Physical and Dosimetric Aspect of Euromechanics Add-on Multileaf Collimator on Varian Clinac 2100 C/D.

BACKGROUND: Before treatment planning and dose delivery, quality assurance of multi-leaf collimator (MLC) has an important role in intensity-modulated radiation therapy (IMRT) due to the creation of multiple segments from optimization process.
OBJECTIVE: The purpose of this study is to assess the quality control of MLC leaves using EBT3 Gafchromic films.
MATERIAL AND METHODS: Leaf Position accuracy and leaf gap reproducibility were checked with Garden fence test. The garden fence test consists of 5 thin bands A) 0.2 Cm width spaced at 2 Cm intervals and B) 1 Cm width spaced at 1 Cm intervals. Each leaf accuracy was analyzed with measuring the full-width half-maximum (FWHM). Maximum and average leaf transmission were measured with gafchromic EBT3 films from Ashland for both 6 MV and 18 MV beams.
RESULTS: Leaf positions were found to be in a range between 1.78 - 2.53 mm, instead of nominal 2 mm for the test A and between 9.09 - 10.36 mm, instead of nominal 10 mm for the test B. The Average radiation transmission of the MLC was noted 1.79% and 1.98% of the open 10x10 Cm2 field at isocenter for 6 MV and 18 MV beams, respectively. Maximum radiation transmission was noted 4.1% and 4.4% for 6 MV and 18 MV beams, respectively.
CONCLUSION: In this study, application of gafchromic EBT3 films for the quality assurance of Euromechanics multileaf collimator was studied. Our results showed that the average leaf leakage and positional accuracy of this type of MLC were in the acceptance level based on the Protocols.

Introduction

Before treatment planning and dose delivery, quality assurance of multi-leaf collimator (MLC) has an important role in intensity-modulated radiation therapy (IMRT) due to the creation of multiple segments from optimization process. Transmission from the MLC should be considered completely when using MLC for the IMRT technique [1-3]. Transmission increases the dose in and out of target volume which is not desirable.

There are two inaccuracies in leaf-positioning, including random and systematic ones. The random leaf-positioning inaccuracies effects is stochastic, moreover, after some treatment fraction, they will be reduced. The effect of systematic leaf-positioning inaccuracies will Cause large dose deviations inside the target volume during treatment of a patient who has been planned by treatment planning system. One of the reasons caused the inaccuracy in leaf-positioning is wrong calibration of leaves. Amount of the wrong calibration can be detected with leaf-positioning tests [4,5].

Several types of MLCs are commercially available. Some of their properties are different from conventional collimators, including design, physical and dosimetric characteristics. MLCs have been used to conform the field of treatment to the target volume for several years. Several studies have investigated the dosimetric characteristic of some MLCs used for static fields [6-8]. Several studies have investigated dosimetric properties of MLC systems using small ionization chambers and radiographic films [9,10], They reported some characteristics such as leaf transmission, leakage and beam penumbra measurements. Small ionization chambers have the limitation due to their finite size and effective active area (EAA), They measure field sizes larger than their EAA precisely, but for field sizes smaller than their EAA, partial volume effects might have an impact on their response. On the other hand, although the advantages of the radiographic films are high spatial resolution and energy independency, they might show an overestimation of beam penumbra due to their over-response to scattered photons with low energy.

In this study, we used EBT3 films which are the newest version of the gafchromic EBT series. The EBT3 is an advanced version of the EBT2 film [11]. One of the advantages of the EBT3 rather than EBT2, is the film scan orientation independency [12]. This is the first installation of 60-leaf PMLC from Euromechanics company and the aim of this study is to assess the quality control of MLC by using the EBT3 gafchromic films.

Material and Methods

A. MLC Description

The Euromechanics PMLC (Euromechanics Medical GmbH, Germany) consists of 60 leaves, each leaf is an independently motor driven and has an independent position measuring system. These 60 leaves are positioned in 2 banks and have 50% over-traveling. The MLC leaves are 63 mm thick and made of high-density tungsten alloy, they are single focus with rounded front end with tongue and grooves design. The physical width for all leaves are 4.3 mm, and project width of around 7 mm at the isocenter depends on the clearance of different LINAC Types.

B. EBT3 Gafchromic Films

The gafchromic EBT3 (Ashland Specialty Ingredients, NJ, USA) is designed for the measurement of absorbed doses of ionizing radiation and appropriate for high energy photons and available in sheets of 8*10 inch. According to the manufacturer specification, it consists of an active layer with approximate thickness 28 µm coated on both sides by a protective 125 µm polyester layer (Figure 1). For these experiments, the films with Lot No. 09071602 were used. Film dosimetry procedures were done based on recommendations of the manufacturer [13] and AAPM TG-55 protocol [14].

Figure1

Structure of Gafchromic EBT3 Dosimetry Film.

C. Mechanical Test (Leaf Position Accuracy)

The leaf position accuracy and leaf gap reproducibility were checked with Garden fence test. The garden fence test consists of 5 thin bands A) 0.2 Cm width spaced at 2 Cm intervals and B) 1 Cm width spaced at 1 Cm intervals. Each leaf accuracy and deviations from planned leaf positions were analyzed with measuring the full-width half-maximum (FWHM) (Figure 2) [15-19]. This fields were irradiated on EBT3 Gafchromic film at SSD 100 Cm placed on the treatment couch without any buildup to have sharper peaks.

Figure2

Exposed EBT3 film and plotted profiles of 5 band in SSD=100 Cm for Garden Fence Test, A) 0.2 Cm width spaced at 2 Cm intervals, B) 1 Cm width spaced at 1 Cm intervals.

D. MLC Transmission

Leaf transmission is the sum of mid-leaf and inter-leaf transmission. Maximum and average transmission were measured with gafchromic EBT3 Films from Ashland for both 6 MV and 18 MV X rays. To find the film response to the radiation, 13 pieces of films (5x5 Cm2) were located in 3 Cm polyester and SSD 97 Cm and were exposed from 0 to 1300 cGy. For measuring the leaf transmission and transmission at the end of the banks (leaf end Transmission), the films were located at SSD 98.5 Cm and depth of 1.5 Cm for 6 MV Photons and SSD 96.5 Cm and depth of 3.5 Cm for 18 MV Photons, the leaves were completely closed and radiated with 1000 MU for each energy (Figure 3). For measuring the leakage between leaves from opposite banks, the same set up were executed and the leaves were in Interdigit position.

Figure3

Exposed EBT3 film with 1000 MU. A: Scanning for measuring the leaf leakage, B: scanning for measuring the end leaf leakage.

All the exposed films and one of un-exposed film were scanned in 48 hours with Microtek scanner model ScanMaker 9800XL Plus (Microtek International, Inc. MRS-3200A3L, China), in three colors (48 bit RGB) with 150 dpi in transmission mode and saved as a .tiff format. The .tiff files were analyzed in ImageJ Software (National Institute of Health, Bethesda, MD).

Net optical densities (Net OD) and the standard deviations (σ) were determined according to the equations (1-2) [12,20,21]:

(1)

(2)

Where I0 and I are intensities measured for un-irradiated films and irradiated films, respectively.

The Net OD values were imported in MATLAB and to retrieve a dose from net OD Value, the best curve was fitted to this OD values according to Devic proposed function (equation 3) [22]. Levenberg-Marquardt algorithm was used for optimization.

D (3)

Where b and c are the fitting parameters and n is responsible for the non-linear saturation process of the film at high doses.

Film dosimetry is accompanied with several sources of uncertainties. There are two main sources of uncertainty, Experimental and fitting, the total uncertainty is the root square of the experimental and fitting uncertainty summation [23]. The estimated experimental uncertainty was calculated as below (equation 4) [22]:

(4)

Where σNetOD is optical densities uncertainty which is defined in equation (2). The fitting uncertainty (σDfit) was calculated as below (equation 5):

(5)

Where σb and σc are the fitting parameter uncertainties. Finally, the total dose uncertainty was calculated through the following equation (equation 6):

(6)

Result and Discussion

A. Mechanical Test (Leaf Position Accuracy):

Table 1 shows the results of the garden fence test for the leaf position accuracy for each peak. The leaf positions were found to be in the range between 1.78 - 2.53 mm, instead of nominal 2 mm and the average of 2.24 mm which correspond to a disposition of 0.24 mm for test A, and between 9.09 - 10.36 mm, instead of nominal 10 mm and the average of 9.77 mm which correspond to a disposition of 0, 23 mm for test B.

Table 1

Results of the measured distance in each Peak in mm for Garden Fence Test, A) 2mm Bands (Top), B) 10 mm Bands (Below).

2 mm bands
	Mean	SD
Peak1	2.69	0.19
Peak2	2.09	0.20
Peak3	2.13	0.22
Peak4	2.19	0.27
Peak5	2.11	0.25
Mean	2.24	0.22
10 mm bands
	Mean	SD
Peak1	9.32	0.29
Peak2	10.13	0.17
Peak3	9.78	0.36
Peak4	9.84	0.28
Peak5	9.75	0.25
Mean	9.77	0.28

In most articles which assessing the leaf position accuracy with the garden fence test, 11 thin bands with 2 mm width have been used. However, we have performed this test with 5 thin bands (Figure 2) due to our field size and over-traveling limitation [24,25]. As we can see in Table 1, All the values for Garden Fence test were considered to be within a safe 2mm limit based on AAPM Reports Task Group 142 [26]. Christos antypas [24] also reported 0.1-0.5 mm disposition for Siemens Oncor with OPTIFOCUS MLC with the garden fence test which is so close to our results in this study. Picket fence is another similar test that assessing the accuracy of the MLC leaf position, it is like the garden fence test, but the Radiation areas are 5 Cm with no intervals. Pipspro (Standard Imaging, Inc, USA) and Artiscan imadose (Aquilab, France) are the new softwares which can assess the leaf position accuracy automatically.

B. Leaf Leakage

When jaws were widely open and MLC Leaves were closed, the Average and Maximum radiation transmission of the MLC leaves were noted 1.79% and 4.1% of the open 10x10 Cm2 radiation-field at isocenter for 6 MV beams and 1.98% and 4.4% for 18 MV beams, respectively (A Direction in Figure 3). The average leaf transmission at the end of the leaves in a closed position (B direction in Figure 3) was noted 18.8% and 19.5% for 6 MV and 18 MV beams, respectively. The average and maximum leaf transmission in an interdigit position were also noted 2.5% and 3.5% for 6 MV beams and 2.6% and 3.7% for 18 MV beams, respectively.

All the transmission measurements were in agreement with acceptance levels based on IPEM 94 [27] and AAPM TG-142 [26] Protocols. Due to the rounded leaf end design, the most transmission was calculated below the leaf end positions. Carlos Daniel Venencia et al. [17] and Thomas Losasso et al. [28] measured the leaf leakage of 120-leaf MLC model Millennium of Varian Clinac21EX 1.5% for 6 MV Beam. Jun Li and et al. [29] have measured the average MLC transmission of ELEKTA model Synergy-s about 2% and the leaf end transmission rate from 25% to 30% for 6MV,10MV and 18MV beams.

Parameters n, b, c in equation 3 were calculated 2, -783.54 and 16297.25, respectively. The total dose uncertainties were between 1-2 %. As shown in Figure 4, the fitting uncertainties were more than the experiment uncertainties for all doses and also the dose uncertainties were reduced by increasing the dose. In TRS-398 document, the dose uncertainty must be within 2% when the protocol is accurately accomplished [23].

Figure4

Standard deviation for different delivered dose to EBT3 film.

Since this MLC was retrofitted to the Varian CLINAC 2100 C/D linear accelerator as an add-on one, and was not replaced with conventional jaws, the clearance from the isocenter was reduced. before such a system can be clinically used, beam characteristics and mechanical aspects should be verified [30]. Leaf transmission, the leaf position accuracy and deviations of leaf position from planned position were evaluated in this study and more investigations like changes in scatter factors, percentage depth doses, off-axis profiles must be evaluated in future studies.

Conclusion

The complete set of the mechanical and dosimetric tests must be done before clinically use of Add-On MLCs. In this study, application of gafchromic EBT3 films for quality assurance of Euromechanics multileaf collimator was studied. This study showed that the average leaf leakage and positional accuracy of this type of MLC were under than the acceptance level based on the protocols.