evaluasi gamma knife

Application of the gamma evaluation method in Gamma Knife film dosimetry

Jeong-Hoon ParkDepartment of Neurosurgery, Seoul National University Bundang Hospital, Seongnam 463-707,Korea and Department of Biomedical Engineering, College of Medicine, The Catholic University ofKorea Seoul 137-701, Korea

Jung Ho Han, Chae-Yong Kim, and Chang Wan OhDepartment of Neurosurgery, Seoul National University Bundang Hospital, Seongnam 463-707,Korea and Department of Neurosurgery, College of Medicine, Seoul National University, Seoul 110-799,Korea

Do-Heui LeeDepartment of Neurosurgery, Asan Medical Center, College of Medicine, University of Ulsan,Seoul 138-736, Korea

Tae-Suk Suha)

Department of Biomedical Engineering and Research Institute of Biomedical Engineering,College of Medicine, The Catholic University of Korea, Seoul 137-701, Korea

Dong Gyu Kim and Hyun-Tai Chunga)

Department of Neurosurgery, College of Medicine, Seoul National University, Seoul 110-799, Korea

(Received 15 November 2010; revised 22 June 2011; accepted for publication 28 August 2011;

published 27 September 2011)

Purpose: Gamma Knife (GK) radiosurgery is a minimally invasive surgical technique for the treat-ment of intracranial lesions. To minimize neurological deficits, submillimeter accuracy is required

during treatment delivery. In this paper, the delivery accuracy of GK radiosurgery was assessed

with the gamma evaluation method using planning dose distribution and film measurement data.

Methods: Single 4, 8, and 16 mm and composite shot plans were developed for evaluation usingthe GK Perfexion (PFX) treatment planning system (TPS). The planning dose distributions were

exported as digital image communications in medicine radiation therapy (DICOM RT) files using

a new function of GK TPS. A maximum dose of 8 Gy was prescribed for four test plans. Irradiation

was performed onto a spherical solid water phantom using Gafchromic EBT2 films in the axial and

coronal planes. The exposed films were converted to absolute dose based on a 4th-order polynomial

calibration curve determined using ten calibration films. The film measurement results and planning

dose distributions were registered for further analysis in the same Leksell coordinate using in-house

software. The gamma evaluation method was applied to two dose distributions with varying spatial

tolerance (0.32.0 mm) and dosimetric tolerance (0.32.0%), to verify the accuracy of GK radiosur-

gery. The result of gamma evaluation was assessed using pass rate, dose gamma index histogram

(DGH), and dose pass rate histogram (DPH).

Results: The 20, 50, and 80% isodose lines found in film measurements were in close agreementwith the planning isodose lines, for all dose levels. The comparison of diagonal line profiles across

the axial plane yielded similar results. The gamma evaluation method resulted in high pass rates of

>95% within the 50% isodose line for 0.5 mm=0.5% tolerance criteria, in both the axial and coro-nal planes. They satisfied 1.0 mm=1.0% criteria within the 20% isodose line. Our DGH and DPHalso showed that low isodose lines exhibited inferior gamma indexes and pass rates compared with

higher isodose lines.

Conclusions: The gamma evaluation method was applicable to GK radiosurgery. For all test plans,planning dose distribution and film measurement met the tolerance criteria of 0.5 mm=0.5% withinthe 50% isodose line which are used for marginal dose prescription VC 2011 American Associationof Physicists in Medicine. [DOI: 10.1118/1.3641644]

Key words: radiosurgery, Gamma Knife Perfexion, film dosimetry, gamma evaluation, gamma

index

I. INTRODUCTION

Stereotactic radiosurgery (SRS) is a noninvasive surgical

technique used for the delivery of highly collimated radia-

tion in small fractions. After its initial introduction in the

1950s by Lars Leksell,1 it was widely spread by the develop-

ment of the linear accelerator SRS technique in the 1980s.2

Exclusive SRS modalities, such as Gamma Knife (GK),

(Elekta AB, Stockholm, Sweden), CyberKnife (CK, Accuray

Inc., Sunnyvale, CA), and Novalis (Varian Medical Systems

5778 Med. Phys. 38 (10), October 2011 0094-2405/2011/38(10)/5778/10/$30.00 VC 2011 Am. Assoc. Phys. Med. 5778

Inc., Palo Alto, CA) system, rendered SRS as a major treat-

ment method in both radiation oncology and neurosurgery.

Tomotherapy is also expanding its application to SRS.36

In particular, GK SRS targets intracranial lesions, such as

brain tumors, vascular diseases, and some functional

diseases.710 The presence of many critical structures and el-

oquent areas in the human brain requires careful SRS treat-

ment planning as well as dosimetric verification of the plan.

The verification between the plan and delivery is achieved

by comparing the planning dose distribution exported from

the treatment planning system (TPS) with the measurement

data obtained using radiation detectors. Although this was

studied extensively in radiation therapy physics, only a few

studies are available on GK SRS. These studies used mainly

a polymer gel to obtain a volumetric dose distribution11,14 or

to measure the output factors of collimators.15 Radiochromic

films also began to be accepted as dosimetric tools in GK.

Yamaguchi et al.16 compared absolute linear dose profiles inhead phantom with TPS data for four collimators of GK using

GafchromicTM (International Specialty Products, Wayne, NJ)

MD-55 films and a flatbed scanner. Novotny et al.17 measuredrelative output factors for small collimators of GK Perfexion

via absolute film dosimetry using Gafchromic EDR2, EBT,

and MD-55 films. However, as the direct acquisition of calcu-

lated dose distribution is not allowed in GK TPS,13 more

detailed comparison studies are not available.

In radiation therapy physics with a general linear acceler-

ator, dosimetric comparison of the treatment plan and deliv-

ered dose distribution is achieved using various

sophisticated techniques and tools. Point dose measurement

using an ion chamber, diode detector, or glass rod detector,

etc., is a basic technique for quality assurance (QA),18 this is

also available in GK SRS. Polymer gel dosimetry allows the

calculation of a three-dimensional dose distribution, despite

the necessity of using magnetic resonance imaging (MRI)

scanning for the readout. Recently, two-dimensional array

detectors such as MAPCHECKTM (Sun Nuclear Corp.,

Melbourne, FL) or MATRIXXTM (IBA Dosimetry America,

Bartlett, TN) started to be used commonly for daily and

patient-specific QA purposes, because of the simplicity of

measurement and high accuracy associated with these meth-

ods.19 They support direct comparison of the detector output

with planning dose distribution from TPS or measured dose

distribution from film measurement. Film dosimetry using

radiochromic films, such as Gafchromic MD-55, EBT, and

EBT2 films, provides a better spatial resolution compared

with other tools. In addition, as radiochromic films have a

water-equivalent electron density and two-dimensional dose

distributions are easy to obtain, they are accepted for both

relative and absolute dosimetry.16,20,21 Moreover, many

commercial software programs, such as OMNIPROTM (IBA Do-

simetry America, Bartlett, TN), FILMQATM (International Spe-

cialty Products, Wayne, NJ), and RAYTM (Standard Imaging,

Inc., Middleton, WI), provide a function that allows compre-

hensive analyses of treatment plan, array detector measure-

ments, and film measurements.

In GK SRS, film dosimetry is the only method that can

measure planar dose distribution accurately, as diode or ion

chamber array detectors for GK are not available currently. In

addition, as the planning dose distribution is only attainable

via screen snapshot of TPS, accurate dosimetric studies were

impossible in GK. The direct export of three-dimensional dose

distributions in the digital image communications in medicine

radiation therapy (DICOM RT) format has become available

with the development of the GK TPS LEKSELL GAMMAPLANVR

(LGP) 9.0 in late 2009. However, the commercial software

available does not support postprocessing of GK DICOM RT

data for dosimetric studies. In this study, the delivery accuracy

of GK SRS was evaluated using film measurement data and

planning dose distribution obtained with the new LGP feature.

The comparison results were quantified using our in-house

software and the gamma evaluation method devised by Low etal.22 The gamma evaluation method quantifies the spatial anddosimetric differences between reference and evaluated dose

distributions using a single metric called gamma (c) index. Weanalyzed the characteristics of GK dose distributions compre-

hensively using various tolerance criteria, including fundamen-

tal qualitative comparison results.

II. MATERIALS AND METHODS

II.A. Test treatment plan

Four test treatment plans were generated in LEKSELL GAM-

MAPLAN (LGP) version 9.0 TPS. As the Leksell Gamma

Knife PerfexionVR

(LGK PFX) system has three collimator

sizes of 4, 8, and 16 mm,23,24 single 4, 8, and 16 mm shots

were set at Leksell coordinates (100, 100, 100) (Fig. 1) for

three individual plans. For the evaluation of composite shots,

FIG. 1. Drawings of Leksell coordinates: the superior posterior right corner of

the patient is the origin (0, 0, 0) and the center coordinate of the Leksell frame

is (100, 100, 100). The x axis has a right to left direction, the y axis has a pos-terior to anterior direction, and the z axis has a cranial to caudal direction rela-tive to the patient. [From Elekta AB, Online Reference Manual Leksell

GammaPlanVR

8.3 (Document number 1008129), Rev. 01, Copyright VC 2008/

06 by Elekta Instrument AB. Reprinted by permission of Elekta AB].

5779 Park et al.: Gamma evaluation method in GK dosimetry 5779

Medical Physics, Vol. 38, No. 10, October 2011

which is a characteristic feature of LGK PFX, a single shot

of the [8 B 4 16 8 B 4 16] collimator sector arrangement was

added as the fourth plan (Fig. 2). The prescription dose was

8 Gy at the 100% isodose line for all plans, as the response

of EBT2 film is more sensitive below the 10 Gy exposure

dose.

For all test plans, three-dimensional dose distributions

inside the dose calculation matrix were exported as an RT

dose module of the DICOM RT standard using the DICOM

RT extension of TPS. The size of dose calculation matrixes

was set to minimum, just covering the 10% isodose line of

each shot, to assure a dose calculation that was as accurate as

possible. Therefore, the exported dose volume ranged from

15 15 15 mm for the 4 mm plan to 60 60 60 mmfor the 16 mm plan. The dose calculation grid size was set to

0.1 mm for the 4 and 8 mm plans and to 0.2 mm for the other

plans, to maintain a similar number of reference points inside

isodose lines.

II.B. Gamma Knife film dosimetry

Film measurement was performed on a spherical phantom

with a diameter of 16 cm to obtain delivered dose distribu-

tions. Elekta manufactures two different phantoms for LGK

PFX: a polystyrene phantom (Fig. 3(a)) and a solid water

phantom [Figs. 3(b), 3(c)].25 Both can hold a film and an ion

chamber for dosimetric purposes. However, they are made

of different materials and have different fixation structures

onto the couch. The polystyrene phantom has a structural

problem that attenuates radiation output from the lateral

direction, as pointed out by Bhatnagar et al.26 It also yields aradiation output that is 2% lower because of the higher elec-

tron density of polystyrene compared with water.27 To over-

come these disadvantages, we used a solid water phantom

that was newly designed for LGK PFX. It is made of pure

solid water material and does not block any collimator sec-

tor. It can hold a maximum of ten measurement films and

can be configured in both the axial (xy) and coronal (xz)planes. It provides a film perforator to align the films in the

stack so that they are compressed evenly. Two perforated

holes are separated by 52 mm at (1006 26, 100, 100) per-pendicular to the cranialcaudal axis [Fig. 3(d)]. These were

used for the alignment with the plans after image processing.

To minimize alignment errors between film and calcula-

tions, we performed another test plan using stereotactic CT

images of the spherical phantom. After defining the Leksell

coordinate of phantom images, the exact center of the film

stack was found in TPS by calculating the center of the fixa-

tion bars. There was no discrepancy between the center of

the film stack and the center of coordinates (100, 100, 100),

which allowed the minimization of alignment errors. There-

fore, the film was guaranteed to coincide with the plan and

only systematic errors occurred. The long-term accuracy and

stability of the positioning in GK have been reported by

Heck et al. as being within 0.2 mm.28

We used Gafchromic EBT2 film in all measurements

[Fig. 3(d)]. The EBT2 film has enhanced physical character-

istics compared with the previous EBT film, such as lower

sensitivity to light, lower postexposure, and stronger resist-

ance to physical damage. The detailed explanation and

experiments using the new EBT2 film are found in Richley

et al.29 and Andres et al.30 The films were cut into a size of60 60 mm along the same orientation as that of the originalfilm to be fitted into the phantom. After perforation, films

were placed on three successive stacks in the center of the

phantom, i.e., y 95, 100, and 105 or z 95, 100, and 105.First, calibration films were irradiated to obtain optical den-

sity to absolute dose correlation. Eight films were exposed to

0.5, 1, 2, 3, 4, 6, 8, and 10 Gy using the 16 mm collimator

and one film was left unexposed. The films were irradiated

using the physics mode of LGK PFX for an exposure time

that was determined as the irradiation dose divided by the

current dose rate. The exposure time was determined to be

within 0.01 min. Next, the four treatment plans developed in

Sec. II A were delivered in the axial (xy) and coronal (xz)configurations of the phantom; thus, eight studies were per-

formed. Both calibration and measurement film examina-

tions were repeated using five film sets to average out

random noise caused by the experimental setup.

Forty-eight hours after the exposure, the films were

scanned as TIFF images at a resolution of 300 DPI (0.08

mm=pixel) and a bit color depth of 48 (16 bit=channel) usingthe commercial flatbed scanner EPSON Expression 10 000

XL (Epson America Inc., Long Beach, CA) with a transpar-

ency unit. After a scanner warming time of 30 min, the cali-

bration and measurement films were scanned five times to

minimize the random noises and uncertainties that occurred

during the scanning procedure. For all calibration films, av-

erage pixel values located within a 3 mm radius from the

center were analyzed with the automatic region of interest

(ROI) function of our software. It maintains a consistent

ROI location for reliable analysis using the coordinate infor-

mation determined from the punch holes. They were con-

verted to optical density and calibration curves of the 4th-

order polynomial were determined for each RGB channel

FIG. 2. Collimator sector configuration for the composite shot, which is a

characteristic feature of GK PFX. The 4, 8, and 16 mm shots and blocked

sectors were mixed.



using the curve fitting function of IMAGEJ (National Institutes

of Health, Bethesda, MD) software. For further analysis, the

measurement films were converted to absolute dose using

the calibration curve. A detailed film dosimetry procedure is

described in Devic et al.,20 we adopted similar procedures.

II.C. Application of the gamma evaluation method

First, the plan and measurement data should be aligned in

the same Leksell coordinate, as they have different centers,

sizes, and rotation angles. We developed an in-house soft-

ware using MATLAB R2010 (The Mathworks, Natick, MA) to

align and scale them into the same coordinates. The exact

center of the measurement film was equivalent to the center

of two punching holes used for fixation, which can be deter-

mined by detecting the perimeter of the circle. In addition,

the rotation angle of the film during the scanning procedure

can be deduced from the line connecting the two centers.

The film images were transformed to Leksell coordinates

using the coordinate center, rotation angle, and known image

resolution. As the center voxel of RT dose data in the

DICOM RT file already corresponded to the center of the

Leksell coordinate in the planning stage, we now had two

dose distributions in the same Leksell coordinate, which

were used for further comparison. Before comparing two

data sets, we applied a Wiener filter with a mask size of

7 7 pixels to measurement data, to minimize image noisescaused by the film itself, while preserving the original

data.20 The punch hole regions were excluded in the analysis

because the cutting edge of these holes has an abnormally

high optical density and the hole itself exhibits an all white

background.

As explained in Sec. I, the gamma evaluation method is a

technique used to quantify the similarity of two dose

distributions as a gamma index. It is a composite analysis

that combines both dosimetric and spatial accuracy. It pro-

vides a reasonable measurement of the quantitative compari-

son of reference and evaluated dose distributions. Many

modified gamma evaluation methods have been developed

to accelerate or improve the original algorithm.31,36

Generally, gamma evaluation is achieved by comparing

three-dimensional planning dose distributions from TPS

with two- or three-dimensional dose distributions obtained

using other plans, film measurement, or an array detector.

According to the definition of the original gamma evaluation

method, the gamma index c at the point rr of the referencesystem (rr, Dr) is calculated from Eqs. (1) and (2) for theevaluation system (re, De).

32

C~re; r!r

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffir!e

r!r2Dd2

De r!

e Dr r!2

Dd2

s(1)

c r!r minfC r!e; r!rg8f r!eg: (2)where Dd and DD are the spatial and dosimetric tolerancecriteria for the comparison systems typically set as 3

mm=3%. In gamma evaluation, if c 1, the reference pointrr satisfies the tolerance criteria relative to the evaluationsystem; otherwise, it fails. If the pass rate, which is a ratio of

points that satisfy the tolerance criteria, is higher than 95%,

the two comparison systems are said to be equivalent, at

least under the criteria. Here, we selected DICOM RT data

as the reference data and film measurement as the evaluation

data.

FIG. 3. Dosimetric phantoms used for GK QA purpose

and irradiation sample of the EBT2 film. (a) Old poly-

styrene phantom and phantom holder; (b) new solid

water phantom and EBT2 film in the axial plane config-

uration; (c) in the coronal plane configuration; and (d)

irradiation sample of Gafchromic EBT2 film.



We applied four criteria of 2.0 mm=2.0%, 1.0 mm=1.0%,0.5 mm=0.5%, and 0.3 mm=0.3% to verify the delivery accu-racy of GK SRS. We used Lows original algorithm to

implement the gamma evaluation method. The evaluation

points within a 5 mm distance from the reference point were

included in the evaluation to reduce calculation time. In the

GK plan, dose prescription is given on a much lower isodose

line, e.g., 50%, compared with the radiation therapy plan,

which uses the isodose line of >95%.37 This means that theGK plan has a more inhomogeneous dose distribution inside

the PTV, ranging from 50 to 100%. Therefore, we calculated

the pass rate of reference points inside the 50% isodose line,

which is clinically important, as well as the 20% isodose

line, which was selected for peripheral dose assessment.

However, as other higher and lower isodose lines are com-

monly used to examine hot spots and normal tissue damage,

a single gamma index inside the 20 and 50% isodose lines

cannot explain heterogeneous delivery characteristics. This

led to the design of the dose gamma index histogram (DGH)

and dose pass rate histogram (DPH), which show the relative

delivery accuracy of the plan. First, isodose levels of

10100% were binned into a 5% interval and reference

points inside each bin were collected. Next, the gamma

indexes and pass rates were calculated separately for each

bin and the results were displayed as dose vs pass rate rela-

tion. The DGH and DPH allow the determination of which

isodose range had relatively good delivery accuracy or not.

Generally, gamma evaluation requires reference data with

smaller, i.e., less data points, or equal dimensions compared

with the evaluation data, to prevent interpolation artifacts.32

Our data satisfied this requirement, as DICOM RT dose data

had a resolution of 0.100.20 mm and film measurement had

a resolution of 300 DPI, which is equivalent to 0.08 mm.Moreover, as the pixel spacing of 0.08 mm in the evaluation

data was smaller than the minimum Dd of 0.3 mm, the datasets had a configuration that was suitable for the prevention

of interpolation artifacts.32

FIG. 4. Optical density vs absolute dose curves of EBT2 film in the 010 Gy dose range for red, green, and blue channels.

FIG. 5. Comparison of isodose lines between planning dose distribution and

film measurement in the axial plane. Solid lines are the 20, 50, and 80% iso-

dose lines from the DICOM RT file. Dash lines are measured isodose lines.



III. RESULTS

III.A. Film dosimetry and qualitative comparison

The optical density of calibration films was calculated

from the pixel values of each red, green, and blue channel.

Their correlation to the absolute dose of the 010 Gy range

was determined as a 4th-order polynomial, as shown in Fig.

4. The red and green channels had an R2 value of 0.9999 in

the 010 Gy range; however, it is well known that the red

channel has a good response under a 10 Gy dose in using

EBT2 film.30 Therefore, we used pixel data of the red chan-

nel for further analyses of measurement films. Figure 4

shows that the correlation curve of the red channel was most

sensitive regarding irradiation dose, compared with other

channels in this range.

After alignment of measurement data and DICOM RT

data into the same Leksell coordinate, isodose lines and

linear dose profiles were compared using a qualitative analy-

sis. Figure 5 shows the planning and measured isodose lines

overlaid on the film measurement image. It shows the 20,

50, and 80% isodose lines in axial (xy) plane at z 100. Forall four plans, the 50 and 80% isodose lines exhibited close

agreement between treatment plan and measurement data.

Measurement data exhibited a slightly broader isodose distri-

bution at the 20% isodose line. The disagreement at low iso-

dose lines tended to worsen as the size of the collimator

increased. In the composite shot, isodose lines had elongated

shapes, with variation of major axes depending on isodose

levels. The dose distribution of the composite shot matched

the measurement closely, even in the blocked sector and in

the 16 mm collimator sector direction. Figure 6 compares

the line profiles of treatment plan and measurement data

along the diagonal direction, i.e., from the anterior-right to

the posterior-left side of the patient in the axial plane.

FIG. 6. Comparison of the line profiles of four test plans and film measurements in the diagonal direction along the anterior-right to posterior-left side of the

patient (top left to right bottom side in Fig. 5). The solid lines correspond to DICOM RT data and the dotted lines correspond to film measurement data.

TABLE I. Pass rates of gamma evaluation within the 20 and 50% isodose lines using four evaluation criteria (2.0 mm=2.0%, 1.0 mm=1.0%, 0.5 mm=0.5%, and

0.3 mm=0.3%; units, %).

20% isodose line 50% isodose line

Plan

2.0 mm

=2.0%

1.0 mm

=1.0%

0.5 mm

=0.5%

0.3 mm

=0.3%

2.0 mm

=2.0%

1.0 mm

=1.0%

0.5 mm

=0.5%

0.3 mm

=0.3%

4 mm axial 100 100 94 60 100 100 96 72

coronal 100 100 90 52 100 100 96 68

8 mm axial 100 100 96 67 100 100 98 78

coronal 100 100 95 67 100 100 95 74

16 mm axial 100 100 95 64 100 100 95 76

coronal 100 100 78 49 100 100 97 68

Composite axial 100 100 98 72 100 100 98 80

coronal 100 100 84 50 100 100 98 70



Similar to what was found in the isodose line comparison,

these values agreed closely with regard to the whole isodose

ranges, without distinct mismatched regions.

III.B. Quantitative comparison using the gammaevaluation method

Table 1 summarizes the pass rates of gamma evaluation

using four evaluation criteria for the eight test studies. Inside

the 50% isodose line, all test plans passed the criteria up to

0.5 mm=0.5% in both axial and coronal planes, with pass rates>95%. Axial studies showed higher pass rates of 9498% for0.5 mm=0.5% criteria even within the 20% isodose line. How-ever, other coronal studies satisfied maximum 1.0 mm=1.0%criteria for the 20% isodose line, rather than 0.5 mm=0.5%criteria. Both the 20 and 50% isodose lines failed to achieve

the last criteria of 0.3 mm=0.3%. Figure 7 shows the gammaindex map for 0.5 mm=0.5% tolerance criteria in axial plane.Four plans showed low gamma index values in the irradiated

areas around the center of the shot. In contrast, high gamma

index values (>2.5) were observed in the unexposed areas.This phenomenon was apparent at the borders of 4 and 8 mm

collimator shots and at the blocked sector side of the compos-

ite shot. In the 4 and 8 mm collimator study, DICOM RT data

with a 0.1 mm dose grid were used for the calculation of the

gamma index and a 0.2 mm grid was used for the visualiza-

tion of a coarse gamma index map. The dose grid of 0.1 mm

yielded a pass rate that was 0.5% higher compared with

that obtained using the 0.2 mm grid.

Figure 8 shows the dose gamma index histogram (DGH)

between the 10105% dose ranges, grouped by 5% isodose

bins. For the 0.5 mm=0.5% criteria, the gamma index waskept under 0.6 in most dose bins. High dose bins had a lower

gamma index compared with low dose bins but the differen-

ces were not large. For the 0.3 mm=0.3% criteria, all plansshowed more irregular shapes, with maximum values

exceeding 1.0. Low isodose bins had a relatively higher

gamma index. Figure 9 illustrates the dose DPH representing

individual pass rates within the bin. Similar to what was

observed for the DGH, the DPH revealed the presence of a

nearly flat pass rate distribution for 0.5 mm=0.5% criteria.The DPH of 0.3 mm=0.3% criteria resulted in a low pass ratethat was

conformed to the lesion margin, with the exception of

extremely small lesions. The delivery accuracy inside this

line affects tumor control directly and is associated with nor-

mal brain complications. The results of the analyses

described above show that the planning dose distribution

based on the TPS was in close agreement with the delivered

dose distribution, at least within clinically important ranges.

Results also showed that low isodose levels had lower pass

rates for 0.5 mm=0.5% and 0.3 mm=0.3% criteria comparedwith higher isodose levels, which was also demonstrated

clearly in our DGH and DPH analyses (Figs. 8 and 9). This

was supposedly caused by the sensitivity characteristics of

the EBT2 film; as this film exhibits a relatively steep

response to irradiation doses in the low dose range, as shown

in Fig. 4, small perturbations in absorbed dose can induce

larger differences in film response in the low isodose range

than in the high isodose range. Alternatively, this may be

caused by the inaccuracy of dose models in the peripheral

region, which should be addressed in a future study.

The advances in the research of GK dosimetry have been

relatively poor compared with those of other radiation ther-

apy modalities. In the early days, polymer gels were used

widely as a dosimetric tool, and films are beginning to be

adopted for various measurement purposes. However, as GK

plan data are accessible only via the digitization of the LGP

planning screen or special support provided by the manufac-

turer, research in this area was limited to a few themes. The

new DICOM RT export function is expected to resolve this

problem and motivate researchers. Although official analyti-

cal tools for GK are not available yet, our experiences can be

referenced in the development of customized software. The

Gafchromic EBT2 film and solid water phantom for the GK

also provided a convenient method of measurement of deliv-

ered dose distribution. We expect that this combination can

be extended as a useful research and QA tool for the compar-

ison of dose distributions in GK radiosurgery. In particular,

it is very desirable to include gamma evaluation testing for

certain tolerance criteria in the annual QA procedure of GK.

FIG. 8. Dose gamma index histogram in 5% isodose bins for 0.5 mm=0.5% and 0.3 mm=0.3% tolerance criteria in the axial plane.



In addition, the built-in film analysis feature needs to be sup-

ported in the LGP TPS for routine and convenient analysis.

V. CONCLUSIONS

A comprehensive comparison of planning and delivered

dose distributions in GK SRS was performed using DICOM

RT data, a solid water phantom, and EBT2 film dosimetry.

For single 4, 8, and 16 mm and composite collimator shots,

the comparison of qualitative isodose lines and line profiles

did not reveal any notable deviations between planned and

film measurements. In addition, the gamma evaluation

method was applied to the verification of the quantitative

delivery of GK SRS. It revealed that GK SRS satisfied toler-

ance criteria of 0.5 mm=0.5% inside the 50% isodose line,which is used for marginal dose prescription, and 1.0

mm=1.0% criteria inside the 20% isodose line. Our DPH andDGH analysis also confirmed that low isodose areas

(1040%) exhibited a relatively inferior accuracy compared

with high isodose areas (50100%).

ACKNOWLEDGMENTS

This work was supported by a grant no. 04-2011-

0320110130 from the Seoul National University Hospital

Research Fund and a National Research Foundation of Korea

(NRFK) grant funded by the Korean government (MEST)

(No. 20110001851). The authors acknowledge Mr. Wook

Choi and Mr. Soo-Hyun Lee of Elekta Korea Inc. for their

support on the DICOM RT export procedure. The authors

have no conflict of interest.

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