Effect of an integral quality monitor on 4‐, 6‐, 10‐MV, and 6‐MV flattening filter‐free photon beams

Abstract Purpose To investigate the effect of an integral quality monitor (IQM; iRT Systems GmbH, Koblenz, Germany) on 4, 6, 10, and 6‐MV flattening filter‐free (FFF) photon beams. Methods We assessed surface dose, PDD20,10, TPR20,10, PDD curves, inline and crossline profiles, transmission factor, and output factor with and without the IQM. PDD, transmission factor, and output factor were measured for square fields of 3, 5, 10, 15, 20, 25, and 30 cm and profiles were performed for square fields of 3, 5, 10, 20, and 30 cm at 5‐, 10‐, and 30‐cm depth. Results The differences in surface dose of all energies for square fields of 3, 5, 10, 15, 20, and 25 cm were within 3.7% whereas for a square field of 30 cm, they were 4.6%, 6.8%, 6.7%, and 8.7% for 4‐MV, 6‐MV, 6‐MV‐FFF, and 10‐MV, respectively. Differences in PDD20,10, TPR20,10, PDD, profiles, and output factors were within ±1%. Local and global gamma values (2%/2 mm) were below 1 for PDD beyond d max and inline/crossline profiles in the central beam region, respectively. The gamma passing rates (10% threshold) for PDD curves and profiles were above 95% at 2%/2 mm. The transmission factors for 4‐MV, 6‐MV, 6‐MV‐FFF, and 10‐MV for field sizes from 3 × 3 to 30 × 30 cm2 were 0.926–0.933, 0.937–0.941, 0.937–0.939, and 0.949–0.953, respectively. Conclusions The influence of the IQM on the beam quality (in particular 4‐MV X‐ray has not verified before) was tested and introduced a slight beam perturbation at the surface and build‐up region and the edge of the crossline/inline profiles. To use IQM in pre‐ and intra‐treatment quality assurance, a tray factor should be put into treatment planning systems for the dose calculation for the 4‐, 6‐, 10‐, and 6‐MV flattening filter‐free photon beams to compensate the beam attenuation of the IQM detector.


| INTRODUCTION
Quality assurance (QA) plays an important role in minimizing and preventing errors in radiation therapy. Dosimetric evaluation for treatment plans has routinely used ionization chambers, thermoluminescent dosimeters, optically stimulated luminescent dosimeters, and films as conventional QA techniques. [1][2][3][4] However, the disadvantage of these QA devices is that they have only been used for offline verification and cannot predict any unexpected errors in vivo dosimetry.
Moreover, the gamma passing rates were reported to be insufficient for prediction of dose errors in QA of intensity-modulated radiation therapy. 5,6 Beam output should be verified during treatment to avoid potential treatment errors such as equipment failure and wrong plan selection. 7,8 Therefore, some advanced in vivo QA methods were introduced to increase the dosimetric accuracy in vivo dosimetry such as point dosimeters, electronic portal imaging device dosimetry (EPID), transmission detectors, linac log file analysis, and dose accumulation methods. [9][10][11][12][13][14][15][16][17][18][19] The point dosimeters and EPID are sensitive to linac and patient errors. 20 Transmission detectors monitor linac performance with high sensitivity in real time. 20 Linac log file analysis is sensitive to plan corruption errors. 20 Dose accumulation methods are used to evaluate intrafraction movements and the multileaf collimator (MLC) tracking systems. 20 A type of new transmission detector called an integral quality monitor (IQM) has been commercialized by iRT Systems GmbH, Koblenz, Germany. The IQM is mounted on a linear accelerator (linac) gantry head and uses an independent system monitor to provide on-line beam monitoring that can detect treatment delivery errors that exceed an acceptance level. 9 The IQM is a wedgeshaped ionization chamber with a continuous spatial resolution and the large sensitive volume that can detect a small range of systematic MLC error compared with 0.6-cc Farmer chamber (PTW 30013, Freiburg, Germany), Delta4 (ScandiDos, Uppsala, Sweden), and 2D-array seven29 (PTW, Freiburg, Germany). 21 However, when the IQM is placed in the beam path, it was reported that the IQM affected to be small yet statistically significant photon beam properties with the increase in the surface dose and beam attenuation of 6-, 10-, 15-, 18-MV, and 6-, 10 -MV flattening filter-free (FFF) X-ray beams. 22 With respect to the effect of IQM on beam quality, Casar et al. 22 evaluated the surface dose, difference in the ratio of percentage depth dose at depths of 20 and 10 cm (PDD 20,10 ), and transmission factors for field sizes from 1 × 1 to 20 × 20 cm 2 for 6-, 10-, 15-, and 18-MV and for two FFF photon beams (6-and 10-MV FFF). Islam et al. 9 evaluated the surface dose, the profiles at 1.5 and 10 cm depths for 30 × 30 cm 2 field and percent depth dose for 10 × 10 and 30 × 30 cm 2 fields, and transmission factor for a field size of 10 × 10 cm 2 for 6-and 18-MV X-ray beams.
Hoffman et al. 23  The previous studies evaluated the influence of the IQM on photon energies of Elekta linacs (Synergy, Precise, and Versa HD (Elekta AB, Stockholm, Sweden)) with 6-MV low-energy photons, 10-MV mid-energy photons, and 15-and 18-MV high-energy photons. 9,22,23 However, in this study, we used an Elekta Infinity linac (Elekta AB, Stockholm, Sweden) with 4-MV low-energy photons, 6-MV mid-energy photons, and 10-MV high-energy photons. In Japan, there is basically a difference in the number of FFF beams between Elekta Infinity and Elekta Versa HD. The Elekta Versa HD has two FFF beams with 10-and 6-MV FFF X-ray beams whereas the Elekta Infinity only selects one of the FFF beams. Outside Japan, the specifications of linac may differ slightly. The Elekta Infinity with Agility MLC (160 leaves with 1 cm leaf width) can be installed depending on the country. In Japan, the 4-MV X-ray beam is used as low-energy photon instead of 6-MV X-ray beam because the 4-MV X-ray beam is routinely used for breasts with small configuration in Japan whereas the 6-MV X-ray beam or higher energy photon is used for large-size breasts in US and Europe. 24 Therefore, the beam shaping inside Elekta linac in Japan is different from global Elekta linac. The Because the configuration of Elekta linacs depends upon the situation of each country, the evaluation of effect of the linac variation for photon beam is necessary and the results obtained for the 6and 10-MV beams in this study should be comparable with published papers.
T A B L E 1 Difference of photon delivery system between the Elekta Linac in Japan and the global Elekta Linac for low-, mid-, and highenergy photons.  transmission factor for a field size was defined as the ratio of the ionization charge with the IQM to that without the IQM at a reference depth of 10 cm in the water phantom. 25,26 The output factor for a certain field size with and without the IQM was defined as the ratio of the measured dose for an actual field size in the water phantom and that for a reference field of 10 × 10 cm 2 at a depth of 10 cm. 25 The differences of output factor with and without the IQM were calculated for all energies and field sizes.

2.C | Evaluation of the effect of IQM on beam quality
The influence of the IQM on the surface and build-up region dose was evaluated by the dose difference from the surface (depth = 0 cm) to depth of dose maximum (d max ) of the PDD curves with and without the IQM. 27 The PDD 20,10 , TPR 20,10 , PDD curves, and inline/crossline profiles, transmission factor, and output factor with and without the IQM were used to assess the effect of the IQM on beam quality of 4-, 6-, 6-MV FFF, and 10-MV X-ray beams beyond d max . The difference between PDD curves and inline/crossline pro- files measured with and without IQM should be within AE1%. The differences at the build-up region dose, PDD 20,10 , TPR 20,10 , PDD curves and inline/crossline profiles, and output factor measured with and without the IQM were defined as follows: where X with IQM and X without IQM are the build-up region dose, PDD 20,10 , TPR 20,10 , PDD curves, crossline and inline profiles, and output factor measured with and without IQM, respectively.
The PDD curves and crossline and inline profiles measured with IQM were compared with the corresponding PDD curves and crossline and inline profiles measured without IQM using a gamma function described by Low et al. 28 We used dose difference (2%) and distance to agreement acceptance criteria (2 mm) for the gamma calculations. The gamma criteria (10% threshold and gamma passing rate above 95% at 2%/2 mm) was used. Local and global dose differences were analyzed for PDD curves and crossline/inline profiles, respectively.       17.2% (6-MV FFF) at a depth of 5 cm, 6.9% (6 MV) to 9.7% (6-MV FFF) at a depth of 10 cm, and 3.3% (6 MV) to 6.5% (4 MV) at a depth of 30 cm (Fig. 10). By gamma function calculations (2%/2  interacted with the phantom and decreased with increasing depth. 10,12,29,30 Therefore, the electron contamination in deeper regions was lower compared with that in the surface region. 31 and colleagues showed that the largest difference in PDD 20,10 for a 6-MV FFF X-ray beam was 0.5%. 22 The differences between TPR 20,10 with and without the IQM did not exceed 0.3%, with the highest value observed for the 6-MV FFF X-ray beam (0.3%) and the lowest value for the 4-MV X-ray beam (0.1%). The differences and gamma values (2%/2 mm) in PDD beyond d max with and without the IQM for all energies and field sizes were within AE1% and below 1, respectively. The gamma passing rates (10% threshold) for all energies and field sizes from 3 × 3 to 30 × 30 cm 2 were above 95% at 2%/2 mm. Islam et al. 9 showed that the differences in PDD in the transient charged particle equilibrium region for a 6-MV X-ray beam were within AE1%. The difference in the surface dose when the field size was 30 × 30 cm 2 for the 6-MV FFF X-ray beam was smaller than those for the 6-and 10-MV X-ray beam because the FFF conditions decreased the electrons contamination. 30 The differences in PDD 20,10 , TPR 20,10 , and crossline/inline profiles of the 6-MV FFF Xray beam with and without the IQM were larger than those of the conventional flattening filter photon beams (4-, 6-, and 10-MV X-ray beams), whereas the differences in output factors were smaller compared with those obtained for beams with other energies. The flattening filter eliminated the primary photons. Therefore, the edge of the field of the FFF beam received a higher head scatter dose compared with that for the edge of beams with the flattening filter.
The differences in surface and build-up dose, PDD 20,10 , and TPR 20,10 of the 4-MV X-ray beam with and without the IQM were smaller than those obtained at other energies. However, the beam attenuation of the IQM for the 4-MV X-ray beam was higher than NGUYEN ET AL.
| 89 that of beams with other energies. Therefore, a tray factor should be put into treatment planning systems for dose calculation for the 4-MV X-ray beam. Casar et al. 22 suggested to configure treatment planning system through tray factors or modify output factors for particular beam energy before using IQM in pre-and intra-treatment QA. Therefore, it is necessary to evaluate the commissioning of the IQM device whether the changes in the beam characteristics and output factors could account for the attenuation of IQM. Some papers showed that the IQM system has the potential in its clinical use. Marrazzo et al. 33 reported the IQM detector is a highly sensitive dose-monitoring device for clinical practice of step-and-shoot IMRT plan and found a good correlation between the measured IQM signal and DVH metrics that is useful for identifying clinical action levels.

| CONCLUSION
We evaluated the difference in the beam quality measured with and without an IQM and found the field-size and beam-energy dependence of the IQM. The influence of IQM on the beam quality (in particular 4-MV X-ray has not verified before) was tested and introduced a slight beam perturbation at the surface and build-up region and the edge of the crossline/inline profiles. To use IQM in pre-and intra-treatment QA, a tray factor should be put into treatment planning systems for the dose calculation for the 4-, 6-, 10-MV, and 6-MV flattening filter-free photon beams to compensate for the attenuation of the IQM detector.

CONFLI CT OF INTEREST
This study was a collaborative research project with APEX Medical, Inc."

AUTHOR CONTRIBUTION STATEMENT
Trang Hong Thi Nguyen, Conceived and designed the analysis, collected the data, contributed data or analysis tools, performed the analysis, wrote the paper, and revised the paper. Haruna Yokoyama; Collected the data, contributed data or analysis tools, performed the analysis, reviewed and revised the paper. Hironori Kojima, Collected the data, contributed data or analysis tools, performed the analysis, reviewed and revised the paper. Naoki Isomura, Collected the data, contributed data or analysis tools, performed the analysis, reviewed and revised the paper. Akihiro Takemura, Collected the data, contributed data or analysis tools, performed the analysis, reviewed and revised the paper. Shinichi Ueda, Collected the data, contributed data or analysis tools, performed the analysis, reviewed and revised the paper. Kimiya Noto, Collected the data, contributed data or analysis tools, performed the analysis, reviewed and revised the paper.