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First published online December 6, 2006
British Journal of Radiology (2007) 80, 446-451
© 2007 British Institute of Radiology
doi: 10.1259/bjr/94582813
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Full paper

Dosimetric evaluation of a new collimator insert system for stereotactic radiotherapy

T Paschalis, PhD 1 P Sandilos, PhD 1,2 E Tatsis, MSc 1 P Karaiskos, PhD 2,3 C Antypas, MSc 1 C Chatzigiannis, MSc 1 K Dardoufas, PhD 1 E Georgiou, PhD 3 and L Vlachos, PhD 1

1 Department of Radiology, Medical School, University of Athens, Aretaieion Hospital, 76 Vas. Sofias Ave, 115 28 Athens, 2 Medical Physics Department, Hygeia Hospital, Kifissias Ave and Erythrou Stavrou, Marousi, 151 23 Athens, 3 Medical Physics Department, Medical School, University of Athens, 75 Mikras Asias, 115 27 Athens, Greece

Correspondence: P Sandilos, PhD, Department of Radiology, Medical School, University of Athens, Aretaieion Hospital, 76 Vas. Sofias Ave, 115 28 Athens, Greece. E-mail: p.sandilo{at}hygeia.gr


   Abstract
Top
 Abstract
Introduction
 Methods and materials
 Results and discussion
 Conclusion
 References
 
The prototype of a stereotactic collimator set developed in our department is evaluated for clinical use. This set consists of three cylindrical blocks mounted on a tray which slides in the wedge insert of a Siemens Primus accelerator. Each block has a collimating hole along its long axis to produce radiation fields of circular cross-section at the isocentre plane with diameters of 15 mm, 20 mm and 25 mm. Different geometric and dosimetric quality assurance tests were performed and results are found within the limits set for stereotactic radiotherapy. Dosimetry results measured using Kodak EDR-2 radiographic film and a pinpoint ion chamber also show good agreement with corresponding results calculated by Monte Carlo simulation of the linear accelerator head and the collimators. Measured dosimetry data were used to adapt a conventional PLATO treatment planning system for stereotactic radiotherapy using the prototype collimator set. Treatment planning system calculations and film measurements for treatment of an intracranial lesion in an anthropomorphic head phantom using coplanar 180° arcs are compared and found to agree within 2 mm. This supports the accuracy of dose delivery using the prototype stereotactic collimators. Despite their increased penumbra (2.5–3.5 mm relative to 2–2.5 mm for commercially available collimators) the ease of construction makes the proposed stereotactic collimators an interesting alternative for accomplishing cost effective stereotactic treatments.


   Introduction
Top
 Abstract
 Introduction
Methods and materials
 Results and discussion
 Conclusion
 References
 
While stereotactic radiosurgery (SRS) combines the use of high energy radiation beams with a stereotactic apparatus for skull fixation to irradiate surgically inoperable lesions in a single treatment of high mechanical accuracy, stereotactic radiotherapy (SRT) maintains the advantage of conventional fractionation through the use of non-invasive re-locatable stereotactic frames [1–4].

Conventional linear accelerators are not suitable for stereotactic irradiation because of the large penumbra resulting from the increased source to skin distance (SSD) between the patient and the collimating system. This limitation can be removed by using an additional collimator mount fixed at the proximal end of the secondary collimator to form stereotactic radiation fields which are commonly circular and small in diameter to offer homogeneous dose distributions.

Stereotactic field dosimetry is complicated by:

  1. the steep dose fall-off in the penumbra region [5]
  2. the lack of electronic equilibrium in all small fields [6, 7]
  3. the significant dependence of output factors on field size for fields below 20 mm in diameter [5, 8]
  4. the relationship between detector size and field dimension [9].

The above factors make the selection of an appropriate dosimeter essential. The sensitive volume of an ion chamber should be smaller than the beam radius in order to adequately resolve the penumbra region of stereotactic fields [10, 11]. Radiographic films exhibit a sufficient spatial resolution for stereotactic beam profile measurements in the penumbra region, suffering however from energy dependence and non-reproducible processing. Monte Carlo (MC) simulation of small stereotactic beams has been shown to provide reliable dosimetry results and is therefore considered the gold standard for inter-comparisons [12–14].


   Methods and materials
Top
 Abstract
 Introduction
 Methods and materials
Results and discussion
 Conclusion
 References
 
Stereotactic collimator set
Three cylindrical blocks of 15 cm height and 8 cm diameter were constructed in-house using Cerrobend (MT-A158, MED-TEC, Iowa, USA) alloy (50% bismuth, 27% lead, 13% tin, 10% cadmium). High accuracy milling was used to create isocentric collimating holes of designated sizes (15 mm, 20 mm and 25 mm at the isocentre) along their long axes. The collimators were inspected for uniformity flaws and collimating aperture alignment using 6 MV linac photon beam radiography. Each individual collimating system consists of a collimator attached on a plate that slides on the gantry head wedge mount of a Siemens Primus linear accelerator (linac) (Figure 1Go). The distance between the wedge tray and the isocentre is 58.7 cm for this particular linac. Each collimator was fitted with a convenient locking system for improved accuracy and safety.


Figure 1
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  		Figure 1. Stereotactic collimator insert.

 
Collimator concentricity test
A collimator concentricity test was performed to ensure the coincidence of the collimating aperture with the central beam axis. Primary collimators were set at 8x8 cm2 for all measurements to maintain a constant head scatter component [7]. A spherical steel ball, 3.0 mm in diameter, was placed at the isocentre. Radiographs were produced for various gantry and couch angle combinations ensuring that tertiary collimators were not mounted off centre [15]. The deviations between the radio-opaque ball's spot and the centre of the tertiary collimator's circular image were recorded. For stereotactic treatments, where a variety of simultaneous gantry and couch rotations are used, these deviations should not exceed 1 mm [1].

Beam size test
Radiographic Kodak EDR-2 films (30.5 cmx25.4 cm) were placed at a depth of 1.5 cm (the dmax for 6 MV photon beams) in a solid water phantom (SP34, Wellhofer Scanditronix GmbH, Schwarzenbruck, Germany) and irradiated using all three collimators to measure the radial profiles of the circular beams along various angular directions. The film model was chosen on the basis of its improved linearity and smaller depth, energy and field size dependence relative to the commonly used Kodak X-OMAT V films [16, 17]. The irradiated films were scanned using a Wellhofer Vidar Scan system (Wellhofer Scanditronix GmbH, Schwarzenbruck, Germany), which employs a 16-bit Vidar Dosimetry Pro CCD scanner (0.178 mm resolution) and the OmniPro-Accept (version 6.1) software. Measured optical density was converted to dose using a calibration curve acquired by irradiating films of the same batch at 10 cm depth using 5x5 cm2 fields to known doses ranging from 20 cGy to 500 cGy [16, 17].

Monte Carlo simulations
The OMEGA/BEAMnrc and DOSXYZnrc user codes of the EGSnrc MC simulation package distributed by the Canadian National Research Council were used to calculate the beam profile at several depths and the percentage depth dose distribution (PDD) of each collimator in a 40x40x40 cm3 water phantom. In addition to each collimator, the Siemens Primus linac head (including the target, primary collimator, absorber, flattening filter, dose chamber, mirror, inner/outer jaw and reticle) were also modelled using technical specifications of geometry and materials as well as energy and energy window values provided by the manufacturer. 25x106 electron histories incident on the target head were simulated to determine the photon spectrum leaving the treatment head, using an energy cut-off value AE = 700 keV. The selective Bremsstrahlung splitting (SBS) variance reduction technique was employed with Nmin = 20 and Nmax = 200. A photon energy cut-off of AP = 0.1 keV was used and the total number of photon histories simulated to generate the depth–dose curves in the phantom was of the order of 500x106 [18, 19]. The scoring voxel size varied from 1x1x1 cm3 to 0.1x0.1x0.1 cm3 at points close to the penumbra region.

Beam parameter measurements
All relevant beam parameters (PDD, off-axis ratios (OAR) at several depths, output and scatter factors) were measured for the 6 MV photon beam formed using each collimator for the purpose of adapting our conventional treatment planning system (TPS) for stereotactic radiotherapy. All measurements were conducted using a Wellhofer CC01 pinpoint ion chamber of 0.01 cm3 sensitive volume that is specifically designed for small circular field measurements. Collimator scatter correction factors (Sc) and total scatter correction factors (Sc,p), were measured relative to a 10x10 cm2 field, under isocentric conditions of 100 cm focus to detector distance (FDD), with the above chamber having its axis parallel to the central beam axis [5, 7]. Sc values were measured in air with an aluminium build-up cap on the pinpoint ion chamber (0.6 cm thickness, 2.7 g cm–2 density), especially designed to reduce the physical size of the cap and provide electronic equilibrium [5]. For Sc,p (measured at dmax = 1.5 cm), PDD and beam profile measurements, the chamber was placed with its long axis perpendicular to the central beam axis, in a Wellhofer Blue water phantom gated with Wellhofer Omnipro Accept 6.1 software. Kodak EDR-2 radiographic films were also irradiated in a solid water phantom for comparison with corresponding ion chamber results.

Treatment planning system adaptation
Measured dosimetric data for the three collimators were imported in a Nucletron Plato SunRise RTS 2.6 TPS. This system relies on the Bortfeld calculation algorithm for basic dose calculations that is basically a convolution of the energy fluence distribution and the pencil beam dose kernel [20]. Each stereotactic collimator was modelled as a set of two blocks placed on a virtual tray at the distal end of the collimator. Each block covers opposing half-fields with a semicircular aperture appropriate to each of the three collimators. Parameter fitting to measured data (PDDs, beam profiles, Scp, Sc, Sp (phantom scatter correction factor) calculated from Sc and Scp measurements [5]) was selected as the most appropriate means of simulating the stereotactic dose distributions. The shape of the TPS calculated profiles was matched to the measured ones by modifying the shape and dimensions of the blocks modelling each collimator. Deviations between calculated and measured profiles during this modification procedure did not exceed 1 mm. The penumbra region for each stereotactic collimator was modelled using separate Gaussian functions for the collimator jaws and each block [21]. Hence modifying the profile shoulder, the penumbra region and the block attenuation factor allowed adaptation of the conventional TPS for stereotactic radiotherapy.

Dose verification
Calculated and radiographic film measured dose distributions were compared for the purpose of evaluating the accuracy in the treatment of an intracranial lesion in an anthropomorphic head phantom (RANDO, Salem, NY, USA) using the prototype collimators and the adapted TPS. CT slices of the phantom were imported to the TPS and isodose curves for several arcs were generated for each plane (Figure 2Go). Planned isodose distributions were digitized using Grab It v.1.5 software (Datatrend Software, Raleigh, USA). Stereotactic treatment was delivered isocentrically (with the aid of markers) using coplanar arcs of 180°. EDR-2 radiographic films were placed parallel to the gantry's rotation plane inside the phantom (Figure 3Go). Any deviations between the planned and measured dose distributions would be largely attributed to potential construction defects in the prototype collimators.


Figure 2
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  		Figure 2. Planned isodose curves for collimator#3 in the humanoid phantom.

 

Figure 3
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  		Figure 3. Stereotactic treatment verification with film dosimetry.

 

   Results and discussion
Top
 Abstract
 Introduction
 Methods and materials
 Results and discussion
Conclusion
 References
 
Table 1Go summarizes the Scp and Sc correction factors measured in water and in air for the three prototype collimators, normalized to a 10x10 cm2 square open field. These measurements are in good agreement with those for commercially available stereotactic collimators [5, 7]. Figure 4Go depicts a typical radiograph taken from the set used for the collimator concentricity assessment. The average displacement of the collimator isocentre from the centre of the radio-opaque ball was measured at 0.4±0.1 mm. This value is within the acceptable limit for SRT, which is set to 1 mm [1]. Geometric accuracy was also investigated by measuring the penumbra region between the 20% and the 80% isodose lines. The average penumbra width ranged between 2.5 mm and 3.5 mm for the three prototype collimators. Although acceptably small, these values are slightly larger than those for commercially available collimators (2–2.5 mm). This is due to the smaller height of the prototype collimators (15 cm relative to 23–27 cm for commercially available ones). The height of 15 cm was chosen to preclude increased collimator weight for safety reasons.


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  		Table 1. Auxiliary collimator scatter factors(Sc) in air and total scatter factors (Scp) in water

 

Figure 4
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  		Figure 4. Collimator concentricity test(G = 0°).

 
Figure 5aGo compares the Kodak EDR-2 film measured and MC calculated beam profiles for the stereotactic collimator of 15 mm diameter at the isocentre. Figure 5bGo presents the same comparison for the stereotactic collimator of 25 mm diameter at the isocentre along with corresponding data measured using the pinpoint ionization chamber which is an acceptable detector for this relatively large beam size although it can be seen to underestimate dose in the shoulder region compared with film results [7]. However, Kodak EDR-2 films are found to be in closer agreement with pinpoint chamber measurements, compared with the commonly used Kodak XV-2 films, due to their superior linearity and smaller depth, energy and field size dependence compared with the Kodak X-OMAT V films [16, 22]. Overall, those results, such as shown in Figure 5Go, indicate field symmetry better than 3% which is within the acceptable limits for SRT and warrants the manufacturing precision of the prototype collimators.


Figure 5
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  		Figure 5. (a) Beam profile for d = 1.5 cm for collimator #1, (b) beam profile for d = 1.5 cm for collimator #3.

 
In Figure 6Go, PDD measurements using EDR-2 film and the pinpoint chamber are plotted along with corresponding MC calculations. The small sensitive volume of the pinpoint chamber leads to increased noise and therefore all pinpoint chamber measurements had to be smoothed in order to avoid fluctuations. On the other hand, the fine spatial resolution of films is a favourable characteristic for PDD measurements of small stereotactic fields.


Figure 6
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  		Figure 6. PDD for collimator#2 at SSD = 100 cm.

 
Figure 7a,bGo compares planned and film measured isodose distributions for a single 180° arc treatment of an anthropomorphic head phantom using the 15 mm and 25 mm stereotactic collimators, respectively. Overall, no systematic offset can be observed between measured and TPS calculated isodose distributions. On closer inspection, Figure 7Go shows that film data generally overestimate the extent of isodose lines, mainly due to the difference in absorption coefficients of the film and phantom materials. The distance to agreement between measured and calculated isodose lines in Figure 7a,bGo ranges between –0.1 cm to 0.2 cm and –0.1 cm to 0.1 cm for the 15 mm and 25 mm collimators, respectively. Greater differences are observed for the smaller 15 mm collimator due to the limitations set in the TPS modelling procedure for this very small field size.


Figure 7
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  		Figure 7. (a) Film verification of the planned isodose curves for collimator #1, (b) film verification of the planned isodose curves for collimator #3.

 

   Conclusion
Top
 Abstract
 Introduction
 Methods and materials
 Results and discussion
 Conclusion
References
 
The aim of this study was to evaluate the efficiency of a simple and cost effective method for implementing stereotactic radiotherapy. After ensuring the geometric accuracy of three custom made stereotactic collimators, their dosimetric parameters were measured to establish the reproducibility in dose delivery. Among the different techniques used, film dosimetry appears to be the most appropriate for dosimetry of narrow stereotactic beams. Their favourable characteristics, including energy independence, linearity of response, high spatial resolution and concurrent measurement in two dimensions, make Kodak EDR-2 films the dosimeter of choice. Monte Carlo simulations for the experimental collimators provided a credible tool for calculating dose profiles. However, the use of more than one dosimeter ensures the necessary reliability of stereotactic beam data for input in treatment planning systems. A conventional, three-dimensional radiotherapy treatment planning system was adapted to facilitate dose evaluation and optimization in stereotactic radiotherapy. Arcs of various angles were delivered to an anthropomorphic head phantom and satisfactory agreement was observed between planned and delivered dose distributions measured using radiochromic film. It is concluded that custom-made auxiliary collimators constitute a simple, accurate and cost-effective alternative for implementing stereotactic treatments of intracranial lesions.


   Acknowledgments
 
This work was partly supported by the EU and the Greek Ministry of Education, in the framework of the Scientific Programme "Herakleitos" and by the Special Research Account of the University of Athens.

Received for publication August 10, 2005. Revision received August 28, 2006. Accepted for publication September 11, 2006.


   References
Top
 Abstract
 Introduction
 Methods and materials
 Results and discussion
 Conclusion
 References
 

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This Article
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