Commissioning and early experience with a new-generation low-energy linear accelerator with advanced delivery and imaging functionalities
© Clivio et al; licensee BioMed Central Ltd. 2011
Received: 29 July 2011
Accepted: 30 September 2011
Published: 30 September 2011
A new-generation low-energy linear accelerator (UNIQUE) was introduced in the clinical arena during 2009 by Varian Medical Systems. The world's first UNIQUE was installed at Oncology Institute of Southern Switzerland and put into clinical operation in June 2010. The aim of the present contribution was to report experience about its commissioning and first year results from clinical operation.
Commissioning data, beam characteristics and the modeling into the treatment planning system were summarized. Imaging system of UNIQUE included a 2D-2D matching capability and tests were performed to identify system repositioning capability. Finally, since the system is capable of delivering volumetric modulated arc therapy with RapidArc, a summary of the tests performed for such modality to assess its performance in preclinical settings and during clinical usage was included.
Isocenter virtual diameter was measured as less than 0.2 mm. Observed accuracy of isocenter determination and repositioning for 2D-2D matching procedures in image guidance was <1.2 mm. Concerning reproducibility and stability over a period of 1 year, deviations from reference were found <0.3 ± 0.2% for linac output, <0.1% for homogeneity, similarly to symmetry. Rotational accuracy of the entire gantry-portal imager system showed a maximum deviation from nominal 0.0 of <1.2 mm. Pre treatment quality assurance of RapidArc plans resulted with a Gamma Agreement Index (fraction of points passing the gamma criteria) of 97.0 ± 1.6% on the first 182 arcs verified.
The results of the commissioning tests and of the first period of clinical operation, resulted meeting specifications and having good margins respect to tolerances. UNIQUE was put into operation for all delivery techniques; in particular, as shown by the pre-treatment quality assurance results, it enabled accurate and safe delivery of RapidArc plans.
KeywordsUNIQUE linear accelerator RapidArc Beam Commissioning
During 2009, a new single-energy linear accelerator for radiotherapy was introduced in clinical operation by Varian Medical System (Palo Alto, CA, USA). This new linac, called UNIQUE™ (UNIQUE in the following), was an evolution of the previous series of low-energy linacs. It incorporated new treatment modalities like Volumetric Modulated Arc Therapy according to the RapidArc® method as well as advances in imaging modalities. UNIQUE also improved gantry mechanical control to allow safe operation of the advanced delivery modes. The world's first installation of UNIQUE took place at the Oncology Institute of Southern Switzerland and the machine started clinical treatments in June 2010.
Purpose of the present report was to summarise commissioning data in terms of main mechanical features as well as beam characteristics. Secondly, the results of the RapidArc commissioning on UNIQUE were presented as well as an overview of the technical aspects of the first clinical treatments. Several protocols and publications exist describing and recommending standardised procedures for beam data commissioning as well as publications on quality assurance procedures (among these, AAPM  or ESTRO  codes of practice), on analysis of results from mono or multi institutional investigations  and on accuracy and precision levels required in radiation therapy in general . The present report, was based on recommendation from the Swiss Society of Radiobiology and Medical Physics  and were tailored to the specific commissioning needs to characterise a delivery system into the Eclipse treatment planning system adopted at author's institute.
UNIQUE linac was designed to generate and deliver a single photon beam of nominal energy of 6MV with a maximum dose rate of 600 (or 400 MU/minute depending on the version), and was developed with a vertical standing wave linac, without bending magnet and steering coils. RF power generation was realised by a conventional magnetron. It was equipped with a Millennium multileaf collimator (MLC) with either 120 leaves (with 0.5 cm resolution at isocentre in the inner 20 cm and 1.0 cm resolution in the outer 20 cm) or with 80 leaves (1.0 cm resolution over the entire 40 cm of maximum field size). The couch top was derived from high energy linacs and adapted for image guidance and rotational therapy (the so-called Exact-IGRT couch top). Mechanical and Enhanced Dynamic Wedges were implemented on this new delivery platform as in other conventional Varian linacs. Mega Voltage Imaging was guaranteed by the amorphous silicon electronic portal imager PortalVision aS1000 (with pixel size of 0.392 mm) or aS1000/2 (with half resolution) operated by the so-called ExactArm, a robotic positioning arm using an active control and position correction system that compensates for gravitational and mechanical undue movements even during rotation. Patient anti-collision safety was implemented by means of a laser-based system (LaserGuard). Optional Image-guided patient repositioning was facilitated through 2D-2D MV image matching (Portal Vision Advanced Imaging (PVAI) application) and by automatic remote treatment couch movement managed by the image review application without the necessity to enter the room for couch operation.
Operational limits for asymmetric jaws were -2 cm overtravel for x jaws and -10 cm for y jaws; similarly, all other mechanical were implemented identical to other existing delivery Clinac platforms from Varian.
Concerning RapidArc implementation on UNIQUE, gantry rotation was controlled in the first generation of machines, by a slipping clutch system. The dose rate control of the UNIQUE accelerator was uses a principle schematically summarised as follows. The gun pulse trigger is always in coincidence with the magnetron pulse; the dose rate is varied by changing the magnetron pulse repetition frequency (PRF). The PRF frequency varies between 50 - 400 pulses/sec depending on the dose rate (up to 600 MU/min).
Every 50 ms, the control system of UNIQUE, compares, in dynamic treatments, the number of cumulative MU (resolution of 0.01 MU) delivered versus prescribed and takes it into account for calculation of the PRF for the next dose rate servo cycle.
A.UNIQUE Commissioning, Anisotropic Analytical Algorithm configuration and periodic quality assurance measurements
Isocentre determination. A conventional star film shot procedure was performed with X-Omat V Kodak films. The specification for the isocentre sphere diameters are 2 mm. The test was repeated for different gantry, collimator and couch angle settings.
Output factors. Output factors were measured for squared and rectangular fields in water at 10 cm depth and data were compared against performed calculations. Field sizes ranged from 3 × 3 to 40 × 40 cm2. Machine calibration was performed at isocentre at 10 cm depth for a field size of 10 × 10 cm2.
Output stability as a function of dose rate (called MU stability) and linearity between output and MU (called MU linearity) were assessed from periodic quality assurance measurements in the range respectively from 100 to 600 MU/minute and from 5 to 300 MU. MU stability was expressed as the ratio of dose measured at a given dose rate to the reference at 300 MU/min delivery. MU linearity was expressed as the ratio of dose measurement per MU (dose/MU) at given MU to the reference 100 MU delivery.
Depth doses and beam profiles in principal x and y axes were measured for a variety of square fields with the same range as at ii)..
Similarly to what performed for open fields, also fields modified by Mechanical and Enhanced Dynamic Wedges were investigated in terms of profiles, depth doses, output factors and wedge transmission factors.
Commissioning beam data measurements were performed in water with ion chambers: 0.125 cm3 (Semiflex, PTW) for profiles and depth doses and output factors or 0.6 cm3 (Farmer, Nuclear Enterprise) for absolute dose calibration. Source to phantom distance SSD was set to 90 cm for all measurements. Depth dose curves (PDD) were normalised to dmax and profiles were normalised at the beam's central axis. A field size of 10 × 10 cm2 was used to determine dmax. Results of periodic quality assurance measurements of beam characteristics, including beam energy check, were reported in this summary, too. These were obtained by means of the portal dosimetry method GLAaS  as implemented in the commercial EPIQA software (Epidos s.r.o., Bratislava, Slovak Republic). For beam profiles analysis, field symmetry was defined as the maximum ratio between symmetric points within the flattened region (80% of the field size): max(D(x)/D(-x)) and expressed in percentage. Homogeneity was defined within the flattened region as (Dmax-Dmin)/(Dmax+Dmin) and similarly expressed in percentage. Field size was defined at 50% beam profile intensity. Tolerances were derived from Swiss regulations on quality assurance on linear accelerators for medical usage .
Beam data measured for machine commissioning, were compared against calculation performed in the Eclipse Treatment Planning System for the Anisotropic Analytical Algorithm AAA version 10.0.25 with a grid size of 2.5 mm. Details on the beam processing for AAA can be found in Fogliata et al . In summary, the AAA configuration phase consisted in the optimisation of parameters and calculation kernels against the measured beam data. The optimisation is performed using objective functions including the gamma index of Low . As an output of the AAA beam configuration phase in Eclipse, plots of the gamma index after optimisation are provided by Eclipse and reported here for depth doses, before and after dmax, and for profiles in the flattened region, within the field edge and outside the field edge.
For some of the parameters, a direct comparison against either published [6, 8], or institutional data for the 6MV beam generated by the high energy Clinac iX available at authors institute was provided to appraise performance of the UNIQUE beam delivery system in the absence of other published references.
B.Imager isocenter accuracy and 2D/2D match and couch shift accuracy
The imager isocenter accuracy QA test evaluated whether the digital graticule generated by the PVAI application coincided with the treatment isocenter. The so-called marker-block phantom (a cubic phantom with one fiducial radiopaque marker at the center) was aligned on the couch with the treatment isocenter using the wall lasers. MV images at different gantry angles were acquired and analyzed measuring the distance between the center of the marker and the digital graticule inside the PVAI application (step 1 of the test).
To test the accuracy of the 2D-2D match procedure, a set of 2 orthogonal images was acquired after a manual pre-defined shift in the 3 directions of the center of the phantom: the 2D-2D match was performed to re-align the phantom, checking the proposed shift respect the expected values (step 2 of the test).
The remote couch shift was applied according to the previous match, and new images were acquired to test the couch shift accuracy (step 3 of the test). This procedure was derived from methods published by Yoo et al . Weekly checks were executed at 90° and 180°, monthly frequency included also 0° and 270° but were not reported here.
To assess overall accuracy and relevance of the gantry sag and imager position (ideally corrected by the arm active control of the Portal Vision system) during rotation, in view of RapidArc commissioning and quality control, tests were performed by measuring the displacement of the center of a narrow field (0.4 × 0.4 cm2) from its expected nominal position at 0,0 cm coordinates (in the imager coordinates system) during an entire arc executed either clock or counter-clock-wise . Measurements were performed with the PortalVision. Comparison with similar measurements on an high energy linear accelerator (Clinac iX), implementing the same arm active control system, were provided for reference.
D. RapidArc commissioning and medium term (1 year) machine performances
RapidArc (details about the principles and the algorithms can be found in Cozzi et al ) commissioning tests were performed according to the procedures described in the seminal work of Ling et al . These tests were performed on the UNIQUE to assess the accuracy of the machine in generating uniform dose delivery with various combinations of dose rate, gantry speed and leaf speed variations during rotational delivery. Tolerance on the acceptable deviation of each dose band generated with a given combination of the above parameters from the baseline (defined as average of all the dose bands) was set to 2%. Results were provided for repeated series of measurements during the first year of UNIQUE operation. Comparison with corresponding measurements on a high energy linac (Clinac iX) were provided for reference. Data were measured by means of portal dose images  and analysed by means of the automatic tool implemented in the Epiqa software.
RapidArc delivery with the UNIQUE was also assessed by investigating the machine dynamic status recorded every 50 msec by the linac control system. These records were saved in the format of dynalog files where each actual dynamic parameter was stored in association to the corresponding expected parameter from delivery steering instructions. Data were recorded and analysed for each MLC leaf position, for the accumulated dose and for the gantry angle. Results were reported for a set of 12 clinical cases from our library of RapidArc plans delivered on the UNIQUE and, for comparison, on the Clinac iX unit.
Quality RapidArc delivery was also assessed at dosimetric level. For reproducibility, the same clinical plan was delivered with a biweekly periodicity while each patient treated with RapidArc on the UNIQUE underwent standard pre-treatment quality assurance measurements. Numerical analysis was performed calculating the 2D gamma of Low  maps from the comparison of calculated and delivered dose distributions at dmax according to the GLAaS method  and scoring the Gamma Agreement Index GAI with Distance to Agreement threshold set to 3 mm and Dose Difference threshold set to 3%. Results from clinical patients included also a limited number of cases treated with fixed gantry IMRT, and data were compared with the corresponding results from other Varian linear accelerators available at authors institute.
Results and discussion
A. UNIQUE Commissioning, Anisotropic Analytical Algorithm configuration and periodic quality assurance measurements
i. Isocenter determination
ii. Output factors
iii. Routine beam output check, MU stability as a function of dose rate and MU linearity
Summary of the results of the periodic radiation beam quality assurance measurements.
Output(% difference from ref.) Tolerance: <2%
-0.3 ± 0.2% [-0.8,+0.2]
-0.1 ± 0.4% [-0.9,+1.0]
%diff. ratio @5.6 cm/dmax
0.0 ± 0.0 [-0.1,0.1]
0.1 ± 0.1 [-0.2,0.6]
%diff. ratio @7.6 cm/dmax
-0.1 ± 0.1 [-0.3, 0.1]
-0.1 ± 0.1 [-0.3, 0.4]
%diff. ratio @11 cm/dmax
-0.1 ± 0.1 [-0.3, 0.1]
-0.0 ± 0.2 [-0.3, 0.6]
EDW _WF(% difference from ref.) Tolerance: <2%
0.0 ± 0.2 [-0.1,0.1]
-0.0 ± 0.3 [-0.1,0.5]
Field Size[cm] 10 × 10 cm2, dmax Tolerance: <2 mm
10.03 ± 0.05(ref.10.02) [10.00, 10.14]
10.06 ± 0.06(ref.10.07) [9.99, 10.13]
10.10 ± 0.03(ref.10.02) [10.08, 10.11]
9.99 ± 0.02(ref.10.04) [9.94, 10.05]
Field Size[cm] 20 × 20 cm2, dmax Tolerance: <3%
20.13 ± 0.01(ref.20.13) [20.10, 20.14]
20.14 ± 0.02(ref.20.13) [20.11, 20.18]
20.21 ± 0.01(ref.20.21) [20.18, 20.24]
20.09 ± 0.02(ref.20.02) [20.02, 20.10]
Flatness[%] 10 × 10 cm2, dmax: Tolerance: <3%
0.8 ± 0.04 (ref.0.7) [0.7, 0.9]
0.9 ± 0.04 (ref.0.9) [0.8, 0.9]
1.2 ± 0.09 (ref.1.1) [1.0, 1.5]
0.8 ± 0.05 (ref.0.9) [0.8,1.0]
Flatness[%] 20 × 20 cm2, dmax: Tolerance: <3%
1.5 ± 0.04 (ref.1.5) [1.4, 1.6]
2.0 ± 0.06 (ref.1.8) [1.9, 2.1]
1.1 ± 0.12 (ref.1.0) [0.9, 1.4]
1.6 ± 0.10ref.1.7) [1.4,1.8]
Symmetry[%] 10 × 10 cm2, dmax: Tolerance: <103%
100.6 ± 0.2 (ref.100.6) [100.4, 100.9]
100.3 ± 0.2(ref.100.4) [100.1, 100.7]
100.5 ± 0.2 (ref.100.3) [100.3, 101.1]
100.5 ± 0.2 (ref.100.3) [100.2, 101.1]
Symmetry[%]20 × 20 cm2, dmax: Tolerance: <103%
101.1 ± 0.1(ref.101.3) [101.0, 101.2]
100.4 ± 0.2 (ref.100.3) [100.2, 100.7]
100.6 ± 0.2 (ref.100.4) [100.1, 101.1]
101.4 ± 0.3 (ref.101.7) [100.5, 101.8]
iv. Depth Doses and beam profiles
Part of Table 1 summarized the results of periodic quality assurance control for field size, profile homogeneity and symmetry in the X and Y directions, and beam energy. The energy check is reported as the ratio between dose measured at different depths in solid water with respect to the corresponding value at dmax. As can be seen, all the findings are within tolerance, the observed range was quite limited and there was a full compatibility of results with data from high energy linac.
v. Mechanical and Enhanced Dynamic wedges
B. Imager isocenter accuracy and 2D/2D match and couch shift accuracy
C. Rotational Stability
D. RapidArc commissioning and medium term (1 year) machine performances
Summary of the stability control and of the pre-treatment patients quality assurance results for RapidArc and IMRT treatments.
GAI [%] constancy on a pre-treatment QA case (1 year data with a periodicity of 2 weeks)
98.5 ± 1.1 [96.7, 99.6]
99.0 ± 0.3 [98.3, 99.4]
99.4 ± 0.1 [99.2, 99.2]
99.0 ± 0.4 [98.2, 99.3]
Clinical pre-treatment RapidArc QA
97.3 ± 1.6 [92.4, 99.9]
97.4 ± 1.8 [91.5, 99.9]
Number of arcs (plans)
348 (192) [12 months]
1186 (797) [31 months]
A new-generation of low-energy linear accelerator, UNIQUE, was recently introduced in the clinical arena (at the moment with the exclusion of USA, Canada and Japan) by Varian Medical Systems. The results of the commissioning tests and of the first period of clinical operation of this new delivery system were presented in this report for beam characterisation and modelling into the treatment planning system, periodic quality assurance tests and RapidArc operations. In all areas, UNIQUE resulted meeting specifications and having good margins respect to tolerances, and was put into operation for all delivery techniques. In particular, as shown by the pre-treatment quality assurance results, it enabled accurate delivery of RapidArc plans and this ended in the interruption of clinical application of IMRT at our institute having replaced the entire fixed gantry IMRT programme with RapidArc now enabled on all delivery systems of our institute.
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