a unique basic beam data set entered into different systems. While the use of a unique beam data set minimizes the influence of uncertainties in the different sets of measurements performed by different users at different sites, there are also uncertainties associated with the beam modelling accomplished by different users as well as the fact that the unique data set is not representative of clinical practice in every department. TG23 [4] has developed a unique standard set of beam data acquired from two treatment units (Varian Clinac-4, 4 MV X-ray beam, Varian, Palo Alto; and AECL Therac-20, 18 MV X-ray beam, AECL, Kanata, Ontario, Canada) as well as a series of test conditions for comparison purposes. Although the TG23 test package can be considered as a complete set of data, there are drawbacks associated with its use [5, 6]. The most important is that it is not presented as an open beam data set to which new devices and new beam features, such as asymmetric settings and multileaf collimators (MLCs) which are currently of common use, can be added. Venselaar et al [5] have presented the Netherlands Commission on Radiation Dosimetry (NCS) test package which consists of a set of beam data measured on the Elekta SL15 and SL20 machines (Elekta, Norcross, USA) and a set of test configurations which adds to that of the TG23 report [4] by including new tests to cover beam features such as missing tissue and asymmetric geometries.

In the present work, the dosimetric performance of a commercial TPS with a three-dimensional calculation algorithm (Plato v2.2.3, Nucletron, Veenendaal, The Netherlands) is investigated using a basic beam data set measured for a 6 MV X-ray beam and a set of test case configurations which are based on the TG23 [4] and NCS test packages and adapted to the routine clinical practice of our department. A new calculation geometry is also developed to check the rebuild-up region under inhomogeneous conditions. The aim is to determine the accuracy of our TPS in dose calculations in a homogeneous phantom as well as in the presence of inhomogeneity, and outline potential limitations of the dose calculation algorithm.


   Materials and methods
Top
 Abstract
 Introduction
 Materials and methods
Results
 Discussion
 Conclusion
 References
 
Treatment planning system
The TPS agreement with the treatment machine is based on IEC 1217 conventions [7] for specifying gantry angle, collimator angle, table angle, wedge orientation and patient orientation. The TPS software operates in a UNIX environment where specific data storage areas can be created and a built-in software utility allows for graphical display and quantitative comparison of measured and calculated profile data (central axis, off axis, any direction).

The photon beam dose calculation algorithm employed by the Plato v2.2.3 TPS has been described by Bortfeld et al [8]. Basically, it comprises a convolution-based approach where the energy fluence distribution is convolved with a dose pencil beam. The dose pencil beam consists of three parts which have a depth-independent width and a depth-dependent relative weight. Thus dose calculation at any arbitrary depth in a homogeneous water phantom involves only one single convolution step for each of the three components. For other depths, the pencil beam components are summed using the depth dependent relative weights. Convolution of the energy fluence distribution with a Gaussian source distribution kernel allows for optimization of the fit between the measured and the calculated edge of the field. Inhomogeneities are taken into account by applying the equivalent tissue–air ratio (ETAR) method introduced by Sontag and Cunningham [9], as described by Yu and Wong [10].

Measurement instrumentation and techniques
Beam data and test point doses were measured for a 6 MV X-ray beam (quality index: {899equ3}) of a Siemens Mevatron linear accelerator (Siemens, Erlangen, Germany). Percentage depth dose (PDD) curves and beam profiles were measured with a fully computerized water phantom (Blue Phantom; Scanditronix Wellhöfer GmbH, Schwarzenbruck, Germany) equipped with thimble type ionization chamber detectors for relative measurements. Absolute dose measurements were performed with an ionization chamber (Farmer 30001 0.6cc PTW, Freiburg, Germany) connected to an electrometer (UNIDOS PTW, Freiburg, Germany). The chamber was calibrated in ND,w (absorbed dose to water calibration factor) according to the IAEA TRS-398 dosimetry protocol [11]. Test point doses were calculated from the integrated signal in irradiations delivering 100 monitor units (MU).

Beam and test data were obtained in different measuring sessions. Self-consistency of relative dose measurements was ensured by comparing a test beam profile (source to surface distance (SSD)=100 cm, 40 cm x 40 cm, depth=dmax) and a test PDD curve (SSD=100 cm, 10 cm x 10 cm) measurement with corresponding reference data obtained during acceptance testing of the linac. The possibility of daily output variation was precluded by output checks and relative measurements at reference conditions.

Basic beam data
The basic beam dataset used for beam modelling (presented with descending input priority) include:

Reference and calibration conditions
Reference conditions were used for determination of output factors and measurement of depth dose data and beam profiles: SSDref=100 cm, reference field FSref=10 cm x 10 cm, reference depth dref=5 cm. The machine was calibrated to deliver 1 cGy MU–1 at the following calibration conditions: SSDcal=SSDref=100 cm, FScal=FSref=10 cm x 10 cm, calibration depth dcal=dmax=1.5 cm.

Energy fluence
Energy fluence is basically represented by the off-axis beam profile of the maximum field size (40 cm x 40 cm) at SSDref and dmax along the major axes of the field (crossplane, inplane).

Depth dose data
Open beam depth dose data along the central axis of square field sizes of: 5, 8, 10, 12, 15, 18, 20, 25, 30, 40 (cm x cm) for depths from 0 up to 30 cm were measured.

Off-axis beam profiles
For each of the aforementioned square fields, five open beam profiles at depths dmax, 5 cm, 10 cm, 15 cm and 20 cm were acquired.

Wedge field data
Depth dose data and five beam profiles for square field sizes of 5, 10, 15 and 20 (cm x cm) were used for each wedge of 15°, 30°, 45° and 60° nominal angle. Differences between calculated and measured profiles were minimized by adjustment of weight factors according to the comparison of the calculated wedge beam profiles at depths 5 cm, 10 cm and 15 cm with corresponding measured values. The comparison and evaluation of differences between calculated and measured beam data was performed in a trial and error fashion using the Beam Data analysis Software (BDAS).

Output factors
The output factor was broken down into a total scatter factor Scp, a head scatter factor Sc and a phantom scatter factor Sp [12], and measured as a function of field size. Scp was measured in water under full scatter conditions at SSDref=100 cm and dref=5 cm and translated to dmax using the appropriate PDD values. Sc was measured using a 3 cm diameter mini phantom made of poly-methyl-methacrylate (PMMA) with the chamber in an upright position. Sp was calculated for each field size as the quotient of Scp and Sc.

Wedge factors, tray transmission, block transmission
Wedge and tray transmission factors were measured at SSDref=100 cm and dref=5 cm as a function of field size. Block transmission was determined using two beam profiles at depths of 10 cm and 20 cm in a wide field arrangement (25 cm x 25 cm).

Test data
The selected test cases represent different aspects of the dose computation process as proposed by AAPM TG23, NCS and other authors [13–16]. Test point measurements correspond to three different depths of 5 cm, 10 cm and 20 cm along the central axis for the reference SSDref=100 cm, unless otherwise stated.

Test case 1
Square fields ranging from 5 cm x 5 cm (the smallest used in our department) up to 20 cm x 20 cm.

Test case 2
Rectangular fields were produced by exchanging the x and y jaws (x x y and y x x) without collimator rotation. Rectangular fields and equivalent square fields were also examined. Field sizes of 5 cm x 22 cm, 6 cm x 12 cm, 7 cm x 9 cm equivalent to 8 cm x 8 cm and 10 cm x 20 cm, 11 cm x 16 cm, 12 cm x 14 cm equivalent to 13 cm x 13 cm were produced.

Test case 3
SSD variation. Three cases of isocentric setup and isocentre position were investigated, [SSD=95 cm, d=5 cm], [SSD=90 cm, d=10 cm], [SSD=85 cm, d=15 cm] for square fields of 5 cm x 5 cm, 7 cm x 7 cm, 10 cm x 10 cm, 12 cm x 12 cm, 15 cm x 15 cm, 18 cm x 18 cm, 20 cm x 20 cm and 25 cm x 25 cm.

Test case 4
Wedge filter. Square fields of 10 cm x 10 cm and 20 cm x 20 cm modified with a 45° wedge filter were investigated. Two measurements were performed for each of the possible wedge orientations. The average measured dose value was then compared with the corresponding calculated value.

Test case 5
Central block. A Cerrobend block of 10 cm x 10 cm dimension at the isocentre and 8 cm thickness resulting in 95% effective attenuation was investigated. Cerrobend is an alloy consisting of bismuth (50% wt.), lead (26% wt.), tin (13% wt.) and cadmium (about 10% wt.). Transmission measurements were performed for square fields of 10 cm x 10 cm, 15 cm x 15 cm and 25 cm x 25 cm at the depth of 10 cm.

Test case 6
Off centre planes. Point dose measurements were performed for a variety of square fields and off-centre planes, i.e. [5 cm x 5 cm, 2 cm], [7 cm x 7 cm, 3 cm], [10 cm x 10 cm, 4 cm], [13 cm x 13 cm, 5 cm], [15 cm x 15 cm, 6 cm], [18 cm x 18 cm, 7 cm], [20 cm x 20 cm, 8 cm]. The average of the four off-centre dose points in the crossplane and inplane directions was used as the mean off centre dose value in both measurements and calculations.

Test case 7
Oblique incidence. The aim of this test was to check the ability of the TPS to account for oblique incidence and skin contour variation. Using an isocentric setup and gantry angles of ±20°, ±30°, ±40°, the dose was determined at two depths of 5 cm and 10 cm along the central beam axis for FSD(Gantry angle=0°)=95 cm and FSD(Gantry angle=0°)=90 cm. Field sizes perpendicular to the beam direction were 10 cm x 10 cm, 15 cm x 15 cm and 20 cm x 20 cm.

Test case 8
Inhomogeneous medium. A low-density material, DOW Styrofoam (a construction material for heat insulation), was used to check the ability of the TPS to account for the presence of inhomogeneities. The inhomogeneity was parallelepiped in shape with a 5 cm x 5 cm side area and 8 cm thickness. Its electron density relative to water ({rho}ed=0.1) was calculated from the measured Hounsfield Number in a CT scan [17]. The inhomogeneity was suspended in the water phantom so that its upper side lay at a depth of 2 cm and the central beam axis passed through its centre. Point dose was measured at 15 cm depth along the central axis for square field sizes of 5 cm x 5 cm, 10 cm x 10 cm, 15 cm x 15 cm and 20 cm x 20 cm.

Test case 9
Inhomogeneity and scatter contribution. The aim of this test was to check the TPS accuracy in scatter contribution calculations. Four dose measurements were performed at a standard depth of 15 cm (i.e. SDD=115 cm) with the aforementioned inhomogeneity slab suspended in the water phantom with its long axis along the central beam axis and its bottom side laying at a distance of either 1 cm, 2 cm, 3 cm or 5 cm above the point of measurement. Measurements were repeated for four different field sizes (5 cm x 5 cm, 10 cm x 10 cm, 15 cm x 15 cm and 20 cm x 20 cm).


   Results
Top
 Abstract
 Introduction
 Materials and methods
 Results
Discussion
 Conclusion
 References
 
Calculated (Dcalc) and measured (Dmeas) dose values were compared for each of the nine test cases. The TPS dosimetric performance was evaluated by calculating the deviation ({delta}) between Dcalc and Dmeas [4, 18] expressed as a percentage of the dose measured locally, i.e. at the specific depth: Go


{899equ1}

In the cases where dose points corresponded to a low dose region, i.e. under a block, the result of the comparison was expressed relative to the dose measured at the same depth, but on the central axis of the open beam Dmeas,cax: Go


{899equ2}

The local dose was preferred for normalization over other alternatives such as the relative Dmax proposed by TG23, since it is considered clinically more relevant and representative of the TPS ability to accurately calculate tumour dose. In total, 190 test point measurements and calculations were compared for the 6 MV photon beam in our department. The final outcome of the comparison is summarized in Table 1Go, in the form of mean, maximum and minimum {delta} deviations. Figure 1Go presents the histogram of deviations for all test cases.


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  		Table 1. Results of the test cases

 


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  		Figure 1. Histogram of the differences between calculated and measured values for all test cases.

 
In test case 1, the TPS calculations for open square fields are in good agreement with measured values presenting a {delta}mean deviation of –0.30%. The maximum deviation, |{delta}|max, was 1.18% and none of the test points exceed the recommended tolerance level of 2% (Figure 2Go). Moreover, results also reveal that as field size increases, deviations become systematically negative, since the TPS tends to underestimate dose in relatively large fields.



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  		Figure 2. Histogram of the differences between calculated and measured values for the test case of square fields.

 
In test case 2, open rectangular fields were investigated. The {delta}mean and |{delta}|max deviations were 0.02% and 1.59%, respectively. All of the test points satisfy the tolerance level of 2% (Figure 3Go). This finding, combined with the dosimetric verification of calculations for rectangular fields, supports the adequacy of the equivalent square method. Moreover, a trend of TPS dose underestimation with increasing field size is also observed for rectangular fields.



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  		Figure 3. Histogram of the differences between calculated and measured values for the test case of rectangular fields.

 
In test case 3, the influence of SSD variation on TPS dose calculations was investigated. The {delta}mean and |{delta}|max deviations were –1.09% and 1.03%, respectively. None of the 24 test point measurements exceeded the tolerance level of 3%. As SSD decreases, absolute deviations were found to increase for all field sizes, being within the acceptable tolerance level. TPS dose calculations were smaller than measured values for most of the points.

In test case 4, the accuracy of TPS calculations was checked for a 45° wedged field. The {delta}mean and |{delta}|max deviations were 1.82% and 2.96%, respectively, while the tolerance level equals 3%. For five of the six points, the TPS was found to overestimate dose.

In test case 5 the influence of a central block on the dose calculation was investigated. The {delta}'mean and |{delta}'|max deviations were 0.37% and 0.52%, respectively, and results for all test points presented positive deviations which were well within the recommended tolerance limit of 4%. This implies proper configuration of the TPS block.

In test case 6, calculation errors for off-centre test points showed a {delta}mean deviation of 0.06%. |{delta}|max was 2.49%. Although increasing with field size, individual point deviations were all within the tolerance level of 3% and verify the TPS point dose module.

In test case 7, the oblique beam incidence was investigated. The {delta}mean deviation between calculated and measured dose was equal to –1.27% with |{delta}|max equal to 1.93%. TPS calculated results for all test points are within the acceptance limit of 3% and lower than measured dose values.

In test case 8, the accuracy of TPS dose calculation in the presence of a 3D inhomogeneity equivalent to air was investigated. Test points were along the central axis for various field sizes and the {delta}mean deviation between measured and calculated dose values was 1.91% with |{delta}|max equal to 5.90%. None of the test points exceeded the criterion of 15%.

Finally, test case 9 verified the accuracy of TPS calculations with respect to scattered radiation under a 3D, air equivalent inhomogeneity. The {delta}mean deviation between measured and calculated results was 1.12% with |{delta}|max equal to 11.45% and thus all of the test points met the tolerance limit of 15%.


   Discussion
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
Conclusion
 References
 
The aim of this work was to test the dosimetric performance of a commercial treatment planning system. Published criteria for the acceptability of TPS dose calculations present significant variation. The first criteria published by Van Dyk et al in 1993 [13] are characterized by increased tolerance limits due to the fact that most of the TPS were using two-dimensional algorithms at the time. The recommendations of AAPM TG53 report in 1998 [4], report 7 of the Swiss Society for Radiobiology and Medical Physics (SSRMP) in 1997 [18] and Venselaar et al in 2001 [5] are generally more strict, but realistic for a properly functioning dose calculation algorithm. When the complexity of the geometry increases, however, tolerance limits may have to be less strict relative to beam modelling geometry. In this work the set of tolerance limits proposed by Venselaar et al [5] was followed. However, it should be noted that only the accuracy of the dose calculation algorithm has been investigated without examining other potential inaccuracies associated with the geometry in the TPS (CT image acquisition and transfer, graphical display of 3D radiation beams etc.). In addition, results of this work are limited to the linac in our department (Siemens MEVATRON 6 MV).

The test cases used can be divided in two groups in terms of increasing complexity of the test configuration. The first group includes simple geometrical test cases (square and rectangular fields, SSD variation, off centre plane and oblique incidence) where dose calculations are performed in a homogeneous phantom for fields without special accessories. The second group includes complex geometrical test cases (wedge, central block and inhomogeneities). The first group of checks (test cases 1, 2, 3, 6 and 7) has also been studied by Alam et al [1] and Venselaar et al [6] for the older PLATO versions 1.21 and 2.01, respectively. Version 1.21 employs a two-dimensional dose calculation algorithm, while version 2.01 is one of the first systems by Nucletron to employ a 3D dose calculation algorithm. Comparing results of these previous studies [1, 6] with those of the present work for version 2.2.3, a continuous improvement of the system is evident. Although older TPS versions also met the tolerance limit for these test cases, a reduction of calculated to measured dose deviations is reported here. The above conclusion assumes ideal modelling of the three systems in the compared studies, or at least that the modelling was performed with comparable accuracy.

In the second group of checks (test cases 4, 5, 8 and 9), differences between the examined TPS and older versions are more significant. This can be explained by the improvements achieved in the algorithm.

In test case 4 of this work involving a wedged field arrangement, deviations are within 1.8% compared with 5% for PLATO version 1.21 and 2.5% for version 2.01. These differences are mainly due to the use of output factor input data corresponding only to open square fields in the older versions. This shortcoming, which added a dose calculation error of about 1–2% for wedged fields in the older versions, has been corrected in our TPS version 2.2.3.

In test case 5 of dose calculation on central beam axis under a central block, all the test points meet the tolerance limit of 4%. The improvement in dose calculation under the central block arrangement is obvious from the results of mean deviations for Plato versions 1.21, 2.01 and 2.2.3 which are 3.9%, 3.3% and 0.4%, respectively.

In test case 8, the mean deviation of calculated and measured dose under a 3D inhomogeneity is within the tolerance level of 15%. The maximum deviation is observed for the smallest field of 5 cm x 5 cm and is due to experimental uncertainty since the thimble type ionization chamber utilized in this work suffers from significant volume averaging in small field sizes. Moreover, for a relatively small photon beam (5 cm x 5 cm) incident on a relatively large air cavity, a lateral electron disequilibrium exists [8, 19]. On the other hand, the pencil beam dose calculation algorithm used by PLATO introduces increased inaccuracy in the presence of an inhomogeneity. However, deviation values reported for the older PLATO version 1.21 by Alam et al [1] for the inhomogeneity test (19.4%) were considerably greater than those in this work.

In test case 9, the maximum deviation of 11.45% also occurred for the smallest field size of 5 cm x 5 cm and a distance of 1 cm between the measurement point and the inhomogeneity. This is due to electronic disequilibrium for the small field size which does not overlap the inhomogeneity, combined with the small distance between the point of measurement and the inhomogeneity (1 cm) as well as a volume averaging error using the thimble ionization chamber in a high dose gradient region. Besides these uncertainties, however, the inhomogeneity correction method in ADAC TPS [19] as well as in PLATO presents a deficiency for dose calculation under air inhomogeneities due to the fact that dose calculation in the rebuild-up region under the inhomogeneity is influenced by dose calculation in the air cavity. It should also be noted that most commonly used dose correction methods for lung inhomogeneity, including that used in PLATO, correct the dose for changes in photon fluence but do not account for changes in charged particle transport and therefore may not accurately predict the dose close to the interfaces. It has been reported in the literature [20, 21] that the point kernel method used by HELAX TMS is superior to pencil kernel methods, especially for higher energies, when it comes to predicting the dose in the low-density region. The percentage differences between PLATO calculations and measurements are presented in Figure 4Go. The TPS calculates the contribution of scattered radiation more accurately as field size increases from 5 cm x 5 cm to 10 cm x 10 cm (Figure 4Go) and the slab of inhomogeneity lies at sufficient distance from the point of measurement to fulfil electron equilibrium conditions, but this change is not observed for field sizes 15 cm x 15 cm and 20 cm x 20 cm.



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  		Figure 4. (a) Deviations of measured from calculated point dose as a function of distance between inhomogeneity and point of measurement (ionization chamber) for field sizes 5 cm x 5 cm, 10 cm x 10 cm, 15 cm x 15 cm and 20 cm x 20 cm. (b) Deviations of measured from calculated point dose as a function of field sizes for the four distances between inhomogeneity and point of measurement (ionization chamber).

 

   Conclusion
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusion
References
 
The use of test data provided by TG23 and Venselaar is useful for quality control and intercomparison of new radiotherapy treatment planning systems since they provide a common reference. Nevertheless the beam modelling and basic data entered in each system depend on the user and the particular features of each system. In this work, basic entry data were measured for a linear accelerator used in clinical practice in our department and thorough quality assurance was performed for a commercially available TPS (Plato version 2.2.3). Measurements require about a month for each new beam quality while the TPS configuration and quality assurance require another couple of weeks. In summary, observed deviations between TPS calculated and measured dose are well within the set tolerance levels in both homogeneous and low density/air inhomogeneous geometries. Comparison with previous TPS versions showed that Plato version 2.2.3 is a significant improvement especially in dose calculations in the presence of low density inhomogeneities.

Received for publication April 19, 2004. Revision received April 5, 2005. Accepted for publication April 21, 2005.


   References
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusion
 References
 

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T Paschalis, P Sandilos, E Tatsis, P Karaiskos, C Antypas, C Chatzigiannis, K Dardoufas, E Georgiou, and L Vlachos
Dosimetric evaluation of a new collimator insert system for stereotactic radiotherapy
Br. J. Radiol., June 1, 2007; 80(954): 446 - 451.
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