Comparison of multi‐institutional Varian ProBeam pencil beam scanning proton beam commissioning data

Abstract Purpose Commissioning beam data for proton spot scanning beams are compared for the first two Varian ProBeam sites in the United States, at the Maryland Proton Treatment Center (MPTC) and Scripps Proton Therapy Center (SPTC). In addition, the extent to which beams can be matched between gantry rooms at MPTC is investigated. Method Beam data for the two sites were acquired with independent dosimetry systems and compared. Integrated depth dose curves (IDDs) were acquired with Bragg peak ion chambers in a 3D water tank for pencil beams at both sites. Spot profiles were acquired at different distances from the isocenter at a gantry angle of 0° as well as a function of gantry angles. Absolute dose calibration was compared between SPTC and the gantries at MPTC. Dosimetric verification of test plans, output as a function of gantry angle, monitor unit (MU) linearity, end effects, dose rate dependence, and plan reproducibility were compared for different gantries at MPTC. Results The IDDs for the two sites were similar, except in the plateau region, where the SPTC data were on average 4.5% higher for lower energies. This increase in the plateau region decreased as energy increased, with no marked difference for energies higher than 180 MeV. Range in water coincided for all energies within 0.5 mm. The sigmas of the spot profiles in air were within 10% agreement at isocenter. This difference increased as detector distance from the isocenter increased. Absolute doses for the gantries measured at both sites were within 1% agreement. Test plans, output as function of gantry angle, MU linearity, end effects, dose rate dependence, and plan reproducibility were all within tolerances given by TG142. Conclusion Beam data for the two sites and between different gantry rooms were well matched.


| INTRODUCTION
Proton pencil beam spot scanning is an emerging technology that is increasingly used in proton centers around the world. Spot scanning provides the ability to modulate the beam in energy as well as intensity as the dose is painted across the target. Spot scanning also negates the use of collimators and compensators, which are sources of neutron dose to the patient. Varian Medical Systems is among the latest vendors entering the proton market with their ProBeam™ spot scanning system. Two facilities in the United States currently use the ProBeam system, and several more are in different phases of construction and commissioning in the United States and Europe. 1 The first center using the Varian ProBeam system in the United States was the Scripps Proton Therapy Center (SPTC, San Diego, CA, USA). The Maryland Proton Treatment Center (MPTC) (Baltimore) was the second. With two centers and three gantries at each currently operational, we compared commissioning data to determine how well different sites can be matched. Commissioning of a spot scanning system with a synchrotron was described previously, 2 but not for a Varian ProBeam system with a cyclotron and not comparing commissioning data across sites. ProBeam systems are exclusively spot scanning systems, with no passive scattering components. Monitor unit (MU) linearity, end effects, dose rate dependence, and reproducibility for the system at MPTC are also discussed here. This study describes dosimetric commissioning tests used for the Varian ProBeam system at MPTC and SPTC and provides results that can be used as a reference for future ProBeam sites. Although the tests described here are not complete, they are similar to those used before [2][3][4] and should provide useful benchmarks.

| ME TH ODS AND MATERIALS
The treatment planning system (TPS) used by both sites is the Varian Eclipse v11, and the treatment machine software version is ProBeam v2.7. To commission the pencil beam proton convolution superposition dose model for the TPS the integrated depth dose curves (IDDs), absolute dose calibration, and spot profiles in air had to be measured, as outlined by the Eclipse reference manual. A complete description of the dose model is given in the Eclipse V11 Proton Algorithm Reference Guide. 5 These measurements were compared for the two facilities.
The TPSs were then verified by dosimetric validation of various test plans, similar to what is suggested in reports from the American Association of Physicists in Medicine and others. [6][7][8] Data from the first clinical gantry were used to commission the TPS beam model at MPTC.
All subsequent gantries at MPTC were compared to the initial data to determine dosimetric equivalence.

2.A | Beamline
The Varian ProBeam system exclusively uses spot scanning gantries that dynamically scan the beam from one spot to another (if distances between spots are less than a few millimeters). The system uses a superconducting isochronous cyclotron with an azimuthally varying field to accelerate hydrogen nuclei. This technology allows proton acceleration to 250 MeV with a maximum of 800-nA extracted current at the exit of the cyclotron. The energy is then modulated continuously (as opposed to a synchrotron system, where the energy is changed discretely) by a double-carbon wedge degrader system, which can reduce the energy continuously to 70 MeV.
Typically a current of 1-2 nA is used during patient treatment, but nozzle current can be as high as 10 nA. Beam losses in the energy selection system can range from 98% (250 MeV) to 99.75% (70 MeV). 9 The beam nozzle contains two steering magnets for the beam, a kapton window to seal the vacuum, an MU chamber, and a strip ionization chamber to verify the beam position. The center of the y-steering magnet is at 256 cm and the x-steering magnet at 200 cm. A source-to-isocenter distance of 228 cm is thus used in the TPS. The maximum field size is 30 (x) by 40 (y) cm at isocenter, where the y axis is aligned in the craniocaudal direction in a Head first supine (HFS) patient with the table at the nominal treatment position. A range shifter can be inserted into the snout, and the snout can move continuously from 3 to 42 cm from the isocenter.
The gantry can rotate 360°, and the couch can rotate from 265°to 95°, with 0°as the nominal position. For pitch and yaw 3°are allowed clinically. The planned energy layer switch time is <1 s, and the minimum time to deliver the minimum weighted spot per energy layer is~3 ms. The spot with the smallest number of MUs needed per layer will thus determine the dose rate. The smallest sigmas of the spots are~4 mm in air at isocenter.
The MPTC has four gantry rooms (TR1, TR2, TR3, and TR4) and one fixed-beam room. SPTC has three gantry rooms and two fixedbeam rooms. SPTC data used here for comparison reflect average data for all their gantries after commissioning.

2.B | Measuring Bragg peaks
The IDDs were acquired with a PTW Bragg peak chamber (PTW-Freiburg, Germany) in a 3D water tank at both sites. The Bluephan-tom2 (IBA Dosimetry, Schwarzenbruck, Germany) was used at MPTC and the PTW 3D water tank (PTW-Freiburg, Germany) at SPTC. The measuring field diameter of the Bragg peak chamber is 8.4 cm, and the active volume is 10.5 cm 3 . The window water equivalent thickness (WET) is 4.0 mm. IDDs were also acquired with a Stingray ion chamber (IBA Dosimetry). The measuring field diameter of this chamber is 12 cm and the window WET is 4.9 mm. The maximum energy that can be delivered in the room is 245 MeV, with a range of~38.5 cm. Bragg peak range measurements were compared to the theoretical calculation by applying the Bortfeld equation (R 80 = 0.00244*E 1.75 ), 10,11 where E represents the energy and | 97 61217 coordinate system, because the water tank was not deep enough to acquire the Bragg peaks from a gantry angle of 0°for the highest energies. Lower energy IDDs were verified with measurements from a gantry angle of 0°. Single pencil beam scans at isocenter were consequently acquired from the side of the water tank, where a 5-mm-thick 20 9 20 cm 2 window was inserted with a WET of 5.5 mm. These scans were acquired every 10 MeV from 70 to 245 MeV. In order to acquire the Bragg peaks in the surface region, the first 10 cm of each IDD was acquired at a gantry angle of 0°a nd then normalized and combined with deeper IDD data. A PTW 7862 chamber was used as reference chamber. The diameter of this chamber is 9.65 cm, with a physical window thickness of 0.2 mm.
The measured IDDs were corrected for the WETs of all the material between the reference chamber surface and the inside surface of the Bragg peak chamber.
Bragg peaks were also verified and compared using a Giraffe multilayer ion chamber (MLIC) device (IBA Dosimetry) that contains 180 air-vented parallel-plate ion chambers with diameters of 12 cm.
The chambers are spaced 2 mm apart. The Giraffe was also used to verify the WETs of the reference chamber, the range shifters, and the water tank window.

2.C | Absolute calibration
The absolute output of the unit was measured using the methodology recommended by the TRS 398 report of the International Atomic Energy Agency 12 for determination of absorbed dose from a proton beam. A PPC05 Markus parallel-plate chamber (IBA Dosimetry) was used in a 10 9 10 cm 2 field of mono-energetic spots spaced 2.5 mm apart, resulting in 1,681 spots per layer with 10 MU delivered per spot. Each energy was measured separately in intervals of 10 MeV (corresponding to the measured IDDs). The window for this chamber has a physical thickness of 1 mm and a WET of 1.8 mm. A point with 2-cm water equivalent depth was then used as the absolute measurement point for all the energies at MPTC and 1.5 cm at SPTC. All data were renormalized to 2 cm for comparison.
The water tank was moved to place the isocenter at the effective point of measurement of the chamber to eliminate the need for a source-to-axis-distance correction, and a gantry angle of 0°was used for these measurements. The corresponding IDDs at MPTC for each energy were then scaled according to this measurement at a 2-cm depth for import into the planning system in units of Gy.mm 2 /MeV. Relative biological effectiveness (RBE) was chosen as 1.1 and was incorporated in our planning system through a depth-dose normalization    The dose rate used was 60,000 MU/min, and the end effect was measured for 70, 160, and 240 MeV for a complete delivery of 3,000 MU and a delivery of 3,000 MU in three separate 1,000-MU deliveries. The end effect was calculated by assuming that ionization M in general is proportional to the sum of n times the set number of monitor units and n times the end effect (T E ), expressed as:

2.D | Spot profiles
where T is the total number of MUs, M 1 is the measurement with no interruptions, and M 3 the measurement with n = 3 interruptions.
The end effect was also calculated by fitting a linear regression through the data measured for the linearity.

2.I | Reproducibility and interrupted treatment
(tolerance: Less than larger of 0.5 cGy or 1% of delivered dose) The gantry was placed at 0°, and the Matrixx PT (IBA dosimetry) planar ion chamber array was placed at the isocenter with a 5-cm buildup (5.4-cm WET) added to place the measuring point at 6 cm.
Three plans were delivered with different ranges and spread-out Bragg peaks (SOBPs) to cover a wide range of energies. Each plan covered a 26 9 26-cm 2 surface to a dose of 500 cGy for varying SOBPs (i.e., R8S4, R15S7, and R22S7, where R represents the nominal range in centimeters and S the length of the SOBP in centimeters). Each plan was delivered three times, and doses at the central chamber were recorded and compared.
To test an interrupted treatment, arbitrary fields were repeatedly delivered on the Matrixx PT array. For the first delivery, the complete field was delivered uninterrupted. For the second field, treatment was interrupted at approximately halfway and then restarted.
Profiles for these measurements were compared and analyzed. Resulting calculations were then compared with measurements acquired with the PPC05 parallel-plate chamber in the 3D water T A B L E 2 Distal fall-off (R80-R20) measured for MPTC gantries TR1, TR3, and TR4 with the IBA Giraffe multilayer ion chamber array. Tolerance is <0.2 g/cm 2 above the physical limit from range straggling of a monoenergetic beam (1.4% of the proton range in water). Water tank data are shown in Fig. 2 TR1, compared to TR4, and 1.55% for TR3, respectively, corresponding to 0.9 and 0.8 mm differences in range in water.

2.J | Test plans
IDDs for the MPTC gantries were also compared with those from SPTC. The data were in good agreement (Fig. 1)  However, for higher energies the IDDs in the shoulder region for TR1 (larger chamber) are slightly higher than those for TR4 (smaller chamber), suggesting that the chamber is not large enough for these higher energies to capture all secondary protons in the halo. This agrees with data from Monte Carlo studies on the halo effect. [13][14][15][16] The increase in dose of the SPTC data was on average 4.5% higher for lower energies compared to that of MPTC. SPTC introduced Monte Carlo-modeled data into their data which take the halo effect more accurately into account. However, this difference occurs in the plateau region, which is only~25% of the maximum dose (i.e., the effect is more on the order of~1% if this region contributes dose to the target volume). We observed no marked differences in comparisons of the TPS plans with measured data inside the planning target volume. This increase in the plateau region decreased as energy increased, and there was no marked difference for energies >180 MeV.
Distal penumbra between R 80 and R 20 in the distal edge of the Bragg peak measured with the Giraffe MLIC are shown in Table 2 for each of the MPTC gantries. These values were also acquired with the water tank and the Bragg peak chamber and are shown in Fig. 2.
The tolerance used was: <0.2 g/cm 2 above the physical limit from range straggling of a monoenergetic beam (1.4% of the proton range in water). All measured values were smaller than the tolerances for and TR3 compared to TR4 were all <0.03 cm.
The sharpest distal fall-off occurred for the lower energies because of less range straggling. The effect of the carbon energy degrader is evident from the almost constant slope for energies >180 MeV. This was first described by Hsi et al. 17 and is only evident in systems with an energy degrader and energy selection slit. This is caused by the increased spread in the energy spectrum for lower energies caused by the degrader. If the full-energy spread generated in the degrader is transported to the treatment location, the width of the peak measured in water should be constant as a function of range. The introduction of the energy slit to reduce the spread in energy causes this continued decrease as the range decrease below 180 MeV. The width of the slit will thus determine where this transition from a constant to a decreasing slope will occur and is fixed for all Varian ProBeam systems. Above 180 MeV the full-energy spectrum is transported but not below that, causing range straggling to become dominant.

3.B | Absolute calibration
Measured absolute doses as functions of energy are shown in Fig. 3 for each gantry as well as for SPTC. The SPTC doses were normalized at 160 MeV to that of the TR4 to eliminate any discrepancies between MPTC calibration and that of SPTC. Calibration of the monitor ion chamber in the snout had to be adjusted for TR3 to achieve better agreement with TR4 values.
The decrease in the output as a function of energy and the eventual slight increase for energies >140 MeV can be attributed to the energy slit, which is used to keep energy dispersion low after the degrader, similar to earlier descriptions. 17 3.C | Spot profiles   4. The FWHM of the profiles is slightly larger for SPTC and TR1 than TR4 at higher energies and smaller at lower energies, but within the 15% tolerance compared to each other. Profiles with the 5-cm range shifter inserted also agree well. In Table 3  The optical solution also changes for energies >180 MeV, which explains the larger RMSE.
In gantries are compared to these values. In Fig. 6 the average values for each gantry over all the gantry angles are shown. In Fig. 7  to the y plane as the gantry rotates 90°. Differences between gantries are largest at higher energies. Because these profiles were measured in air, these spots will contribute to shallower dose. For higher energies at depth, however, the difference will have a reduced impact, because spot size there depends on elastic scattering and any difference will "smear out." Impacts on plan quality and equivalence of beam models were evaluated for different rs and were 3.C.2 | Spot symmetry (Tolerance: ≤5%) Table 4 shows symmetry between r x and r y for TR1, TR3, and TR4.

3.I | Test plan comparison
The distribution of percentage differences between the point doses for the test plans calculated with the TPS and those measured are shown in Fig for TR1 The distribution of measurements was skewed higher than the calculated TPS doses for TR4 for a majority of the plans. For TR1, measurements were more evenly distributed across an average of 0%.
F I G . 8. Distribution of percentage differences between test plans run on each gantry and the treatment planning system (TPS). Left panels show percentage differences between doses calculated by the TPS and absolute dose measurements. Right panels show cumulative distribution functions.

3.J | Range shifters
The largest differences between the range shifter WETs measured with the Giraffe and the theoretical values for R90 were within 0.88%. This corresponds to a 0.02-cm difference in the WET for the 2-cm range shifter. The WETs used for the range shifters in the TPS were 5.7, 3.42, and 2.28 cm. The WET for the kVue One Proton couch base was measured as 7 mm. The Hounsfield units of the couch base obtained from a CT image were changed to represent the measured WET and inserted into each patient's CT image.

| CONCLUSION
We found reasonable agreement between the TPS and measurements without using a double Gaussian function to model the spot profiles in the TPS as suggested by other authors. 1,[13][14][15][16] Although this was surprising, including a double Gaussian function in the TPS model did not markedly increase agreement between measured and calculated data from the TPS. The same beam model could be used to model all the MPTC gantries, and the machines were declared dosimetrically equivalent. We also showed agreement between spot profiles and IDDs measured at different sites for this system.

CONFLI CT OF INTEREST
Dr. Dong reports personal fees from Varian Medical Systems, outside the submitted work; Other authors have no conflicts of interest to report

R E F E R E N C E S
T A B L E 4 Symmetry: |(r x -r Y )|/(r x +r Y )*100 (%) for MPTC gantries TR1, TR3, and TR4 for each energy between the spot rs at a gantry angle of 0°at isocenter.