Commissioning an in‐room mobile CT for adaptive proton therapy with a compact proton system

Abstract Purpose To describe the commissioning of AIRO mobile CT system (AIRO) for adaptive proton therapy on a compact double scattering proton therapy system. Methods A Gammex phantom was scanned with varying plug patterns, table heights, and mAs on a CT simulator (CT Sim) and on the AIRO. AIRO‐specific CT‐stopping power ratio (SPR) curves were created with a commonly used stoichiometric method using the Gammex phantom. A RANDO anthropomorphic thorax, pelvis, and head phantom, and a CIRS thorax and head phantom were scanned on the CT Sim and AIRO. Clinically realistic treatment plans and nonclinical plans were generated on the CT Sim images and subsequently copied onto the AIRO CT scans for dose recalculation and comparison for various AIRO SPR curves. Gamma analysis was used to evaluate dosimetric deviation between both plans. Results AIRO CT values skewed toward solid water when plugs were scanned surrounded by other plugs in phantom. Low‐density materials demonstrated largest differences. Dose calculated on AIRO CT scans with stoichiometric‐based SPR curves produced over‐ranged proton beams when large volumes of low‐density material were in the path of the beam. To create equivalent dose distributions on both data sets, the AIRO SPR curve's low‐density data points were iteratively adjusted to yield better proton beam range agreement based on isodose lines. Comparison of the stoichiometric‐based AIRO SPR curve and the “dose‐adjusted” SPR curve showed slight improvement on gamma analysis between the treatment plan and the AIRO plan for single‐field plans at the 1%, 1 mm level, but did not affect clinical plans indicating that HU number differences between the CT Sim and AIRO did not affect dose calculations for robust clinical beam arrangements. Conclusion Based on this study, we believe the AIRO can be used offline for adaptive proton therapy on a compact double scattering proton therapy system.

these reasons that it is customary to include a range uncertainty margin on the proton beam to ensure that the target is covered. In our practice, a margin of 3% of the range is added plus an additional 3 mm to ensure adequate target coverage including uncertainties in the stopping power of the protons. When comparing two different CT scanners for dose comparison, any changes in the CT values and changes in the calculated stopping powers can lead to changes in the dose along the proton path or to a change in the range of the proton therapy.
The purpose of this study was to characterize the AIRO relative stopping power curve in preparation of adaptive proton therapy on a Mevion S250 compact double scattering proton therapy system and to assess the dosimetric implications of adaptive planning with the AIRO imaging system.

2.A | CT number comparison between AIRO and CT Sim
Images of an electron density CT phantom (Gammex RMI 467; Gammex Inc., Middlenton, WI, USA) containing 16 rods of 13 tissue substitute materials were acquired on the CT Sim and on the AIRO (Table 1) with varying plug patterns, table heights, and mA with fixed 120 kV. Thirteen images with various rod placements and table positions were averaged to acquire CT numbers (mean AE standard deviation). For each of the AIRO CT scans, the mobile CT scanner was moved into the proton treatment room and the proton treatment couch was used as the imaging couch top. Analysis of all images was performed in MIM v.6.6.7 software (MIM Software Inc, Cleveland, OH, USA). For each plug, in each scan, the average and standard deviation of the CT numbers for a 1 cm diameter region-of-interest (ROI) was acquired. The CT numbers for each plug were compared between the two different scanners. The CT constancy for the AIRO was also tested over several months for the clinically selected protocols.
Images were acquired for the following individual plugs: brain, lung 300, lung 450, cortical bone, adipose, breast, liver, solid water, and true water. The plugs were individually scanned in the Gammex phantom with solid water plugs in all other holes and with no plugs in the other holes to simulate the effect of lung-like scatter conditions ( Fig. 1). This study was performed to evaluate changes in the mean and standard deviations of each plug when scanned alone or with other plugs in place. The mean and standard deviations of the CT numbers were compared between the AIRO and the CT Sim to determine if there were differences in the CT number depending on the scanner used.
The Gammex RMI 467 was scanned as described above with the AIRO in the proton treatment room, as would be used for localization and/or adaptive scanning. To acquire SPRs, averaged CT numbers were entered into a stoichiometric SPR calculation algorithm.
The resulting AIRO SPR vs. CT number curve for scans (CT calibration curve) with 120 kV tube-voltage was entered into our Pinnacle treatment planning software v. 16.0 (Philips Medical Systems, Fitchburg, WI, USA).

2.B | Treatment planning and dosimetric analysis
The last step of commissioning was to confirm the dosimetric equivalence for dose calculated on CT scans from the CT Sim and the AIRO. Treatment planning was done in Pinnacle using a double scatter beam module that had been previously commissioned for treatment planning. All treatment planning was done using the scans from the CT Sim with the clinically used CT Sim SPR curve. CT scans of a RANDO anthropomorphic thorax, pelvis, and head phantom as well as CIRS thorax and head phantoms were used for planning.
Heterogeneous, single-field, nonrobust plans were developed on each phantom in order to test the accuracy of dose for proton beams traversing large areas of heterogeneous media. Additionally, clinically realistic plans were generated to test the accuracy of the adaptive system for use in common clinical scenarios and to evaluate clinical metrics such as dose-volume histogram (DVH) changes between the two CT scans. Phantoms were used to ensure that dosimetric changes were only due to changes in the CT scan and not due to clinical changes in the images. Each phantom was scanned twice with AIRO in the proton treatment room.
Using the dynamic workflow module in the Pinnacle treatment planning system, the proton beams were locked from changes to the original, treatment planning CT scan and the beams were copied to the AIRO CT scans following rigid registration. Dose was calculated on the AIRO CT scans with whichever SPR curve was to be tested. To visually inspect the dosimetric comparison, the isodose lines from the treatment planning scan were converted to contours so they could be displayed on the AIRO CT scan. Dose was calculated on the AIRO CT scan using the following SPR curves: (a) CT Sim SPR curve, (b) AIRO-specific stoichiometric SPR curve (acquired in 2.A), and (c) dose-adjusted SPR curve (adjusted after visual inspection of isodose lines and CT numbers).  This change in CT numbers depending on plug configuration was most notable for the low-density plugs and also for the high-density plug. Most of the soft tissue plugs did not change with configuration. Table 2 is a summary of the mean and standard deviation of the CT numbers for each plug with different configurations.
The final CT-SPR curves for the CT Sim and the AIRO are plotted in Fig. 3. These curves appear similar although there are visible differences in the low CT number region (less than 800) and high CT number region (above 1400).

3.B.2 | Water-equivalent thickness comparisons
Water-equivalent thickness (WET) (gm/cm 2 ) values along the PA lung beam on the RANDO thorax phantom demonstrated results shown in Table 3 for the CT Sim, Stoichiometric AIRO, and doseadjusted SPR curves. WET for the beam on the pelvis RANDO phantom demonstrated results shown in Table 4 for the CT Sim, Stoichiometric AIRO, and dose-adjusted SPR curves. Average percent F I G . 2. Low-and high-density plug CT number comparison between AIRO and clinical CT Sim scanners. (a) Discrepancies for LN300 and LN450, (b) discrepancies for adipose, breast, solid water, brain, and liver, and (c) discrepancies for cortical bone. Solid water surround is shown in Fig. 1(a). Air surround is shown in Fig. 1(b).

3.B.4 | Clinical plans
The clinical treatment plans are shown in Fig. 5  commissioned CT Sim. Therefore, in principle, the system can be used for adaptive proton treatment planning. Prior to this study, we developed many preliminary tests to determine which factors most affected the AIRO's CT numbers. 3 We tested various plug configurations, mA, table positions, reconstruction kernels, FOV, and phantoms. We found that reconstruction artifacts were minimal and that CT numbers were mostly affected by the surrounding materials and plug patterns. Thus, we decided to use the plug pattern suggested by Gammex when building the AIRO's SPR curve and we tested the AIRO CT numbers with various surrounding materials.
We found that AIRO HU values of both small lung plug volumes changed up to 6.3% (LN 450) when surrounded by solid water-which was not reproduced with our CT Sim (up to 1.7% difference). The reason for this phenomenon with AIRO scans is still unclear but could be related to the reconstruction of the images and beam hardening. This could also be related to the fact that the AIRO has bigger bore compared to a conventional CT simulator which introduces more scatter issues. The magnitude of these differences in air-like media is quantified. 3 Therefore, we believe that dosimetric equivalency testing using

CONCLUSI ONS
We present a methodology for developing a stopping power calibration curve on a new in-room AIRO mobile CT scanner for the purpose of adaptive proton therapy. Our methodology is based on reiterative dose-based mapping of SPR values between the clinically commissioned stationary CT scanner and AIRO for a variety of phantom, This approach yielded better overall dosimetric equivalency to the conventional stoichiometric method based on dose calculations in various heterogeneous phantoms. We show that the AIRO CT system can be a viable alternative to conventional CT Sim for the purpose of adaptive planning in proton therapy.

CONF LICT OF I NTEREST
The authors declare no conflict of interest.