Surface‐guided tomotherapy improves positioning and reduces treatment time: A retrospective analysis of 16 835 treatment fractions

Abstract Purpose In this study, we have quantified the setup deviation and time gain when using fast surface scanning for daily setup/positioning with weekly megavoltage computed tomography (MVCT) and compared it to daily MVCT. Methods A total of 16 835 treatment fractions were analyzed, treated, and positioned using our TomoTherapy HD (Accuray Inc., Madison, USA) installed with a Sentinel optical surface scanning system (C‐RAD Positioning AB, Uppsala, Sweden). Patients were positioned using in‐room lasers, surface scanning and MVCT for the first three fractions. For the remaining fractions, in‐room laser was used for setup followed by daily surface scanning with MVCT once weekly. The three‐dimensional (3D) setup correction for surface scanning was evaluated from the registration between MVCT and the planning CT. The setup correction vector for the in‐room lasers was assessed from the surface scanning and the MVCT to planning CT registration. The imaging time was evaluated as the time from imaging start to beam‐on. Results We analyzed 894 TomoTherapy treatment plans from 2012 to 2018. Of all the treatment fractions performed with surface scanning, 90 % of the residual errors were within 2.3 mm for CNS (N = 284), 2.9 mm for H&N (N = 254), 8.7 mm for thorax (N = 144) and 10.9 for abdomen (N = 134) patients. The difference in residual error between surface scanning and positioning with in‐room lasers was significant (P < 0.005) for all sites. The imaging time was assessed as total imaging time per treatment plan, modality, and treatment site and found that surface scanning significantly reduced patient on‐couch time compared to MVCT for all treatment sites (P < 0.005). Conclusions The results indicate that daily surface scanning with weekly MVCT can be used with the current target margins for H&N, CNS, and thorax, with reduced imaging time.


| INTRODUCTION
Accurate, reproducible, and fast setup of the patient is of great importance for a successful radiotherapy treatment, and in particular in helical tomotherapy due to the treatment complexity and number of degrees of freedom. The treatment margins are defined or calculated based on the uncertainties associated with the treatment, 1,2 and hence, affect the size of the treated volume. Helical tomotherapy 3 is an established treatment technique where the patient is treated on a slice by slice basis using a rotating linac, megavoltage (MV) photons and a continuous couch translation. The TomoTherapy can treat targets of up to 135 cm in length in one field. 4 Megavolt beam imaging is used for image guidance of the patient setup. 5 The treatment beam is combined with an on-board single row computed tomography (CT) detector array and the captured projection images are used to reconstruct a volumetric MVCT image of the patient. 6 Daily imaging using MVCT contributes to absorbed dose outside the treatment volume. 6 Also, MVCT is time consuming which decrease the patient throughput, and contributes to an increased risk of intra-fraction patient movement. 7 To reduce the amount of MVCT images while keeping an accurate patient setup several imaging strategies have been adopted, such as weekly MVCT imaging with daily patient setup using in-room lasers. 8 A recent strategy is to use surface guided radiotherapy (SGRT), where the patien'ts skin surface is scanned by an optical surface scanning (OS) system for patient setup. 9 The OS system compare the patient's surface at treatment setup to a reference surface and accurately calculates the patient position. 9 The advantage of using surface scanning is that the information from the surface can improve the patient setup compared to in-room lasers. [10][11][12] Also, a surface scan takes seconds, in comparison to minutes for MVCT. Thus, surface scanning has the potential to increase the accuracy, without substantially adding time for setup compared to setup with in-room lasers. The surface can be correlated to the MVCT images with a similar method as the in-room lasers. In this study, the Sentinel surface scanning system (C-Rad, Uppsala, Sweden) 9 was used to position the patients at a TomoTherapy HD (Accuray, Madison, US) linac between 2012 to 2018. Crop et al has previously reported improved patient setup for breast cancer patients using SGRT at tomotherapy 12 ; however, in this study an extensive number of targets in head and neck (H&N), intra-and extracranial (CNS), thorax and abdomen were included. The aim of this study was to retrospectively investigate the potential improvements of surface guided setup compared to in-room lasers, both verified by weekly MVCT. Also, the potential time gain using SGRT compared to daily and weekly MVCT was to our knowledge investigated for the first time.

2.A.1 | Surface scanning
The Sentinel OS system is a laser-based OS system that acquires a three-dimensional (3D) surface image of the patient over several seconds. The daily surface scanned is registered to a reference surface and the patient's position is calculated using rigid registration. 9 The scanner is mounted in the ceiling, at the end of the treatment couch. To avoid shadowing of the surface due to the closed bore of the TomoTherapy, the patient setup was carried out at the virtual isocenter, 700 mm longitudinal outside the bore. The Sentinel OS system has been found to be reproducible to < 1 mm and < 1°of rotation. 13 The Sentinel system and the TomoTherapy lack communication, and thus for safety any couch shifts that were carried out based on the OS system was followed by a second surface scan to verify that the shifts were carried out correctly.

2.A.2 | Megavoltage computed tomography
The standard imaging modality on the TomoTherapy is MVCT acquired using a built-in detector array with the treatment beam at 3.5 MV energy. The collimator is positioned in the longitudinal direction and was set to 4 mm width for imaging. Images were acquired slice-by-slice and using a pitch set to fine, normal, or coarse. The reconstruction interval was 2 or 4 mm optionally. Transversal slice spatial image resolution for MVCT imaging was ≤ 1.6 mm per pixel at 512 × 512 pixels. The scan length for MVCT imaging was chosen to include the PTV in the longitudinal direction. The MVCT image was reconstructed and compared to the reference CT using automatic registration with manual adjustment. The patient was repositioned if the automatic registration resulted in a rotation of more than 2°. If the patient was repositioned, a second scan was acquired.
The registration was further performed with only translational axis, the correction was applied, and the couch was moved from the control room. Prior to 2012, the couch was controlled solely from inside the treatment room, which increased the setup and imaging time. for MVCT scans performed once weekly, Fig. 1. The procedure has been derived from the work of Månsson, 14 which concluded that weekly imaging with laser setup and three initial imaging verification procedures were sufficient with the used imaging protocol. The threshold for deviation between MVCT and surface scanning was 2 mm in any direction. A deviation larger than 2 mm prompted MVCT the following treatment fraction, as did any large anatomical changes. The protocol at the time of the study was a NAL protocol 8 with action limits of 2 mm for H&N and CNS, for thorax and abdomen patients the action limit was 3 mm. CTV to PTV margins differ between sites and diagnosis, but was generally 5-7 mm for CNS and H&N and 7-10 mm for thorax and abdomen patients.

2.A.4 | Positioning data statistics
The MVCT was registered with the planning CT as reference. The resulting translational couch movement from the original position to the registered position was defined as the setup correction vector,

2.C | Data selection
The data were collected between January and April 2018. Data Only patients with more than three treatment fractions were included and patients positioned using in-room lasers, surface scanning, and MVCT performed on more than three fractions. Data was extracted from the Sentinel database and from the TomoTherapy archive using an in-house developed C# program. The resulting data were analyzed using Python (Version 3.6, Python software foundation, 2019).

| RESULTS
A total of 696 patients with 894 plans were analyzedin total 16 835 treatment fractions. Of the 894 plans, 78 plans were undefined treatment sites or treatment sites other than H&N, CNS, thorax, or abdomen and thus omitted from the analysis. followed by the longitudinal direction. For surface scanning, the largest error was mostly in the longitudinal direction followed by the lateral direction. The residual error was further separated into a systematic and a random error, 16 Table 2. The random and systematic error was substantially larger for in-room lasers than optical surface scanning, for all sites. The number of MVCT scans that prompted a rescan due to the difference between surface scanning and MVCT was over the action limit was for H&N.

3.B | Imaging time
The

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We found that with surface scanning only 1.7% of the setup deviations was larger than 5 mm for H&N and CNS, which was the target margin for patients immobilized with a thermoplastic mask.
To the authors knowledge, this is the only setup protocol to achieve this accuracy apart from daily imaging with MVCT. Other studies have investigated the residual errors for different treatment sites with daily in-room lasers, 18,21-24 using no action limit protocols (NAL) 8 and determined the residual deviation after daily inroom and NAL to be 2.6-14.2% for head and neck patients, depending on the number of fractions for evaluation and action limit. This would imply that a protocol with weekly MVCT imaging using daily surface scanning is as good or better than setup with in-room lasers and NAL protocol. Our positioning data was found similar to published data with an older laser scanning system. 19 To improve the positioning with surface scanning, a NAL protocol could be implemented based on the first three fractions, or by evaluating similar to methods on conventional linac. 8 Despite the high accuracy, there will be a few fractions that will be outside the treatment margins. Similar to population based margin recipes were the margins is deducted were 90% of the population receives 95%