Clinical validation of ring‐mounted halcyon linac for lung SBRT: comparison to SBRT‐dedicated C‐arm linac treatments

Abstract Stereotactic body radiotherapy (SBRT) of lung tumors via the ring‐mounted Halcyon Linac, a fast kilovoltage cone beam CT‐guided treatment with coplanar geometry, a single energy 6MV flattening filter free (FFF) beam and volumetric modulated arc therapy (VMAT) is a fast, safe, and feasible treatment modality for selected lung cancer patients. Four‐dimensional (4D) CT‐based treatment plans were generated using advanced AcurosXB algorithm with heterogeneity corrections using an SBRT board and Halcyon couch insert. Halcyon VMAT‐SBRT plans with stacked and staggered multileaf collimators produced highly conformal radiosurgical dose distribution to the target, lower intermediate dose spillage, and similar dose to adjacent organs at risks (OARs) compared to SBRT‐dedicated highly conformal clinical noncoplanar Truebeam VMAT plans following the RTOG‐0813 requirements. Due to low monitor units per fraction and less multileaf collimator (MLC) modulation, the Halcyon VMAT plan can deliver lung SBRT fractions with an overall treatment time of less than 15 min (for 50 Gy in five fractions), significantly improving patient comfort and clinic workflow. Higher pass rates of quality assurance results demonstrate a more accurate treatment delivery on Halcyon. We have implemented Halcyon for lung SBRT treatment in our clinic. We suggest others use Halcyon for lung SBRT treatments using abdominal compression or 4D CT‐based treatment planning, thus expanding the access of curative ultra‐hypofractionated treatments to other centers with only a Halcyon Linac. Clinical follow‐up results for patients treated on Halcyon Linac with lung SBRT is ongoing.

the Radiation Therapy Oncology Group (RTOG) 0813 trial provides recommendations for the dosing, contouring, treatment planning, and delivery of SBRT. 11 This protocol is commonly used as a riskadapted prescription for tumors located adjacent to critical structures such as ribs. Traditional lung SBRT techniques are associated with long treatment times and patient inconvenience. [6][7][8][9][10] Currently, lung SBRT is delivered via volumetric modulated arc therapy (VMAT). [12][13][14] VMAT provides a more conformal dose distribution with faster dose fall-off outside the target, better sparing of organs at risk (OAR) and quicker treatment delivery. The dosimetric advantages of VMAT for lung SBRT are enhanced by using a flattening filter free (FFF) beam instead of a traditional flattened beam. 15,16 These include higher dose rates, reduction of out-of-field dose, less head scatter, and less electron contamination. 15,16 These advantages directly improve the patient's treatments by improving target dose coverage at the lung-tumor interface and shortening the treatment time. 13,14 Treatment time reduction potentially leads to improved patient comfort and decreased intrafraction target motion.
Varian recently introduced a single energy ring-mounted linear accelerator, the Halcyon Linac (V2.0), for image-guided radiation therapy. 17 The compact Halcyon Linac is equipped with 6MV FFF beam and is capable of rotating the gantry at a speed of four rotations per minute. 18,19 The mean energy and the nominal depth of (kV-CBCT) imaging system that includes a high-quality iterative CBCT reconstruction algorithm (iCBCT). 22,23 Daily patient setup times are significantly reduced on the Halcyon with a new one-step setup that will automatically apply couch shifts during a patient's setup. Thus, eliminating the need to manually apply isocenter shifts.
Some investigators have shown fast and effective treatment delivery is possible with Halcyon for conventionally fractionated breast, head and neck, and prostate treatments. [24][25][26] For hypofractionation schemes, Knutson et al. reported a retrospective dosimetric study of fractionated intracranial stereotactic radiation therapy (SRT) using the Halcyon. 27 In a 20-patient study (30 Gy in five fractions), they demonstrated acceptable plan quality for brain SRT using Halcyon coplanar geometry. Another study by Li et al. demonstrated the Halcyon V2.0 can generate plan quality comparable to a C-arm Linac for 6-10 brain tumors (diameter >1.0 cm), with a single-isocenter VMAT approach for intracranial radiosurgery. 28 While these retrospective planning studies demonstrated acceptable plan quality, they failed to include setup uncertainties associated with plan delivery and did not use their plans for patient treatment. Our study is the first to focus on the evaluation and clinical implementation of Halcyon Linac for lung SBRT. In this report, we have evaluated plan quality, treatment delivery efficiency, and accuracy of SBRT for lung tumors using Halcyon Linac by comparing with high-quality noncoplanar clinical VMAT plans on our SBRT-dedicated Truebeam Linac. This study provides the support for lung SBRT treatments with Halcyon. Lung SBRT has been clinically implemented in our clinic based on these findings.

2.A | Halcyon Linac for SBRT
After installation of a Halcyon V2.0, it was confirmed the machine met manufacturer specifications through both initial acceptance testing and commissioning. These high-quality lung SBRT plans are created with Eclipse using the advanced AcurosXB algorithm 29-32 as the final dose calculation as it has been shown it better predicts dose distribution by accounting for tissues heterogeneities (e.g., the SBRT board and Halcyon couch insert). Independent validation was performed using the MD Anderson's SBRT credentialing service by delivering a SBRT prescription dose of 6.6 Gy to the provided phantom. All dosimetric criteria established by IROC for SBRT treatments were satisfied. Currently, tumor motion is managed via abdominal compression and 4D CT-based target delineation or both as needed.

2.B | Patient characteristics
After Institutional Review Board approval, 15 consecutive early stage I-II NSCLC patients with centrally located tumors who underwent lung SBRT treatments using highly conformal noncoplanar clinical VMAT plans on a SBRT-dedicated Truebeam Linac for 50 Gy in five fractions were included in this retrospective study. Per institutional protocol, our physicians predominately treat lung SBRT patients using a noncoplanar geometry to ensure maximal target conformality and OAR sparing.

2.C | Imaging and target definition
Patients were immobilized using the Body Pro-Lok TM platform USA) and coregistered for tumor delineation. An internal target volume (ITV) was created using the 4D-MIP and the planning target volume (PTV) was generated by adding a 0.5 cm symmetric margin around the ITV per RTOG-0813 recommendation. 11 The relevant critical structures that were contoured included bilateral lungs excluding the PTV (normal lung), spinal cord, heart, trachea/bronchus, esophagus, ribs, and skin.

2.D | Clinical Truebeam VMAT plans
Our clinical highly conformal VMAT lung SBRT plans were generated in Eclipse TPS using 3-6 (mean, 4) partial noncoplanar arcs (with to medium reporting mode was applied. Planning objectives followed the RTOG-0813 requirements for prescription isodose surface coverage, target dose heterogeneity, high and low dose spillages, and dose limiting organ constraints. 11 These patients were treated on Truebeam Linac every other day per lung SBRT protocol.

2.E | Halcyon VMAT plans
For comparison, all clinical plans were reoptimized in Eclipse TPS using the same numbers of partial arcs, identical collimator rotations and identical arc geometry (including Truebeam VMAT arc length).
All Halcyon arcs were coplanar due to geometric limitations. Additionally, the Halcyon couch and SBRT board was inserted. Optimization objectives were identical to Truebeam VMAT plans. Identical dose calculation algorithm, dose reporting mode, CGS and PO MLC optimizer were used for Halcyon plans as described above. Halcyon VMAT plans were normalized identically to clinical Truebeam VMAT plans as described above, and the ITV hot spots were limited to those of the respective Truebeam VMAT plans.

2.F | Plan comparison and statistical analysis
The clinical Truebeam VMAT and Halcyon VMAT plans were compared via RTOG-0813 SBRT protocol for target dose conformity (CI), tumor dose heterogeneity (HI), gradient index (GI), and dose to OAR.
Additionally, delivery efficiency and accuracy were recorded. The DVHs of all treatment plans were evaluated following RTOG-0813 high and intermediate dose spillage parameters as follows 11 : 1. Conformity index, CI: ratio of prescription isodose volume to the PTV. CI less than 1.2 is desirable; CI = 1.2-1.5 is acceptable with minor deviations.
2. Gradient index, GI: ratio of 50% prescription isodose volume to the PTV. GI must be smaller than 3-6, depending on the PTV.

3.
Maximum dose at any point 2 cm away from the PTV margin in any direction, D 2cm : D 2cm must be smaller than 50-70%, depending on the PTV size.

4.
Percentage of normal lung receiving dose equal to 20 Gy or more, V20Gy: Per protocol, V20Gy should be less than 10%, V20Gy less than 15% is acceptable with minor deviations.

5.
Heterogeneity index, HI: HI = Dmax/prescribed dose was used to evaluate the dose heterogeneity within the PTV.
6. Gradient distance, GD: GD is the average distance from 100% prescribed dose to 50% prescribed dose, which indicates how sharply the dose falls off. The GD is used to evaluate dose sparing to normal lung volume. The smaller the GD, the faster the dose fall-off around the target.

3.B | Dose to OAR
The dosimetric differences (mean, standard deviation and range) between clinical Truebeam VMAT and Halcyon VMAT plans for the OAR (spinal cord, heart, esophagus, trachea/bronchus, skin, ribs and normal lung) are listed in Table 2

3.D.2 | Delivery efficiency and accuracy
Pretreatment PD QA pass rate was 95.3% with a 2%/2mm gamma passing criteria. The net treatment time (from first beam on until last beam off, including second and third beam preparation, but no couch kick time) was about 4.0 min as described above. Recorded mean couch time for this patient on Halcyon Linac was less than 10 min. This patient was initially positioned using external marks and in-room lasers, followed by the one-step patient setup and a 15 s pretreatment free-breathing kV-CBCT scan. An in-house SBRT/IGRT protocol was applied to coregister the pretreatment kV-CBCT with the planning CT scans (see Fig. 3). Image registra-

| DISCUSSION
We have evaluated the plan quality, treatment delivery efficiency, and accuracy of lung SBRT treatment using the Halcyon platform. All VMAT lung SBRT plans generated using coplanar Halcyon beam geometry had similar dosimetric plan quality compared to our stan-