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ORIGINAL ARTICLE
Year : 2015  |  Volume : 38  |  Issue : 3  |  Page : 102-108  

Acceptance testing and quality assurance of Simulix Evolution radiotherapy simulator


1 Department of Radiotherapy, Uttar Pradesh Rural Institute of Medical Sciences and Research, Saifai, Uttar Pradesh, India
2 Department of Radiotherapy, King George Medical University, Lucknow, Uttar Pradesh, India
3 Roentgen SAIMS Radiation Oncology Centre, Sri Aurobindo Institute of Medical Sciences, Indore, Madhya Pradesh, India

Date of Web Publication10-Nov-2015

Correspondence Address:
Om Prakash Gurjar
Roentgen SAIMS Radiation Oncology Centre, Sri Aurobindo Institute of Medical Sciences, Indore, Madhya Pradesh - 453 111
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0464.169386

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  Abstract 

The success of radiotherapy depends on precise treatment simulation and proper patient positioning. The simulator is a conventional radiographic and fluoroscopic system which emulates the geometrical positions of radiotherapy treatment unit. Hence, the acceptance tests and quality assurance (QA) of the simulator are important prior to its commissioning for the safe and precise clinical use. The verification of mechanical and optical readouts, field size, isocenter, optical and radiation field congruence were performed. The X-ray beam parameters were tested for kVp , mAs and consistency of radiation output. The flat panel detector performance was checked with respect to resolution, low contrast sensitivity (LCS), automatic dose rate control (ADRC), and gray image resolution (GIR). Gantry, table, and imaging system collision possibility was checked. Radiation survey around the room was also performed. The field size test for digital readout and on graph paper, the results of isocenter checkup for rotation of gantry, collimator, and couch, and the deviations observed in auto stop for various movements were found within the tolerance limits. Optical field and radiation field was found congruent. All the lasers were found aligned with the established isocenter. Maximum deviation for set and measured kV was found to be 3% in fluoro mode. The maximum deviation observed in mAs was 1.5% in 3-point as well as in 2-point film exposed mode. The X-ray output was found consistent. The results of tests for resolution, LCS, ADRC, and GIR of the flat panel detector were within tolerance limits. All the six safety interlocks were found working. Radiation level around the room was found within the acceptable limits. All the tests carried out were found within the tolerance limits. The data which has been taken in this study will provide basic support to the routine QA of the simulator.

Keywords: Acceptance test, flat panel detector, quality assurance, Simulix Evolution simulator


How to cite this article:
Sinha A, Singh N, Gurjar OP, Bagdare P. Acceptance testing and quality assurance of Simulix Evolution radiotherapy simulator. Radiat Prot Environ 2015;38:102-8

How to cite this URL:
Sinha A, Singh N, Gurjar OP, Bagdare P. Acceptance testing and quality assurance of Simulix Evolution radiotherapy simulator. Radiat Prot Environ [serial online] 2015 [cited 2020 Sep 25];38:102-8. Available from: http://www.rpe.org.in/text.asp?2015/38/3/102/169386


  Introduction Top


Radiation treatment process is very complex. One of the important steps in the treatment process is labeled as "simulation." Treatment simulation includes the localization of the target volume which is to receive the planned dose and nearby critical organs and normal tissues which should receive a minimum possible dose. If there is any error in the simulation, it will affect the entire course of treatment. Karzmark and Rust [1] have studied the advantages of radiotherapy treatment simulators from a cost viewpoint and quality of patient care. It is a machine that mimics the geometry of the treatment unit but uses diagnostic quality of X-rays to perform the localization and verification of the patient in treatment position. The acceptance testing and quality assurance (QA) of radiotherapy simulator have been discussed by McCullough and Earle [2] and Ravindran et al.[3] QA of a simulator consists of a series of specific tests necessary to provide adequate confidence that this product will satisfy the quality requirements needed in radiotherapy treatment. [4] Hence, well-programmed QA will ensure the optimum operation of the simulator and its imaging system. [5]


  Materials and methods Top


Acceptance tests are usually performed immediately after the machine is fully installed. Simulix Evolution radiotherapy simulator (Nucletron Pvt., Limited., The Netherland) has two modes, i.e., fluoroscopy and digital radiography. The gantry of the simulator can rotate about an angle of ±190°. It also has a provision to change the focus to axis distance (FAD) and axis to film (which in this simulator is the flat panel imager) distance (AFD). It is also possible to rotate the collimator through an angle of ±185°. In both the modes, one can get a real-time image of the anatomy of the patient on the monitor in the control console room. The simulator comes with minimum 2.5 mm Al X-ray beam filter. In the fluoro mode, exposure parameters are 40-120 kVp and 0.24-12.0 mA, and in the digital radiography mode 40-150 kVp and 10-40 mA. Exposure factors can be used in both the auto and manual mode. Both the modes use the flat panel detector, which is mounted on one of the arms of the gantry.

The flat panel is 41 cm × 41 cm X-ray sensitive detector and has over 1 million photodetectors. These detectors are the photodiodes and are connected to an integrating circuitry. The useful image area is 40 cm × 40 cm, and the useful photo elements are 1000 × 1000 pixels. Each pixel can represent 65,000 gray scales (depending on the intensity of the light energy received) and on average, 2.5 pixels are present in 1 mm. Each detector element is approximately 0.4 mm apart. The 41 cm × 41 cm flat panel is an assembly of 16 blocks of detectors, each block has a different level of sensitivity, and each block has its own 16 bit a/d converter. The 16 outputs are multiplexed and are directly interfaced to the acquisition card in the workstation.

Couch of the simulator is identical to the couch of the teletherapy machine, i.e., with vertical, lateral, and longitudinal motions. The head of the simulator consists of collimator in three stages: Primary collimator, built in the tube itself; secondary collimator, made of dark lead material gives the shutter size; and third, the tertiary collimator i.e., jaws (X 1, X 2, Y 1, and Y 2 ) gives the desired simulation field.

Acceptance tests carried out on the Simulix Evolution consist of checks on (i) mechanical and optical parameters, (ii) parameters of X-ray beam, (iii) performance of imaging system, and (iv) safety and radiation protection.

The mechanical and optical parameters were checked for accuracy of their readout. The system has both digital and mechanical readouts for collimator angle, couch angle, and gantry angle. Under the mechanical and optical parameters check, mechanical and optical readouts, field size verification, isocenter verification, and congruence between optical and radiation field were carried out. Field size verification was performed using graph sheet. Field congruence test was carried out with six metal washers. Check on X-ray beam parameters included the verification of the accuracy of kVp, mAs, and consistency of X-ray output. The kVp, mAs, and X-ray output were verified using Unfors - kVp meter, Unfors - mAs meter, and calibrated X-ray dosimeter (Unfors, Inc., Sweden) which for the measurement of these quantities have accuracies better than 5%. The imaging system performance was checked for resolution, low contrast sensitivity (LCS), automatic dose rate control (ADRC), and gray scale resolution. Resolution was checked using Leeds TOR Phantom 18FG (Leeds Test Objects Ltd., North Yorkshire, UK). LCS test was carried out using Leeds Test tool. Leeds TOR Phantom 18FG is used for two-dimensional X-ray imaging QA, this Phantom contains several dark circles which are visible in X-ray. The gray scale resolution was checked with a step wedge.

The International Organization for Standardization norms for QA and International Electrotechnical Commission norms for conventions was followed.

Mechanical and optical parameters

Verification of field size


To check the accuracy of field size, both the shutters and the jaws position were checked for accuracy of their read out. This was performed by affixing a graph sheet on the couch at isocenter. Now the field size was increased from 5 to 40 cm. The measured value on the graph sheet and the digital readout values were compared. This was carried out for two values of FAD, i.e., 80 and 100 cm.

Isocenter check

The isocenter was checked by the American Association of Physicists in Medicine (AAPM) test tool. AAPM Phantom is used for computed tomography data and imaging evaluation. The Phantom is shown in [Figure 1]. Gantry was set to 0°, and the AAPM test tool was placed on the table. Under fluoro, the table was moved longitudinally and laterally until the central hole of the tool is exactly in the cross-wire. The table longitudinal value was noted; then gantry moved to 180°. With fluoro, the table longitudinal value is adjusted until the central hole is exactly matching in the cross-wire. The difference between set and measured values was noted.
Figure 1: American Association of Physicists in Medicine computed tomography Phantom

Click here to view


Isocenter checks at 90° and 270°

Gantry was set to 90°, FAD = 100 cm and AFD = 50 cm, the AAPM test tool was placed on the table top. Under fluoro, the table is adjusted vertically and laterally until the central hole of the tool exactly matches the cross-wire, and the table height was noted. Now the gantry was set to 270°, and with fluoro, the table was moved vertical and longitudinal if necessary to match the cross-wire in the central hole of AAPM tool, and the table height was noted. The deviation is the difference between two table height readings.

For isocenter check during couch rotation, the gantry was positioned at 0° with FAD = 100 cm and a sheet of paper was fixed on the table top centered to the field light cross-wire projection. The table was rotated to 0°, 45°, 90°, −45°, and − 90° and for each angle the field light cross-wire projection was marked on the paper. Then the circle was drawn around all the cross-wire centers. The system had preprogrammed auto stop for various movements, such as flat panel detector, couch, and gantry rotation. The deviations in various movements were measured and noted.

Congruence between optical field and radiation field

For verifying the optical and radiation field congruence, a sheet of white paper was fixed on the table top. FAD = 100 cm was set and then a symmetric field of 15 cm × 15 cm was opened. For congruence test, six metal washers were placed on the paper, further aligned so that their centers coincided with the heart line of the wires. After taking a fluoro image, the difference between the actual wire position and the centers of the washers (using the known dimension of washers as a reference) was measured.

Laser positioning system

AAPM test tool was used for verifying the alignment of the laser. All these alignments were matched to the established isocenter.

X-ray beam parameters

The accuracy of kVp and mAs were measured using unfors kVp and mAs meters. The set value, limit values, and results were noted.

Consistency of X-ray output

For verifying consistency of X-ray output, the exposure were measured using a calibrated exposure meter (Unfors) at the isocenter (FAD = 100 cm) with the tube settings of 70 kV, 10 mA, and 200 ms. The measurement was repeated with 120 kV, 10 mA, and 200 ms with same isocenter (FAD = 100 cm).

Performance of flat panel detector

Resolution


Spatial resolution is the line group where the lines are just visible without aliasing. For checking resolution, the contrast Leeds TOR Phantom 18FG was used. The imaging was performed on fluoro mode at 50 kV and 1 mA without any filter material. Phantom was placed for horizontal resolution, and then it was rotated to 90° for vertical resolution.

Low contrast sensitivity

LCS check was carried out using Leeds Test tool containing eighteen small radio-opaque circular discs. Test tool was exposed to fluoro at 70 kV and 2.2 mA with 1 mm Cu filter. Now the visible small discs were counted and noted.

Automatic dose rate control

For checking ADRC in fluoro mode, the calibrated unfors X-ray dosimeter was used. There were two controls anti-lock braking system (ABS1) (full form of ABS) and ABS2. First, the ABS1 control was set and the dose measured at the entrance plane of the flat panel detector. Now, for same control ABS1, the image quality was verified. Leeds TOR Phantom was exposed with 70 kV, 2 mA, and with 2 mm Cu filter for verifying contrast sensitivity.

The Leeds Phantom was again exposed with the same settings and filter in ABS2 control, and the dose was measured on the imager entrance plane. The image quality was also verified.

For ADRC in digital radiography mode, image quality was verified by contrast Phantom, i.e., Leeds TOR Phantom 18FG.

Gray image resolution of the flat panel detector

Step wedge provided by Nucletron was exposed directly on the grid, and exposure was given in fluoro mode at 70 kV, 2 mA, and with 1 mm Cu filter; nine steps (prescribed limit >8) were seen.

Safety and radiation protection

Collision safety


The gantry, table, and imaging system, i.e., flat panel detector movements were checked for the collision safety requirements. The imaging system has a complete stop if it approaches the floor. In lateral movement, there was no collision between table and detector at gantry angle of 90° and 270°. There was a mechanical touch guard in collimator and flat panel detector. If they were activated, all the movements were stopped.

Radiation protection

Radiation protection survey was carried out using fluke biomedical survey meter (Fluke Biomedical, Ohio, USA). Simulator has two modes, i.e., fluoro and digital radiography. At the maximum exposure conditions, radiation survey of the simulator room was performed in both the modes. Readings for radiation levels at different places, i.e., door, on the outside walls, at the viewing window, in the control console, and other occupied areas were taken.


  Results and discussion Top


Mechanical and optical parameters

Verification of field size


The difference between digital readouts and observed value on the graph was <0.1 mm for jaws and 0.2 mm for lead shutters. The results are shown in [Table 1].
Table 1: Field size verification, isocenter verification and deviations in auto-stop movements


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Isocenter check

The difference between the two table longitudinal readings on the table is 1.4 mm conforming accuracy of isocenter with gantry positions at 0° and 180°. The results are shown in [Table 1].

Isocenter checks at 90° and 270°

The radiographs obtained for isocenter checkup at gantry angles 90° and 270° are shown in [Figure 2]a and b, respectively. The dimensions were observed to be within tolerance limit [Table 1].
Figure 2: Radiograph for isocenter checkup at gantry angle (a) 90° and (b) 270°

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On repeated rotation of the collimator by ±90°, the dimensions of cross-wire projections from its position were within the tolerance limits as shown in [Table 1].

For isocenter check during couch rotation, the radius of the circle formed by the cross-wire centers was found to be within tolerance limit, as shown in [Table 1].

The deviations observed in the auto stop for various movements, such as flat panel detector, couch, and gantry rotation are given in [Table 1].

Congruence between optical field and radiation field

The difference between the actual wire position and the centers of the washers was <1 mm (tolerance value ≤1 mm). The radiograph obtained is shown in [Figure 3].
Figure 3: Radiograph taken for congruence between optical and radiation field

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Laser positioning system

All the lasers viz., lateral left, lateral right, sagittal, and overhead lasers were found well-aligned with the established isocenter. The measured values for all were <±0.5 mm (tolerance limit).

X-ray beam parameters

The set value, limit values, and results of kVp and mAs are given in [Table 2]. Maximum deviation for set kV and measured kV was found to be 3% in fluoro mode, i.e., in pulsed mode. The maximum deviation observed in mAs was 1.5% in 3-point film exposed mode as well as in 2-point film exposed mode.
Table 2: kV accuracy, mAs accuracy, and consistency of X - ray output


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Consistency of X-ray output

The X-ray output was consistent (within the prescribed limit) as shown in [Table 2].

Performance of flat panel detector

Resolution


The numbers of resolution patterns shown horizontally and vertically are 21 lines (tolerance limit ≥9) which are shown in the [Figure 4]a and b. From the pattern table given in log book (Simulix-HP Acceptance Protocol (Section 2), Netherlands) of spatial resolution using Leeds Phantom, resolution is 1.6 line pair per millimeter (lp/mm). For both vertical and horizontal patterns, it is 1.6 lp/mm (tolerance limit ≥1.25), which is within the tolerance limit.
Figure 4: Radiograph for checking (a) horizontal and (b) vertical resolution

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Low contrast sensitivity

The visible small discs were counted as shown in [Figure 5]. A total number of discs visible were 12 (tolerance limit ≥ 12) and the corresponding sensitivity from the table (given in log book) was 2.7% (tolerance limit ≤2.7%).
Figure 5: Low contrast sensitivity check image on flat panel detector

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Automatic dose rate control

The Leeds Phantom was exposed with 2 mm Cu filter in ABS1 control. The dose at the entrance plane of the flat panel detector was measured as 1.086 μGy/s, which was within the tolerance limits (0.8 μGy/s-1.6 μGy/s). For verifying contrast sensitivity, the number of discs visible were eighteen (tolerance limit ≥9 discs) and the sensitivity was found to be 0.9% (tolerance limit ≤4.5%) from the table (given in log book).

The Leeds Phantom was again exposed with 2 mm Cu filter in ABS2 control, the dose measured on the imager entrance plane was 1.34 μGy/s, which is within the tolerance limit (0.8 μGy/s-1.6 μGy/s). The image quality was verified by Leeds TOR Phantom 18FG with 70 kV and 2 mA. A number of discs visible were eighteen (tolerance limit ≥9 discs), and the measured sensitivity was found to be 0.9% (tolerance limit ≤4.5%).

[Figure 6]a and b shows the contrast sensitivity verification for ADRC in fluoro mode for ABS1 and ABS2 control, respectively.
Figure 6: Contrast sensitivity verification for automatic dose rate control in fluoro mode for (a) anti-lock braking system 1 and (b) anti-lock braking system 2 control

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For ADRC in digital radiography mode, image quality was verified by contrast Phantom as shown in [Figure 7], the number of discs visible were eighteen (tolerance limit ≥11 discs) and sensitivity was 0.9% (tolerance limit ≤3.3%).
Figure 7: Contrast sensitivity verification for automatic dose rate control in digital radiography mode

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Gray scale image resolution of the flat panel detector

Step wedge provided by Nucletron was exposed directly on the grid, and exposure was given in fluoro mode at 70 kV, 2 mA, and with 1 mm Cu filter; nine steps (prescribed limit >8) were seen. The step wedge is shown in [Figure 8].
Figure 8: Step wedge for gray scale image resolution of flat panel detector

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Safety and radiation protection

Collision safety


There were a total of six working safety interlocks viz., (1) between gantry and floor, (2) for the simulator, (3) collision stop for the flat panel detector (imaging system), (4) emergency stop button on the walls, (5) emergency offload for patient, and (6) door interlock.

Radiation protection

Readings for radiation level at different places, i.e., door, on the walls, at the window, and in the control room was very low, and hence, the doses to radiation workers and general public was well within the tolerance limits. [6],[7] The detailed survey data are shown in [Table 3] (1 mR/h, i.e., 40 mR/week, which corresponds to an annual dose of 20 mSv/year, permitted for radiation workers).
Table 3: Radiation level around simulator room


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  Conclusions Top


The various acceptance tests carried out for mechanical, X-ray beam parameters, the performance of flat panel detector, collision safety, and radiation safety showed the results are within the prescribed limits and tolerances recommended by the international bodies. This ensures the reliable performance and operation of the simulator, necessary for later transfer of the correct anatomical image data with tumor localization to a radiotherapy unit for successful patient treatment.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

1.
Karzmark CJ, Rust DC. Radiotherapy treatment simulators and automation. A case for their provision from a cost viewpoint. Radiology 1972;105:157-61.  Back to cited text no. 1
    
2.
McCullough EC, Earle JD. The selection, acceptance testing, and quality control of radiotherapy treatment simulators. Radiology 1979;131:221-30.  Back to cited text no. 2
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3.
Ravindran BP, Singh RR, Raj SB, Raj DV. Acceptance testing and commisioning of ximatron radiotherpy simulator. J Med Phys 2001;26:51-9.  Back to cited text no. 3
    
4.
Hendee WR, Ibbot GS, Hendee EG. Radiation Therapy Physics. 3 rd ed. Hoboken, NJ: John Wiley and Sons, Inc. Publications; 2005.  Back to cited text no. 4
    
5.
Van Dyk J, Munro PN. Simulators. In: Van Dyk J, editor. The Modern Technology of Radiation Oncology. Madison, WI: Medical Physics Publishing; 1999. p. 95-129.  Back to cited text no. 5
    
6.
Atomic Energy Regulatory Board (AERB). Safety Code for Medical Diagnostic X-ray Equipment and Installations. Available from: http://www.aerb.gov.in. [Last accessed on 2014 Aug 25].  Back to cited text no. 6
    
7.
Gurjar OP, Jha VK, Sharma SD. Radiation dose to radiotherapy technologists due to induced activity in high energy medical electron linear accelerators. Radiat Prot Environ 2014;37:25-9.  Back to cited text no. 7
  Medknow Journal  


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]


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