Linac Twins in Radiotherapy

In a radiotherapy department having more than one linear accelerator, it is rather common to match the dose output of all machines. In particular, the recently developed flattening filter free mode requires new investigations regarding the feasibility of matching and the consequences for quality assurance and workload. This refers also to the beam model of the radiotherapy treatment planning system. Our results show that matching is possible not only for flat beams but also for flattening filter free mode. Therefore, the machines can substitute each other in the case of breakdown or service without new treatment planning even in the case of complex intensity-modulated radiotherapy or volumetric-modulated arc therapy. The quality assurance is reduced to only one data set for both the linear accelerators and the radiotherapy treatment planning system.


Introduction
Electron linear accelerators (linacs) are the most common treatment machines in radiotherapy. Having two (or more) equal linacs (linac twins or also called matched linacs) enables a radiotherapy department to facilitate the workflow and to reduce the amount of quality assurance. The major part of the German standards (DIN) regarding quality assurance of medical linear accelerators has been reworked or has been published for the first time in the recent years due to technical developments. Similar updates have been published in other countries, e.g., by The draft of the German standard for consistency tests of RTPS DIN 6873-5 [20] requires calculations for each treatment machine. Probably part 1 of DIN 6873 for commissioning of RTPS which is in development will demand this too. A technical report of the International Atomic Energy Agency (IAEA) [21] recommends checks for each photon and electron beam used in clinical planning and therefore each beam model and treatment machine. Having only one treatment machine model reduces time and effort for quality assurance and commissioning.

RTPS and water phantom
For commissioning of the linac model in the RTPS Oncentra® 4.3 (by Nucletron an Elekta Company), a set of geometrical data, absolute, and relative dose measurements have been measured [22] using a water phantom of the type Blue Phantom² of IBA company. It has been operated by the software OmniPro Accept 7.4, including a module for data export for Oncen-tra®. The data are processed by Elekta to create a model of the treatment unit, which takes several weeks according to our experience. Once the model is delivered by the company, it has to be validated by the customer. The RTPS comprises for this purpose the Beam Data Tool, which allows not only to compare measured and calculated dose distributions but also to adapt the size of the focus and the transmission of the collimator for final optimization. One aim of the study is to investigate if this procedure can be reduced to the validation process for the second linac.
been installed and commissioned according to earlier experiences [12] and has been running in the clinical routine for several months, but initially not FFF. The desktop software is Integrity 3.1. For the second linac, the installation has been completed in June 2014. The manufacturer specifies a routine for acceptance testing of linear accelerators [13], which refers to matched machines only pertaining to the beam quality.  The evaluation tools of the water phantom software were used to compare the measured dose distributions of both machines. For the graphical demonstrations, smoothing with least square algorithms was performed to get rid of some noise, and the curves were renormalized to the dose maximum on the central beam. The Beam Data Tool of the RTPS was applied for comparisons of measured and calculated dose distributions. For the calculations, the collapsed cone algorithm was used. The European Society for Radiotherapy and Oncology (ESTRO) gives tolerances as confidence limits for calculated doses [23]. The specifications of the manufacturer of the RTPS Oncentra® [24] are not always comparable, e.g., the shoulder region is not defined exactly, or dose deviations refer to different points, or different units are used (Table 1). We referred to the manufacturer's specifications in our evaluations as they have to be kept by the manufacturer's beam model and used the ESTRO's-modified to a gamma evaluation of 10% and 2 mm in the sharp gradient area-only for further investigations. Distance to agreement, dose deviation, and gamma evaluation [25] are the integrated evaluation options in the Beam Data Tool. The depth dose distributions and profiles in in-plane and cross-plane direction in depths of 5, 10, and 20 cm were evaluated for square field sizes of 2, 5, 10, 15, 20, 30, and 40 cm.
From the measured depth dose curves of fields of 10 × 10 cm² at a source surface distance (SSD) of 100 cm, the beam quality was derived as Q = D200/D100, with D200 the dose in 20 cm depth and D100 in 10 cm.

Breakdown concept
The German directive "Strahlenschutz in der Medizin" [26], paragraph 2.3.4, requires a concept to ensure patient treatment even during machine down times (e.g., maintenance or breakdown). Linac twins allow shifting all patients from one machine to the other without calculating new treatment plans. Sjöström et al. [8] provided this as main argument for matching linacs. Depending on the tumor type and the patient state, such a transfer is also advised by the Board of Faculty of Clinical Oncology of the Royal College of Radiologists [27].
The record and verify system (Mosaiq®, version 2.50) can be configured in a manner that fields for one machine can be delivered at the other without warnings or password confirmation. Some VMAT and IMRT plans (FB and FFF) calculated with the beam model for the first machine were measured on the other to verify the exchangeability using the 2D array Matrixx Evolution phantom and software OmniPro-I'mRT, version 1.7 of IBA.
The procedure for these plan verifications has been described in detail in earlier publications [12,28]. The patient plans were transferred to a cuboid phantom of solid water (RW3 of the PTW company) with the Matrixx phantom in the center in a horizontal plane. The dose distribution was recalculated without modification of any parameter. When the phantom was irradiated with the original plan, the software recorded the dose. The measured dose was compared to the calculated dose by gamma evaluation [25] with a dose tolerance of 3% of the maximum dose and a distance to agreement of 3 mm. The first clinical IMRT plans with 6MV and for testing of the feasibility some single IMRT plans with 6MV FFF and VMAT plans were evaluated.

Beam quality and measured dose distributions
The results of the beam quality evaluation of the percentage depth dose curves are given in Table 2.  Table 3. Evaluation of the field size of 6 MV photons in cross-plane direction as given by the profiles in Figure 5.
Linac Twins in Radiotherapy http://dx.doi.org/10.5772/60427   The second group of figures shows profiles in the cross-plane direction in a depth of 10 cm. Others with further field sizes, in different depths, and in in-plane direction have been measured with similar results. The difference between the flattened and the flatness filter free mode is obvious; as the angular distribution of the bremsstrahlung is not compensated, the maximum dose is always on the central beam. The high congruence of the corresponding curves makes often only one of them visible, sometimes in a mixed color. Numerical values of the field size evaluation are given in Table 3.

RTPS commissioning
As described above, no model for the second linac has been created in the RTPS, but all calculations were done with the first linac model and compared to the measurements of the second. Figure 8 shows the results of such a validation at an example of FFF profiles. The left Evolution of Ionizing Radiation Research ordinate refers to the measured and calculated profiles (green and orange), the right to the validation criterion. In the first step, both curves are compared using the dose deviation of the calculated dose from the measured dose. The horizontal orange lines show the limits of ±3%. They are not valid in the region of the field borders (high gradient). Therefore, the evaluation was repeated plotting the distance to the next point with the same dose with a distance to agreement of 3 mm in the second step. Here all points in this region are within the limits.

3.2.
RTPS commissioning As described above, no model for the second linac has been created in the RTPS, but all calculations were done with the first linac model and compared to the measurements of the second. Figure 8 shows the results of such a validation at an example of FFF profiles. The left ordinate refers to the measured and calculated profiles (green and orange), the right to the validation criterion. In the first step, both curves are compared using the dose deviation of the calculated dose from the measured dose. The horizontal orange lines show the limits of ±3%. They are not valid in the region of the field borders (high gradient). Therefore, the evaluation was repeated plotting the distance to the next point with the same dose with a distance to agreement of 3 mm in the second step. Here all points in this region are within the limits.  In the upper part, a dose evaluation is shown (also in orange) referring to the right ordinate, in the lower part a distance to agreement evaluation. All depth doses for 6MV, 15MV, and 6MV FFF were within the tolerances given by the specifications of the manufacturer. One example is shown in Figure 9. For a total amount of 42 analyzed profiles per energy, the specifications were met in the central region in every case. They were only excessed in single points in the gradient region of 6 MV for the smallest field size (2 cm square). The ESTRO recommendations were failed in single points of the two largest field sizes for the sharp gradient regions.
Linac Twins in Radiotherapy http://dx.doi.org/10.5772/60427 Figure 9. Depth dose curves of 6MV photons (calculation in red, measurement in green) for a field size of 2 cm square. The dose difference curve (also in red) refers to the right ordinate. Figures 10 and 11 show the evaluations of a VMAT plan with 6 MV and of an IMRT plan with 6MV FFF as it is presented on the screen of the OmniPro-I'mRT software. The upper left corner demonstrates the calculated dose distribution in the measurement plane, below the measured dose distribution can be seen. The upper-right corner presents profiles in both planes (calculated in red, measured in green). The position and the direction of these profiles are variable.

IMRT and VMAT plan verifications
The lower-right corner shows the gamma evaluation. Pixels in blue and white passed the evaluation. The number of pixels representing a specified value is given in the histogram below. Plans with a passing rate of 95% or more are accepted.

Discussion
The conformity of the depth dose distributions for the linac twins could be shown in the beam quality and the depth dose curves. The differences are in the order of repeated measurements at the same device. The quite unchanged beam quality of 6MV and 6MV FFF shows that not only the linacs are matched but also the FFF mode is matched to the flattened mode [29]. This is not self-evident; the beam hardening of the flattening filter must be compensated by the energy selection [30] that has been performed by the manufacturer.
The slight difference in the field size for the larger fields is within the specifications of the manufacturer [13]. However, this could be adjusted by the service engineer. From a practical point of view, the measured dose distributions are equivalent.
Therefore, the results allowed to continue with the validation of the model of the first machine in the RTPS for the employment at the second one. Deviations from the calculated dose above Evolution of Ionizing Radiation Research 180 the specifications are exceptions in single points and have been assessed clinically to be acceptable. For example, exceeding the gamma criterion of 10% and 2 mm for the largest field size in the sharp gradient area can be traced back to the slightly different field size calibration Figure 10. Plan verification of a VMAT plan with 6MV calculated for one machine and treated at the other. The gamma image was evaluated with 3% and 3 mm and showed a passing rate of 99%, which is indicated in the histogram below. Figure 10. Plan verification of a VMAT plan with 6MV calculated for one machine and treated at the other. The gamma image was evaluated with 3% and 3 mm and showed a passing rate of 99%, which is indicated in the histogram below.
Linac Twins in Radiotherapy http://dx.doi.org/10.5772/60427 181 of both machines, which has also be seen in the comparison of the measure data. Nevertheless, the specification of the manufacturer with a distance to agreement of 3 mm was met. Figure 11. Plan verification of an IMRT plan with 6MV FFF, calculated for one machine and treated at the other. The gamma image was evaluated with 3% and 3 mm and showed a passing rate of 97% which is indicated in the histogram below. Figure 11. Plan verification of an IMRT plan with 6MV FFF, calculated for one machine and treated at the other. The gamma image was evaluated with 3% and 3 mm and showed a passing rate of 97% which is indicated in the histogram below.

Evolution of Ionizing Radiation Research
Thus, the compatibility of the linac twins has been proven for the depth dose distributions and profiles on both main axes in different depths and for the full range of collimator apertures. The acceptance test of the manufacturer for matched linacs, which only refers to the beam quality, is only a first step and runs too short as it has also been reported for the matching process of another manufacturer [8].
The evaluations of different IMRT and VMAT plans, which had been calculated for the first machine and irradiated at the second one, showed very good results. The passing rates were in the same range as they had been seen for verifications at the "original" machine. The beam model in the RTPS of the first machine has been demonstrated sufficient for the second machine even for plans of very high complexity as IMRT with 6MV FFF or VMAT. This means that the second linac can substitute the first one in cases of breakdown. Chang et al. [10] had similar satisfying results for three matched Varian linacs. However, they first measured the data of all three linacs and combined them by averaging to get composite beam data. Having a time interval of several months or more between the installation of different linacs, as it is given in the case of replacement of old machines, this procedure is not applicable.

Conclusion
It has been shown that the dose distributions for all photon energies and modes could be adjusted equivalent. The new FFF mode presents no exceptions. Plan verifications of complex IMRT and VMAT plans demonstrate the exchangeability of the linacs also for the FFF mode, allowing continued therapy during downtimes, e.g., service works. Our results confirm that the time and effort for commissioning and quality assurance can be reduced for linac twins: • There will be only one set of quality checklists, including the tolerance values.
• One beam model for both machines is sufficient in the RTPS.