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 Table of Contents  
REVIEW ARTICLE
Year : 2018  |  Volume : 14  |  Issue : 2  |  Page : 260-266

Comparison of full width at half maximum and penumbra of different Gamma Knife models


1 Department of Medical Radiation, Engineering Faculty, Central Tehran Branch, Islamic Azad University, Tehran, Iran
2 Department of Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran
3 Department of Radiotherapy Oncology, Cancer Research Center, Cancer Institute, Tehran University of Medical Sciences; Department of Medical Physics and Biomedical Engineering, Faculty of Medicine and Radiation Oncology Research Centre, Cancer Institute, Tehran, Iran

Date of Web Publication8-Mar-2018

Correspondence Address:
Dr. Hassan Ali Nedaie
Department of Radiotherapy Oncology, Cancer Research Center, Cancer Institute, Tehran University of Medical Sciences, Tehran
Iran
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0973-1482.189248

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 > Abstract 


As a radiosurgical tool, Gamma Knife has the best and widespread name recognition. Gamma Knife is a noninvasive intracranial technique invented and developed by Swedish neurosurgeon Lars Leksell. The first commercial Leksell Gamma Knife entered the therapeutic armamentarium at the University of Pittsburgh in the United States on August 1987. Since that time, different generation of Gamma Knife developed. In this study, the technical points and dosimetric parameters including full width at half maximum and penumbra on different generation of Gamma Knife will be reviewed and compared. The results of this review study show that the rotating gamma system provides a better dose conformity.

Keywords: Full width at half maximum, Gamma Knife, penumbra


How to cite this article:
Asgari S, Banaee N, Nedaie HA. Comparison of full width at half maximum and penumbra of different Gamma Knife models. J Can Res Ther 2018;14:260-6

How to cite this URL:
Asgari S, Banaee N, Nedaie HA. Comparison of full width at half maximum and penumbra of different Gamma Knife models. J Can Res Ther [serial online] 2018 [cited 2018 Sep 19];14:260-6. Available from: http://www.cancerjournal.net/text.asp?2018/14/2/260/189248




 > Introduction Top


In radiotherapy, the methods of treatment are becoming increasingly more complex and sophisticated in terms of delivery techniques and treatment precisions.[1] One of the most important objectives of treatment planning is to deliver the maximum dose to the tumor and the minimum dose to the surrounding tissues.[2] Advancement in radiation oncology over the past two decades travels from conventional radiotherapy to advanced forms of treatment. These techniques include three-dimensional conformal radiation therapy, intensity modulated radiation therapy, image-guided radiation therapy, and stereotactic radiosurgery (SRS).[3] The concept of SRS as a precise delivery of a single-fraction high-dose radiation to an imaging-defined target was originally developed by Swedish neurosurgeon Lars Leksell in 1951.[4] SRS with Leksell Gamma Knife (LGK) is a noninvasive intracranial technique based on the principles of the Leksell stereotactic system for open deep brain surgery.[5] The Gamma Knife delivers a single dose of ionizing radiation which is mechanically focused on the target and spares healthy areas of the brain from high-dose exposure to radiation and eliminates many of the risks inherent in traditional invasive surgery.[6],[7] Different generation of Gamma Knife had been developed after the Leksell invention. The aim of this study is to compare the full width at half maximum (FWHM) and penumbra of different Gamma Knife models.


 > Historical Aspects Top


Larson along with Leksell designed the first multicobalt unit in the late 1960s. Two collimators collimated the photons from the cobalt, and the secondary collimator was attached to the treatment couch. There were two exchangeable secondary collimator helmets collimated the beams. Therefore, 50% of the maximum dose fell within an ellipsoid volume of 3 mm × 3mm × 7 mm. The other helmet could produce a similar volume of 5 mm × 5 mm × 11 mm.[8]

The first Gamma Knife was placed in Sophiahemmet in Stockholm in 1968. However, the first patient (with a craniopharyngioma) was treated by Leksell and Backlund in 1968 at the site of the Gamma Knife manufacturing site in Linkoping.[9] This prototype was designed to produce slit like radiation lesions for functional neurosurgical procedures to treat pain, movement, or behavioral disorders that did not respond to conventional treatments. This model used the advantages of geometric focusing, multiple sources disbursed over a large area, and patient positioning to produce very small, disk-shaped lesions in brain nuclei, and white matter tracts. The success of this first unit led to the construction of the second device, which has been used at Karolinska Institute since 1975. Containing 179 cobalt-60 (Co-60) sources, the second Gamma Knife unit was designed to produce spherical lesion to treat brain tumors and intracranial arteriovenous malformations.[10],[11] In some respects, this machine was still a prototype, but it has been proved to be very reliable and easy to use.[12] Leksell cobalt unit evolved into a modern commercially available gamma unit incorporating 201 focused beams obtained from 201 sources. The output spectrum of radiation source consisting of two photon peaks 1.17 and 1.33 MeV. For the gamma unit, the dose falloff outside the target volume is quite rapid, the unit has several drawbacks most notably a restricted purpose, a very high capital cost and complicated source changing procedure which is required typically every 5 years.[13],[14]

Model U

The first LGK (Model U) entered the therapeutic armamentarium at the University of Pittsburgh in the United States on August 1987. The first patient, harboring vestibular schwannoma was treated in Pittsburgh in 1987. The Gamma Knife itself consists of a permanent 18,000 kg shield surrounding a hemispheric array of 201 sources of Co-60 with an average activity of 30 ci each. The central beam is fixed at an angle of 55° to the horizontal plane. The other beams are arranged in an arc and reach a focal point 403 mm from the sources. Four interchangeable collimators with diameters of 4, 8, 14, and 18 mm are used to vary the target volume.[15],[16] In the first year of operation, during which 152 patients were treated, all measured exposure levels were below the As Low As Reasonably Achievable limits established by the University of Pittsburgh and stated in NRC license application.[17]

The disadvantages of Model U is that some sources at the superior part of the patient are usually plugged to protect optic chiasm.[18] Another drawback is that loading and reloading of Co-60 sources are time-consuming.[19] To facilitate the reloading of the Co-60 sources, the unit was redesigned (Model B) so that the sources were arranged in a circular configuration.[20]

In 1990, Wu et al. measured FWHM with microchamber. The dimensions of the 50% isodose, or FWHM were measured 4.0, 8.4, 14.0, and 18.0 mm for 4, 8, 14, and 18 mm collimator helmets, respectively.[10]

Model B

Model B was installed first in Bergen in Norway in 1988. This model makes three-dimensional, complex treatment planning possible using modern imaging techniques.[21] The Gamma Knife Model B unit is made up of 201 Co-60 sources. They are directed to a single focal point at a source to focus distance (SFD) of 401 mm. The 201 sources are distributed a long five parallel circles on a hemispherical source housing separated from each other by an angle of 7.5°.[22] Primary collimators fixed in the main body of the Gamma Knife shape the primary beam while interchangeable helmets provide secondary collimators mated to each of the primary collimators and associated 201 Co-60 sources. Four helmets with different size collimator are available with nominal clinical beam size of 4, 8, 14, and 18 mm. In 2002, Bank was used a microchamber, PTW Pinpoint, and film to measure FWHM and penumbra for an Elekta Gamma Knife, Model B. [Table 1] shows the clinical beam widths at the 50% isodose lines (FWHM) and the penumbra widths as measured with film for the four helmets.[23]
Table 1: Beam widths and penumbras in X direction in the transverse plane for the four helmets measured with film and the PinPoint ion chamber. Beam width at 50% isodose line, penumbra from 20% to 82% isodose line

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In Model U, in keeping with original design, the patient couch was pulled into the machine and then upward by means of hydraulic power. This was done to improve radiation safety with Model B, replacing the hydraulic system with electric motors driving the couch straight into the radiation unit was a significant simplification. We could understand the illustration of the collimators in the two models (U and B) as shown in [Figure 1].[24]
Figure 1: The distribution of 201 collimators in the Gamma Knife Models U and B

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Model C

The first updated Model C Gamma Knife unit was installed in Germany in 1999.[25] The LGK with automatic positioning system (APS) differs from all LGK installed before 1999 by offering computer control of the treatment process.[26] Due to this advantage, Gamma Knife unit was changed from purely mechanical instrument to a more sophisticated electro-mechanical and electronic system.[27] The APS is a robot that moves the patient's head using motors from the position of one isocenter to the next without the intervention of the treating personnel in the treatment room. With the previous models, the staff had to enter the treatment room between each isocenter and set the next isocenter manually. This was a time-consuming procedure. It appears that the Model C with APS provides a better quality of treatment.[28]

Models C and B have similar source geometry and dimensions as a Model U, but Model U has larger latitude angels; thus, some sources at the superior portion of the patients are frequently plugged to protect the optic chiasm. The Models B and C are the widely used Gamma Knife units. For both models of Leksell (B and C), the dose is delivered with 201 Co-60 sources each of the Co-60 sources consists of a 20 cylindrical Co-60 pellets 1 mm diameter and 1 mm length. The problem of manually setting of the target position is solved by introducing Model C. Models B and C have same geometries and radiation units; however, Model C has a motorized APS.[19],[29],[30]

[Figure 2] shows the geometry of a single source in the Gamma Knife unit Model C.[31]
Figure 2: The geometry of a single source in the Gamma Knife unit Model C

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Model 4C

The Model 4C is the fourth generation of Gamma Knife, which was introduced in 2004. For the LGK Model 4C, the size of the high-dose volume delivered by one shot is determined by one of four possible collimator helmets with collimators ranging in size 4, 8, 14, and 18 mm in diameter. In addition to the APS, this model is equipped with software, which enables the coregistration of imaging data taken without frame fixation during treatment planning. Due to this improvement, integrating many kinds of imaging data became feasible for treatment planning. As mentioned previously, APS is an advantage of both Models C and 4C, but the APS took up space and reduced the range of couch's motion, and consequently, limited the access to certain regions of the skull. To avoid these limitations, the Perfexion Model was introduced.[19],[32],[33]


 > Perfexion Top


The world's first LGK Perfexion for radiosurgery started its operation at Timone University Hospital in Marseille on July 10, 2006.[34] The LGK Perfexion is a Co-60-based medical device, capable to deliver targeted radiation on a specific area of the brain. The Perfexion Model has a completely redesigned radiation unit with respect to its predecessors. In previous Gamma Knife models, the beam collimation system consisted of two separate parts: internal collimators arranged in a hemispherical pattern and embedded within the shield, and interchangeable collimator elements located in hemispherical external helmets. The most striking differences between Perfexion and its predecessors is the absence of the external part of collimation system.[35] For Perfexion system, the range of collimator size (beam size) is changed from the previous Gamma Knife models. Only three collimator sizes (4, 8, and 16 mm) are now available. The 120 mm thick tungsten collimator ring is subdivided in to eight identical sectors, each sector containing 72 collimators (24 collimators for 4 mm, 24 collimators for 8 mm, and 24 collimators for 16 mm). The beam size for each sector is changed automatically by moving 24 sources over the selected collimator set. With automatic selection of built in collimation, the need for the time consuming manual collimator installation required with previous model is eliminated. A total of 192 sealed sources of Co-60 are arranged on eight movable sectors, each sector accommodating 24 sources. The arrangement of sources differs substantially from the previous hemispherical arrangements and results in a different SFD for each ring varying from 374 to 433 mm.[33],[36],[37] A detailed illustration of the Perfexion collimator system is provided in [Figure 3].[38]
Figure 3: Leksell Gamma Knife Perfexion radiation unit and collimator system. (a) Cross-section of the Perfexion radiation unit. (b) Detailed view of sectors. Each sector holds 24 cobalt-60 sources and can be moved independently of other sectors in desired position to define a collimator size or block beams

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In the LGK Models C and 4C systems, the stereotactic treatment coordinates are setup using the APS. This has been replaced by a patient positioning system (PPS) in the LGK Perfexion system. In PPS instead of moving only the patient's head, the whole body of the patient lying on the PPS is moved into the preselected stereotactic coordinates. The PPS provides more comfortable position for the patient during treatment. The majority of the treatment with LGK Perfexion can be completed in just one run.[38] The beam-on times are comparable between various Gamma Knife models, and the setup time is generally 20 min less compared to the 4C Model.[20] There are some potential limitation with the Perfexion unit. The system is designed in such a way that a manual treatment mode is no longer possible. This is probably a relative limitation given the extensive testing, but in general it can be said that the reliability of the device is remarkable.[39]

In 2008, Novotny et al. were used Kodak EDR2 films for both Model 4C and Perfexion to measure FWHM and penumbra. A comparison of FWHM for all collimator is given in [Table 2]. A similar comparison of penumbra (defined as the distance between the 20% and 80% isodose lines) for all collimators and along the three stereotactic axes is given in [Table 3]. Excellent agreement (typically within 0.5 mm) was observed between experimental data (FWHM and penumbra) obtained from film measurements and calculated data obtained from treatment planning system. A difference of 0.2 mm in FWHM for the 4 mm collimator and 0.6 mm difference for the 8 mm collimator was observed. As a criterion for a good agreement for FWHM, one can take as reference the radiophysical acceptance test required by the manufacturer, where 1.0 mm agreement is required between measured FWHM and FWHM calculated by the treatment planning system during the LGK system acceptance. For the penumbra, the Perfexion system showed a 0.3 mm larger penumbra along the Z-axis for the 4 mm collimator and a 0.5 mm larger penumbra for the 8 mm collimator. An identical penumbra for the 4 mm collimator was observed along the X and Y stereotactic axes for both models; a 1.0 mm larger penumbra for the 8 mm collimator was observed along the X and Y axes for the Gamma Knife Model 4C. This can be explained based on the geometric penumbra, which is a function of the distance from the collimator to the isocenter (the penumbra increases with distance). Whereas for the Gamma Knife Model 4C, the distance for all sources is constant from collimator to isocenter while in the Perfexion this distance varies.[36]
Table 2: Comparison of calculated and measured full width at half maximum for all collimators along the X, Y, and Z stereotactic axes for Gamma Knife Perfexion and Gamma Knife 4C*

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Table 3: Comparison of penumbra for all collimators and all three stereotactic axes for Gamma Knife Perfexion and Gamma Knife 4C*

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 > Rotating Gamma Systems Top


In 1996, the first rotating gamma system (RGS) OUR was installed in china. The RGSs Gamma Art-6000 and its Chinese equivalent (OUR) are new radiosurgery systems that use rotating Co-60 sources instead of the 201 static sources.[18] The OUR RGS unit utilizes 30 Co-60 gamma radiation sources of initial activity, 200 curies or more (activity of the previous models is 30 curies) arranged symmetrically in 30 concentric circles, grouped in 6 spirals spaced at 60° intervals about the axis of rotation. Each Co-60 radiation source is doubly encapsulated in stainless steel and surrounded by a tungsten collimator. The source-to-focal point distance is 366 mm. A selection of four collimated apertures is available, producing nearly spherical volumes of collimated radiation with nominal diameters of 4, 8, 14, and 18 mm. A newer design of the RGS unit will offer radiation beam diameter of 3, 5, 10, 12, 16, 20, and 24 mm. The user could select 4 of the 7 available sizes prior to the manufacture and delivery of the unit. The patient's exposure to leakage radiation is reduced compared to the LGK, which does not have an internal shutter mechanism.[40] In 2003, American Radiosurgery has successfully installed the RGS Gamma Art-6000 at the advanced Radiation Oncology Center in Gurnee, Illinois, USA.[41] The unique device is a hybrid between the older multisource Co-60 radiosurgery units and LINAC radiosurgical units.[42]

Some advantages of RGSs are mentioned below:

  1. The radiation beams are collimated and converged from greater solid angle to the target while the healthy tissues surrounding the target receive less radiation
  2. Secondary collimators are built-in internally. This feature eliminates the need for changing the massive external secondary helmets to use the collimators of different sizes
  3. The use of 30 Co-60 sources decreases the cost of exchanging the sources and eliminates downtime.[43]


[Figure 4] shows the source body of Gamma Art-6000.[43]
Figure 4: Source body of Gamma Art-6000

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In 2006, Cheung and Yu were used the Monte Carlo system Parameter Reduced Electron-Step Transport Algorithm version of the electron gamma shower 4 computer code. [Table 4] shows the widths at 50% or FWHM of both Model C and Gamma ART-6000. [Table 5] shows the penumbra width for the 50% and 20% diameters between two cases using static sources (Models B or C) and RGS with the SFD of 400 mm.[18]
Table 4: Comparison of the widths at 50% level (full width at half maximum (between the Model C and Gamma Art-6000

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Table 5: Comparison of penumbra for all collimators between the cases using static and rotating sources

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The Gamma Knife system using static sources has a better beam profile penumbra than the Gamma Knife systems using rotating sources in the X or Y direction. The reason is that the scattered radiation increases along the X or Y direction in the beam profile as the number of beams using rotating sources increase mainly in the X or Y direction.[18]

In 1999, Goetsch et al. were used a prototype PTW Model 31006 Pinpoint Chamber to measure penumbra with SFD 366 mm. [Table 6] gives the measured field diameters for both the Leksell Model U Gamma Knife (Piedmont Hospital Gamma Knife Center, Atlanta, GA, USA) and the OUR RGS (Auhai Radiosurgery Research Center) for the 90% and 50% diameters.[40]
Table 6: Measured field diameters from radiochromic film for the Piedmont Hospital Gamma Knife Center Leksell Model U Gamma Knife and the Auhai Radiosurgery Research Center OUR Rotating Gamma System

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[Table 6] also gives the penumbra (the difference between the 90% and 50% field diameters). The 90% and 50% of measured field diameters for the Piedmont Leksell Model U gamma unit are within 1.1 mm of the corresponding results reported by McLaughlin et al. for the two Leksell Model U gamma units measured with radiochromic film and reported in 1994. This shows good consistency of both the measurement technique and the gamma units as well. The differences between 90% measured field diameters of OUR RGS and Leksell Model U is −1.6 mm, this quantity is larger for the 4 mm diameter. The OUR RGS and the Leksell Model U differed by 2.3 mm or less for 50% measured field diameters. The OUR unit had larger measured 50% diameters for both profiles at 4 and 8 mm, and for the X-Y profiles for the 14 and 18 mm diameters, but smaller measured 50% diameters than the Leksell Model U gamma unit for the X–Z profiles at 14 and 18 mm diameters. The difference for the 90% to 50% penumbra between the two units is 2.6 mm or less, with the Leksell unit the same or somewhat sharper at 4, 8, and 14 mm diameters, and with mixed results at 18 mm diameter.[40]

In 2002, Kubo and Araki reported the differences between the Chinese version of the RGS at Auhai Radiosurgery Center and new modified RGS unit installed at UC Davis Cancer Center in [Table 7]. The Gafchromic films were used for penumbra measurements. The dosimetry and mechanical accuracy of the modified RGS is found to be similar to those of the original RGS.[44]
Table 7: The first column indicates the collimator size and the plane of the film. The units of the data are all in millimeters. The large second and third columns are the results of Goetsch's Auhai RGSc data and Kubo and Araki's Auhai RGSc data, respectively. The last large column shows the results of our RGSu data

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 > Conclusion Top


The comparison of FMHM and penumbra of different models of Gamma Knife shows that the Gamma Knife systems using rotating sources have a better beam profile penumbra in the Z direction. On the other hand, Gamma Knife systems with static sources have a better beam profile penumbra in the X or Y direction.

It may be said that the modified RGS should behave similarly in terms of dosimetric and mechanical accuracy to the LGK but rotating Gamma Knife is a better option financially and provides a better quality of treatment with better dose conformity, shorter treatment time, and less exposure of staff to radiation than other models.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7]



 

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