Characterization of a prototype MR-compatible Delta4 QA-system in a 1.5 tesla MR-linac

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1 Physics in Medicine and Biology ACCEPTED MANUSCRIPT Characterization of a prototype MR-compatible Delta QA-system in a. tesla MR-linac To cite this article before publication: J H Wilfred de Vries et al 0 Phys. Med. Biol. in press Manuscript version: is the version of the article accepted for publication including all changes made as a result of the peer review process, and which may also include the addition to the article by IOP Publishing of a header, an article ID, a cover sheet and/or an Accepted Manuscript watermark, but excluding any other editing, typesetting or other changes made by IOP Publishing and/or its licensors This is 0 Institute of Physics and Engineering in Medicine. During the embargo period (the month period from the publication of the Version of Record of this article), the is fully protected by copyright and cannot be reused or reposted elsewhere. As the Version of Record of this article is going to be / has been published on a subscription basis, this is available for reuse under a CC BY-NC-ND.0 licence after the month embargo period. After the embargo period, everyone is permitted to use copy and redistribute this article for non-commercial purposes only, provided that they adhere to all the terms of the licence Although reasonable endeavours have been taken to obtain all necessary permissions from third parties to include their copyrighted content within this article, their full citation and copyright line may not be present in this version. Before using any content from this article, please refer to the Version of Record on IOPscience once published for full citation and copyright details, as permissions will likely be required. All third party content is fully copyright protected, unless specifically stated otherwise in the figure caption in the Version of Record. View the article online for updates and enhancements. This content was downloaded from IP address on //0 at 0:

2 Page of Characterization of a prototype MR-compatible Delta QAsystem in a. Tesla MR-Linac J.H.W. de Vries, E. Seravalli, A.C. Houweling, S. Woodings, R. van Rooij, J.W.H. Wolthaus, J.J.W. Lagendijk and B.W. Raaymakers Department of Radiation Oncology, University Medical Center Utrecht, Utrecht, The Netherlands Department of Radiology, University Medical Center Utrecht, Utrecht, The Netherlands Corresponding author: Department of Radiation Oncology, University Medical Center Utrecht, tel: + 0, w.devries-@umcutrecht.nl Abstract AUTHOR SUBMITTED MANUSCRIPT - PMB-0.R To perform patient plan-quality assurance (QA) on the newly installed MR-Linac (MRL) there was a need for having an MR-compatible QA-device. An MR compatible device (MR-Delta) was developed together with Scandidos AB (Uppsala, Sweden). The basic characteristics of the detector response such as short-term-reproducibility, dose linearity, field size dependency, dose rate dependency, dose-per-pulse dependency and angular dependency were investigated for the Clinical DeltaPT as well as for the MR compatible version. All tests were performed with both devices on a conventional linac and the MR compatible device was tested on the MRL as well. No statistically significant differences were found in short term reproducibility (< 0.%), dose linearity ( 0.%), field size dependency (<.0% for field sizes larger than x cm ), dose rate dependency (<.0%) and angular dependency for all phantom/linac combinations. Dose-per-pulse dependency (< 0.%) was found to be significantly different between the devices. This difference can be explained by the fact that the diodes of the Clinical DeltaPT have been irradiated with a much larger dose than the MR-DeltaPT ones. The absolute difference between the devices (< 0.%) was found to be small, so no clinical impact is expected. For both devices, the results were consistent with characteristics of the DeltaPT device reported in literature (Bedford et al. (00), Sadagopan et al. (00)). We found that the characteristics of the MR compatible Delta phantom are found to be comparable to the clinical used one. Also the found characteristics do not differ from previous reported characteristics of the commercially available non-mr compatible DeltaPT phantom. Therefor the MR compatible Delta prototype was found to be safe and effective for use in the. Tesla magnetic field of the Elekta MR- Linac

3 AUTHOR SUBMITTED MANUSCRIPT - PMB-0.R Page of Introduction At the UMC Utrecht an MRI-Linac (MRL) has been installed combining Magnetic Resonance Imaging (MRI) with a linear accelerator (linac). This MRL is an improved version of the one described by Raaymakers et al. (0). The MRL is equipped with a MV Flattening-Filter-Free (FFF) Linac (Elekta AB, Stockholm Sweden) and a. Tesla MRI-scanner (Philips Medical Systems, Best The Netherlands). By combining an MRI scanner and a linac, it is possible to simultaneous irradiate and create a high quality image with soft-tissue contrast of the target area (Lagendijk et al. (0)). Before starting patient treatments, extensive quality assurance (QA) of treatment plans should be performed to make sure the planned dose distributions are deliverable with the required accuracy. In our institute, also for the MRL QA of pre-treatments plans is performed, although this plan will be adapted to the position and anatomy of the patient at the time of treatment at the MRL. Post-treatment QA of the delivered plan is also performed in our institute. To perform such verification measurements, nowadays several commercial QA-systems are available (Hussein et al. (0)). However, at the moment of writing the only commercially MR-compatible device available is the ArcCHECK-MR (Model 0-MR, Sun Nuclear Corporation, Melbourne USA). The performance of this device in the MRL is evaluated by Houweling et al. (0). In our department a Delta-PT phantom (Scandidos AB, Uppsala Sweden) (Clinical Delta) is used for the QA of treatment plans. The performance of this device was investigated previously by Bedford et al. (00) and Sadagopan et al. (00). Together with the Research & Development department of Scandidos AB (Uppsala, Sweden) a prototype MR compatible Delta phantom (MR-Delta) was developed. This MR-Delta is based on the commercially available Delta-PT. The effect of these changes on the characteristics of the phantom are expected to be small but are unclear. Furthermore, basic design characteristics, like small air volumes around the diodes, might influence the measurements due to the Electron Return Effect (ERE) effect as well (Raaijmakers et al. (00), Reynolds et al. (0)) The purpose of this study was to characterize the MR-Delta phantom and compare these results to the characteristics of the Clinical Delta. Therefore, the short-term-reproducibility, dose linearity, field size dependency, dose rate dependency and dose-per-pulse dependency of both phantoms were evaluated. All of these tests were performed on a conventional linac with unflattened beam (FFF) for both the Clinical Delta and the MR-Delta. To study the effect of the presence of a. T magnetic field all of the tests were repeated with the MR-Delta in the MRL.. Material and Methods.. Linacs The conventional linac (Precise Treatment System, Elekta, Stockholm Sweden) is equipped with a MV Flattening-Filter-Free (FFF) photon beam. The Source-Axis Distance (SAD) of this linac is 00 cm, with a multi-leaf collimator (MLC) with 0 leaf pairs of cm leaf width at SAD. The dose maximum for this beam is at mm depth and the TPR 0/0 is 0..

4 Page of The MRL is a combination of a modified MV FFF (Elekta, Crawley UK) accelerator and a modified. T MR system (Philips Ingenia, Best, The Netherlands). The SAD of the MRL is. cm, and it is equipped with 0 leaf pairs of 0. cm width at SAD. The dose maximum of this beam is at mm depth and where TPR 0/0 is 0.0. Both accelerators were calibrated to deliver Gy per 00 MU at dose maximum, according to the NCS- Code of Practice (Nederlandse Commissie voor Stralingsdosimetrie (Stralingsdosimetrie", January 00).. Phantoms The Delta-PT phantom is a cylindrical polymethylmethaacrylate (PMMA) phantom containing orthogonal detector boards with in total 0 p-type Si-diodes (Scandidos). To be used in the MRL, the device had to be made MR-compatible. Therefore the feet of the device were taken off, adaptations were made to the network sockets and the power supply cables were extended to position the power supply outside of the gauss line of the magnetic field area. In figure A the device is setup in the MRL. The phantom consists of two crossing arrays which are fixed in a cylindrical geometry. No differences between the arrays are expected. For the purpose of this study, the main detector board of the Delta-PT phantom was taken out of the cylindrical phantom and embedded in the rectangular calibration phantom (Scandidos AB, Uppsala, Sweden), made of PMMA (figure B). This allowed the insertion of an ionization chamber (IC) (Farmer NE A ionization chamber, Qados, Sandhurst, United Kingdom), in a parallel orientation to the magnetic field (Fig B), in the phantom at the same time for each measurement. The standard measurement setup at the conventional linac was a Source-Skin-Distance (SSD) of.0 cm, resulting in a Source-Detector-Depth (SDD) of. cm (. cm geometrical depth in PMMA which is equal to.0 cm water equivalent depth). The standard setup at the MRL was an SSD of. cm, with an SDD of.0 cm. (A) (B) Figure : MR-Delta phantom in a regular QA setup (A) and the standard setup for performing all characterization measurements (B). All measurements were performed with the Clinical Delta and MR-Delta at a conventional linac, and with the MR-Delta at the MRL... Methods AUTHOR SUBMITTED MANUSCRIPT - PMB-0.R No differences in performance between different detectors are expected. Therefore, only the central diodes on the mainboard were investigated. For each test the three central diodes of the detector board

5 AUTHOR SUBMITTED MANUSCRIPT - PMB-0.R Page of were measured along with a simultaneous IC measurement (figure b). For each measurement the three diodes were averaged and then the average diode reading was divided by (ie normalized to) the corresponding averaged IC reading to eliminate possible linac output fluctuations. Deviations were calculated from the readings under reference conditions. The reference field size used was 0x0 cm, dose rate was set to maximum available, and 00 monitor units per measurement were given. The standard Farmer chamber, field size, dose rate and number of monitor units were applied unless otherwise noted... Short term reproducibility The short term reproducibility was examined by performing 0 consecutive measurements for each linacphantom combination. The reading of the central diodes was normalized to the average reading of the 0 ionization chamber measurements. The standard deviation and maximum deviation of the readings were calculated... Dose linearity The dose linearity was investigated by measuring the reading for,, 0, 0, 0, 00, 00, 00, 000 and 000 MU. All measurements were normalized to the 00 MU measurement. The standard deviation and maximum deviation of the readings were calculated... Field size dependency To investigate the field size dependency, 00 MU were delivered per measurement for field sizes of x cm, x cm, 0x0 cm, 0x0 cm and x cm. All measurements were normalized to the 0x0 cm field measurement. For field sizes equal and smaller then x cm, a small volume ionization chamber (IC-0, IBA Dosimetry GmbH, Schwarzenbruck Germany) was used instead of the Farmer chamber... Dose rate dependency The dose rate dependency was assessed, by varying the Pulse-Rate-Frequency (PRF) on the linac. The investigated dose rate range depended on the type of linac and modality of photon beam. For the flattening filter free (FFF) photon beam at the conventional linac,,, and MU/min were applied. At the MRL dose rates of 0,,,,,,, and MU/min were used. All measurements were normalized to the intermediate dose rate (e.g. and MU/min) measurement... Dose per pulse dependency The dose-per-pulse dependency was evaluated by changing the Source Detector Distance (SDD) per measurement. On the conventional linac SDDs of 0, 0, 00, 0, 0, 0,. and 0 cm were applied. All measurements were normalized to the measurement for an SDD of 00 cm. By design, the tabletop of the MRL is not able to move in height and the SDD could not be changed. Therefore, this test was not performed at the MRL.

6 Page of AUTHOR SUBMITTED MANUSCRIPT - PMB-0.R.. Angular dependency Each system was irradiated with the conventional linac from a range of gantry angles to assess the angular dependency of the detectors. Particular attention was given around gantry angles 0 and 0 degrees, where the change in angular sensitivity was expected to be greatest. Measurements were also performed with the MRI-linac at angular increments of 0 degrees (gantry angles were limited at the time of measurement). All measurements were normalized to the reading of the gantry angle 0... Statistical analysis For the short-term reproducibility and the dose rate dependency, because the samples were completely independent, the differences between the devices and linacs were tested for significance using the Kruskal-Wallis test. For all other experiments, where independency was not assured, a Friedman test was performed (IBM SPSS statistics, version ).. Results.. Short term reproducibility There was no significant difference (p=0.) in the short term reproducibility for both the Clinical- Delta and MR-Delta on both the conventional linac and the MRL (figure ). The maximum deviation was 0.%, which was observed for the MR-Delta phantom at the conventional linac. Deviation (%) 0.0% 0.0% 0.00% -0.0% -0.0% -0.0% 0 Number of Measurement Clinical Delta Conventional Linac MR-Delta Conventional Linac MR-Delta MR-Linac Figure : Short term reproducibility Clinical and MR-Delta on a conventional linac and MR-Delta on the MRL. Per phantom/linac combination the reading is normalized to the average of the 0 measurements.

7 AUTHOR SUBMITTED MANUSCRIPT - PMB-0.R Page of Dose linearity The largest deviations were observed for the MU measurements for all phantom/linac combinations. However, no significant difference (p=0.) was observed between the different devices. Deviation (%).0%.%.0% 0.% 0.0% -0.% -.0% -.% Figure : Dose linearity of the Clinical Delta and MR-Delta on a conventional linac and MRL. Per phantom/linac combination the reading is normalized to the measurement of 00 MU; the X-axis is displayed on a log scale... Field size dependency Amount of MU Clinical Delta Conventional Linac MR-Delta Conventional Linac MR-Delta MR-Linac An increasing deviation was observed when the field size decreases. There was no significant difference observed (p=0.) between all the phantom/linac combinations for the field size dependency. Deviation (%) 0.% 0.0% -0.% -.0% -.% -.0% -.% -.0% Field edge length (cm) Clinical Delta Conventional Linac MR-Delta Conventional Linac MR-Delta MR-Linac Figure : Field size dependency of the Clinical Delta and MR-Delta on a conventional linac and MRL. Per phantom/linac combination the reading is normalized to a 0x0 cm field size.

8 Page of AUTHOR SUBMITTED MANUSCRIPT - PMB-0.R.. Dose rate dependency There was no significant difference found (p=0.) between all phantom/linac combinations for dose rate dependency, although there is an outlier for the MR-Delta in the MRL (.%). However, if this outlier is not taken into account, all other points are within % deviation. Deviation (%) 0.0% 0.0% 0.0% 0.0% 0.0% -0.0% Figure : Dose rate dependency of the Clinical Delta and MR-Delta on a conventional linac and MRL. Per phantom/linac combination the reading is normalized to an intermediate dose rate (around 0 MU/min, depending on the linac)... Dose-per-pulse dependency MU/min Clinical Delta Conventional Linac MR-Delta Conventional Linac MR-Delta MR-Linac The dose-per-pulse dependency, measured by increasing the SDD (0-0 cm), is presented in figure. All values were normalized to the reading at an SDD of 00 cm. All readings were within a 0.% difference. There is a significant difference found (p=0.00) between the samples from the clinical Delta and the MR-Delta. The largest difference between measuring points (at the same SDD) is 0.% for a SDD of 0 cm. The clinical Delta shows on all dose-per-pulse variations a larger deviation than the MR- Delta phantom. Deviation (%).00% 0.% 0.0% 0.% 0.00% -0.% -0.0% 0 Clinical Delta Conventional Linac SDD (cm) MR-Delta Conventional Linac Figure : Dose-per-pulse dependency of the clinical Delta and MR-Delta on a conventional linac. Per phantom/linac combination the reading is normalized to the measurement at a SDD of 00 cm.

9 AUTHOR SUBMITTED MANUSCRIPT - PMB-0.R Page of Angular dependency The angular dependency, measured by varying the gantry angle, is presented in figure. All values were normalized to the reading at a gantry angle of 0. There was no significant difference found (p=0.) between all phantom/linac combinations for angular dependency. The largest difference between measuring points (at the same gantry angle) is.% for a gantry angle of 0. Figure : Angular dependency of the clinical Delta and MR-Delta on a conventional linac and MRL. Per phantom/linac combination the reading is normalized to the measurement at gantry 0.. Discussion Clinical Delta Conventional Linac MR-Delta Conventional Linac MR-Delta MR-Linac The characteristics of an MR-compatible Delta-PT phantom were investigated in a transverse. T magnetic field at the MRL. These were compared with the results of the same tests performed on a conventional linear accelerator. Moreover the results were compared to the clinical version of the Delta- PT. No significant differences between the all phantom/linac combinations were found for short term reproducibility, dose linearity, field size dependency and dose rate dependency. Sadagopan et al. (00) and Feygelman et al. (00) showed that the short term reproducibility of the detector response was 0.%, which is consistent with our measurements. For dose linearity Bedford et al. (00) reported a response of the Delta for MU better than 0.%. Sadagopan et al. (00) showed for a dose of cgy ( 0MU) and higher a response within 0.%. In this study the results for MU were consistent with Bedford et al. (00) and Sadagopan et al. (00). The largest deviation for dose linearity was +.% for the MR-Delta at the MRL. However this deviation was found for the MU measurements. As seen in standard clinical practice, the lower the number of MU, the larger the deviation will be. For all readings at MU the deviation is not larger than +/- %, which is the clinically accepted tolerance for this small amount of MU. Therefore this deviation is not clinically significant

10 Page of For dose rate dependency Feygelman et al. (00) showed a variation in Delta response by no more than 0.%. In this work a difference of % was observed at PRF-values of 0 Hz (0 MU/min) for the MR- Delta on the MRL. Excluding this point, the response variation for dose rate dependency was consistent with the literature ( %). The outlier in the dose rate dependency for the MR-Delta device in the MRL can be explained by the fact that these measurements were performed without a trigger signal, whereas all the measurements at the conventional linac were performed with the device connected to the triggering-signal of the linac. The MRL has by design no point where a trigger cable can be connected since the gantry can rotate continuously. When there is no trigger cable connected to the device, the device switches to so called search mode and waits until it detects radiation on the detectors. The sampling window of this search mode is set at 00 Hz +/- 0 Hz, which results in a sampling time of ms. The PRF to get a dose rate of 0 MU/min at the MRL is 0 Hz, which gives a period of. ms. Calculations on these period times showed that every st, th, th, th and 0 th radiation pulse is not detected by the phantom. Since the sampling window for dose is set at 00 HZ, the same amount of missed pulses can be expected with PRF values for 0 and Hz (double or half of 0Hz). On a clinical linac, Delta measurements are made in a triggered mode, so for the outcome of QA measurements of Volumetric Modulated Arc Therapy- (VMAT-)plans there is no error expected. When measuring on a MRL, the measurement has to be made in an untriggered way, but since the width of the detected peak is small, about 0Hz what corresponds to an actual doserate difference at the machine of 0 MU/min, also the effect on VMAT plans is not expected. Feygelman et al. (00) showed a variation in dose-per-pulse dependency of less than 0.%. They varied the SDD from 0. cm to. cm. A comparison of these results in the same range of SDD, shows a similar variation for all phantom/linac combinations. For the larger SDD Sadagopan et al. (00) found a variation of 0.%, with a standard deviation of 0.%. The measurements in this study fit within this range. There was a significant difference observed for the dose-per-pulse dependency between the clinical Delta -PT and the MR-compatible version. However, the maximum absolute difference between the two phantoms is less than 0.%. This statistically significant difference between the two devices can be explained by historical irradiation. The Clinical delta has been used in our institute for six years. This may have caused a decrease in sensitivity for these detectors. The absolute reading, when no correction factors applied, for all MR-Delta measurements is on average % higher than for the Clinical Delta device. If absolute calibration factors were applied, this difference would be removed. In the detector boards, small air volumes, up to a few mm in size, around the diodes are evident in a CT scan. It is expected that the Lorentz force, acting on electrons within the air cavities will change the diode readings, dependent on the precise shape and size of the air cavities (Raaijmakers et al. (00)). Also the incident direction of radiation with respect to the magnetic field may cause a relative response difference (Reynolds et al. (0), Raaijmakers et al. (00)). This is a likely explanation of the observed variation in angular sensitivity.. Conclusion AUTHOR SUBMITTED MANUSCRIPT - PMB-0.R Short term reproducibility, dose linearity, field size dependency, dose rate dependency dose-per-pulse dependency and angular dependency were investigated on a commercially available Delta phantom, as well as for the modified MR-compatible phantom. Both devices were tested in a non-mr environment (conventional linac) and the MR-compatible phantom was tested in a MR-environment (. T MRL).

11 AUTHOR SUBMITTED MANUSCRIPT - PMB-0.R Page 0 of For the majority of tests no significant differences were observed for both devices in conventional linac and MRL, however for the dose-per-pulse dependency a significant difference was observed. This difference was within the clinical tolerances. Therefore it is concluded that the modifications applied to the original Delta-PT phantom to make it MR-compatible did not result in any significant changes. The prototype system is suitable for safe and effective use in the. T magnetic field of the MRL.. Acknowledgements We would like to thank Scandidos AB for their collaboration and providing us with a MR-compatible Delta-PT phantom. Especially Daniel Nystrom and Thomas Matzen of Scandidos AB for their contribution to this study.. References BEDFORD, J. L., LEE, Y. K., WAI, P., SOUTH, C. P. & WARRINGTON, A. P. 00. Evaluation of the Delta phantom for IMRT and VMAT verification. Phys Med Biol,, N-. FEYGELMAN, V., FORSTER, K., OPP, D. & NILSSON, G. 00. Evaluation of a biplanar diode array dosimeter for quality assurance of step-and-shoot IMRT. J Appl Clin Med Phys, 0, 00. HOUWELING, A. C., DE VRIES, J. H., WOLTHAUS, J., WOODINGS, S., KOK, J. G., VAN ASSELEN, B., SMIT, K., BEL, A., LAGENDIJK, J. J. & RAAYMAKERS, B. W. 0. Performance of a cylindrical diode array for use in a. T MR-linac. Phys Med Biol,, N0-. HUSSEIN, M., ROWSHANFARZAD, P., EBERT, M. A., NISBET, A. & CLARK, C. H. 0. A comparison of the gamma index analysis in various commercial IMRT/VMAT QA systems. Radiotherapy and Oncology, 0, 0-. LAGENDIJK, J. J., RAAYMAKERS, B. W. & VAN VULPEN, M. 0. The magnetic resonance imaging-linac system. Semin Radiat Oncol,, 0-. RAAIJMAKERS, A. J., RAAYMAKERS, B. W., VAN DER MEER, S. & LAGENDIJK, J. J. 00. Integrating a MRI scanner with a MV radiotherapy accelerator: impact of the surface orientation on the entrance and exit dose due to the transverse magnetic field. Phys Med Biol,, -. RAAYMAKERS, B., SMIT, K., BOL, G. H., VAN DEN BOSCH, M. R., CRIJNS, S. P. M., KOK, J. G. M. & LAGENDIJK, J. J. W. 0. Sp-0 Dosimetry in Strong Magnetic Fields; Issues and Opportunities. Radiotherapy and Oncology, 0, S0. REYNOLDS, M., FALLONE, B. G. & RATHEE, S. 0. Dose response of selected solid state detectors in applied homogeneous transverse and longitudinal magnetic fields. Med Phys,, 00. SADAGOPAN, R., BENCOMO, J. A., MARTIN, R. L., NILSSON, G., MATZEN, T. & BALTER, P. A. 00. Characterization and clinical evaluation of a novel IMRT quality assurance system. J Appl Clin Med Phys, 0,. SCANDIDOS. Delta PT True Volumetric Pre-treatment Verification. Available: r.pdf. STRALINGSDOSIMETRIE, N. C. V. January 00. Code of Practice for the Absorbed Dose Determination in High Energy Photon and Electron Beam. Available:

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