Validation of a method for in vivo 3D dose reconstruction in SBRT using a new transmission detector

Abstract Stereotactic body radiation therapy (SBRT) involves the delivery of substantially larger doses over fewer fractions than conventional therapy. Therefore, SBRT treatments will strongly benefit patients using vivo patient dose verification, because the impact of the fraction is large. For in vivo measurements, a commercially available quality assurance (QA) system is the COMPASS system (IBA Dosimetry, Germany). For measurements, the system uses a new transmission detector (Dolphin, IBA Dosimetry). In this study, we evaluated the method for in vivo 3D dose reconstruction for SBRT using this new transmission detector. We confirmed the accuracy of COMPASS with Dolphin for SBRT using multi leaf collimator (MLC) test patterns and clinical SBRT cases. We compared the results between the COMPASS, the treatment planning system, the Kodak EDR2 film, and the Monte Carlo (MC) calculations. MLC test patterns were set up to investigate various aspects of dose reconstruction for SBRT: (a) simple open fields (2 × 2–10 × 10 cm2), (b) a square wave chart pattern, and (c) the MLC position detectability test in which the MLCs were changed slightly. In clinical cases, we carried out 6 and 8 static IMRT beams for SBRT in the lung and liver. For MLC test patterns, the differences between COMPASS and MC were around 3%. The COMPASS with the dolphin system showed sufficient resolution in SBRT. For clinical cases, COMPASS can detect small changes for the dose profile and dose–volume histogram. COMPASS also showed good agreement with MC. We can confirm the feasibility of SBRT QA using the COMPASS system with Dolphin. This method was successfully operated using the new transmission detector and verified by measurements and MC.


| INTRODUCTION
Recently, there has been increased clinical use of stereotactic body radiation therapy (SBRT), where extremely large doses of radiation are delivered in 1-8 fractions to one or more small targets of diseased tissue. The SBRT approach of delivering a few large doses guarantees that any error made in treatment delivery has a greater radiological impact on the patient compared to the same error made during a conventional treatment regimen. Therefore, SBRT requires robust quality assurance (QA).
The ideal verification technique for SBRT is one that is applied during patient treatment by employing either entrance or exit dose measurements. A number of publications 1,2 have suggested that online verification is a prudent step to ensure correct delivery and maintain public assurance.
For in vivo entrance dose measurements, the commercial QA platforms 3,4 which are able to correlate the delivered dose to the patient's anatomy are available. Also, the COMPASS system (IBA Dosimetry, Germany) is an in vivo dosimetry system, which provides dose-volume histograms (DVHs) based analysis for each structure. 5,6 For measurements, the system uses a new transmission detector (Dolphin, IBA Dosimetry) for in beam measurements.
Since the Dolphin detector is a new device, a detailed analysis of its accuracy is mandated. Thoelking et al. 7 have already reported on the characterization of the Dolphin and its influence on 6 MV photon beam characteristics. They showed the increase rate of surface dose, the change in percentage-depth-dose (PDDs), the transmission factor, and the comparison to the clinical IMRT plan. However, those results are not specialized for SBRT, moreover, a major concern with these devices is that their large pixel size will blur the measurement of the incident SBRT fluence. It is well known that the spatial resolution of a multielement detector is limited by the size and spacing of individual detector elements, 8 as well a large detector element will result in poor resolution, and large spacing may result in missed information. In particular, SBRT QA requires a high-resolution detector for small fields.
In this paper, we evaluate the method for in vivo 3D dose reconstruction with SBRT using the new transmission detector developed for in vivo dose verification in intensity-modulated radiotherapy (IMRT). Specifically, we validate the detection capability of the COMPASS transmission detector for multi leaf collimator (MLC) test patterns and clinical cases using Monte Carlo (MC) calculations.

| MATERIALS AND METHODS
All measurements were performed using a Synergy (Elekta, Stockholm, Sweden) linear accelerator equipped with an agility MLC. On this linear accelerator, all measurements were carried out with 6 MV photon energy. The treatment planning system (TPS) dose calculations were performed by a Pinnacle 3 ver. 9.2 hr (Philips Radiation Oncology Systems, WI, USA) equipped with a superposition calculation algorithm. The calculation grid size was 1.0 9 1.0 9 1.0 mm 3 for the MLC test cases and 2.0 9 2.0 9 2.0 mm 3 for clinical plans, respectively. Regarding the correction of the transmission detector, we registered the value of 0.91 with TPS as the tray factor. 5

2.A | The COMPASS system
The COMPASS system provides 1-model-base dose computation, 2-measurement-based dose reconstruction using a 2D-array for pretreatment QA (Matrexx, IBA Dosimetry), and 3-measurement-based dose reconstruction using a new transmission detector for in vivo dosimetry (Dolphin, IBA Dosimetry). In this study, we evaluated the measurements-based dose reconstruction using the Dolphin detector.
The COMPASS system in this study consists of the Dolphin detector and an integrated software solution comprising of a superposition algorithm 9 that models the linear accelerator head. The 3D dose distributions were calculated from 2D measured fluence and the scatter kernel (collapsed corn) using the superposition formula. Regarding beam modeling and the reconstruction algorithm including the correction kernel, in particular, we did not modify them for SBRT because we used it for conventional IMRT and VAMT. We confirmed small fields (1 9 1, 2 9 2, 3 9 3, and 5 9 5 cm 2 ) using the dose profiles for SBRT commissioning. The detector assembly is mounted in a holder attached to the treatment head of the Synergy linear accelerator (see distance of 60 cm. Outside the 140 9 140 mm 2 area the center-tocenter distance of the chambers ranges from 5 to 10 mm. The new Dolphin detector has a higher detector density than the previous model. The beam attenuation and the increasing rate of surface dose for the Dolphin detector have already reported that they are about 10% and less than 0.1% at source-to-surface distance 100 cm, respectively. 7 analyzed with a DD system (R-tech, Tokyo, Japan). In the center of a 300 9 300 9 100 mm 3 solid water phantom (Gammex-RMI), point dose measurements were performed with a thimble type ionization chamber (PTV30013, 0.6 cc) to be able to convert the dose distribution measured with film to the absolute dose. We made the optical density-absolute dose curve using 12 step irradiations (0-300 cGy) for the ionization chamber and EDR2 film for dose calibration.

2.C | Clinical SBRT cases
We confirmed the accuracy of COMPASS with Dolphin using clinical SBRT cases. We used a step and shot delivery IMRT beams for SBRT in the lung and liver (Fig. 3 and Table. 1). The prescribed doses for lung and liver are 48 Gy/4 fractions and 36 Gy/5 fractions for clinical target volume (CTV), respectively. For organs at risk (OARs), we used our hospital constraints. In clinical cases, we compared not only the dose profiles and DVHs, but also the global gamma evaluation (criteria, DTA/DD:2 mm/2%, threshold 10%).

2.D | MC simulation
To verify the accuracy of the COMPASS system, the dose profiles and the dose distribution for lung and liver plans were also calculated by the EGSnrc/BEAMnrc 10,11 and DOSXYZnrc 12 user-codes.       cases. For the lung case, the 2D pass rates for the COMAPSS and MC were 97% and 98% at the coronal planes of isocenters, respectively. As well as in the liver case, the 2D pass rates for the COMAPSS and MC were 99% and 98% at the coronal planes of isocenters, respectively. As well as the gamma pass rate, we confirmed other gamma parameters (maximum and deviation gamma value, 3D gamma, etc), however, there was no significant difference between the COMPASS and MC.   A disadvantage of 2D detector arrays is that their resolution is generally much lower than what is obtainable with film and MC. The work of Opp et al. 17 suggests that a 5 mm resolution (at an isocenter) is sufficient to detect errors in IMRT delivery. Also, Asuni et al. 18 reported the spatial resolution of the COMPASS transmission detector using narrow slit fields. Although the detector was the previous version (Ver. 2) where the distance between the detectors is larger than the Dolphin, the COMPASS system showed acceptable resolution for IMRT. However, for SBRT, higher resolutions are required.

3.B | Clinical SBRT cases
The detection of slight MLC position errors is essential in SBRT due to the use of smaller fields.
For MLC error position tests, the absolute values of errors were 0.24 mm which includes systematic errors. The accuracy of the static MLC position for the Synergy linac is around 0.1 mm. 19 Therefore, considering the systematic error of MLC, the detectability of a COMPASS system is around 0. EPID and the log-file system are easy to use, and effective. However, the EPID shows some dependences. Also, the log-file system is not dosimetry and log-files include an electrical delay. A QA procedure for COMPASS is a little troublesome compared to the other two methods. However, for accuracy and robustness, the fluence measurement-based system using an ionization chamber is advantageous.
Recent radiotherapy, SBRT is performed on many sites due to the outcomes. 21,22 But, the number of fractions is smaller, and the delivered dose is larger than in conventional treatment. Therefore, QA during treatment is very important. Sharma et al. 23 reported that dynamic MLC (DMLC) output factor, which is a MLC positioning test, was reproducible within AE 0.5% over a period of 14 months.
Monitoring is the best QA method for SBRT because the position of the MLC changes during treatment. We can confirm the feasibility of SBRT QA using the COMPASS system with Dolphin. We suggested a method for in vivo 3D dose reconstruction for SBRT. This method was successfully implemented using a new transmission detector and verified by measurement and MC.

| CONCLUSION S
We have implemented a method for in vivo 3D dose reconstruction for SBRT using a new transmission detector. In a phantom study, the differences between COMPASS and MC were around 2-3%. The