Prostate SBRT using O ‐ Ring Halcyon Linac — Plan quality, delivery ef ﬁ ciency, and accuracy

Cone beam CT ‐ guided prostate stereotactic body radiotherapy (SBRT) treatment on the recently installed novel O ‐ ring coplanar geometry Halcyon Linac with a single energy 6MV ‐ ﬂ attening ﬁ lter free (FFF) beam and volumetric modulated arc therapy (VMAT) is a fast, safe, and feasible treatment modality for early stage low ‐ and intermediate ‐ risk prostate cancer patients. Following the RTOG ‐ 0938 compliance criteria and utilizing two ‐ full arc geometry, VMAT prostate SBRT plans were generated for ten consecutive patients using advanced Acuros ‐ based algorithm for heterogeneity corrections with Halcyon couch insert. Halcyon VMAT plans with the stacked and staggered multileaf collimators (MLC) produced highly conformal SBRT dose distributions to the prostate, lower intermediate dose spillage and similar dose to adjacent organs ‐ at ‐ risks (OARs) compared to SBRT ‐ dedicated Truebeam VMAT plans. Due to lower monitor units per fraction and less MLC modulation through the target, the Halcyon VMAT plan can deliver prostate SBRT fractions in and overall treatment time of less than 10 minutes (for 36.25 Gy in ﬁ ve fractions), signi ﬁ cantly improving patient compliance and clinic work ﬂ ow. Pretreatment quality assurance results were similar to Truebeam VMAT plans. We have implemented Halcyon Linac for prostate SBRT treatment in our institution. We recommend that others use Halcyon for prostate SBRT treatments to expand the access of curative hypofractionated treatments to other clinics only equipped with a Halcyon Linac. Clinical follow ‐ up results for patients who underwent prostate SBRT treatment on our Halcyon Linac is underway. detector has an active area of 400 mm × 400 mm with a high ‐ resolution pixel size of 0.34 mm at a 150 cm nominal source to imager distance. The mean and standard deviation for each dose metric was compared using a two ‐ tailed paired student ’ s t ‐ test (p value of < .05 statistically signi ﬁ cant) for all dosimetric parameters, target coverage, OAR doses, and treatment delivery parameters.

modulation radiation therapy (IMRT). 11 The Radiation Therapy Oncology Group (RTOG) 0938 report provides the most recent recommendations for prostate SBRT dosing scheme, contouring, treatment planning, and delivery criteria 12 in addition to the standard SBRT guidelines. 13 Due to the relatively longer traditional treatment time and patient discomfort, [1][2][3][4][5][6][7][8][9][10][11] prostate SBRT has recently been delivered using a SBRT-dedicated C-arm Linac via volumetric modulated arc therapy (VMAT). 14-16 VMAT provides a highly conformal dose distribution to the prostate while maintaining fast dose fall-off outside the target, better sparing of organs-at-risk (OAR) and much faster treatment delivery. The dosimetric advantages of VMAT prostate SBRT can be enhanced by utilizing flattening filter free (FFF) beam because of its higher dose rates, reduction in out-of-field dose and decreased head scatter and electron contamination with respect to traditional flattened beams. 17,18 Linac-based five fractions prostate SBRT treatment have been studied by a few researchers. [19][20][21] For instance, in a recent phase II study by Alongi et al, 20 25,26 Halcyon provides an improved penumbra with a smaller dosimetric leaf gap (DLG) of 0.1 mm. In addition to MVcone beam CT imaging, the Halcyon Linac is equipped with a fast 15-second kilovoltage cone beam CT (kV-CBCT) imaging system that includes a high-quality iterative CBCT reconstruction algorithm (iCBCT). 27,28 This Linac is designed for a "one-step" patient setup that automatically applies couch shifts after an image-guidance procedure. 22 This eliminates the need for therapists to manually apply isocenter shifts in the room and will decrease overall treatment time.
We have recently installed a Halcyon V2.0 Linac in our institution and initial acceptance testing and commissioning data confirmed that the machine met manufacturer specifications as described above. [22][23][24][25][26][27][28] We originally installed the Halcyon Linac for conventionally fractionated treatments but decided to commission it for extracranial SBRT treatments because of the superior image quality and performance capabilities aforementioned. To improve dose calculation accuracy, we have used the advanced AcurosXB algorithm [29][30][31] to better account for heterogeneities including SBRT board and Halcyon couch insert. For mobile tumors, we account for tumor motion using both abdominal compression and 4D-CT-based target delineation. We performed an end-to-end test and independent validation test by delivering a SBRT prescription dose of 6.6 Gy to the MD Anderson's VMAT/IMRT credentialing phantom in treatment mode. All dosimetric criteria established by the IROC for SBRT treatments were satisfied.
A few early researchers have shown a fast and effective treatment delivery is possible using Halcyon Linac for conventionally fractionated breast, head, and neck, and prostate treatments with no reduction in plan quality when compared to other treatment modalities. [32][33][34][35] In a hypofractionated study, Knutson et al. reported a retrospective dosimetric analysis of intracranial stereotactic radiation therapy (SRT) treatments using the Halcyon Linac. 36 In a 20patient study with a fractionation scheme of 30 Gy in five fractions, they demonstrated that acceptable plan quality for brain SRT is achievable using Halcyon coplanar geometry. Another recent study by Li et al. demonstrated that the Halcyon V2.0 can generate plan quality comparable to a SBRT-dedicated C-arm Linac for 6-10 brain tumors (diameter >1.0 cm), using a single-isocenter approach for intracranial radiosurgery. 37 While these retrospective planning studies demonstrated acceptable plan quality, they did not use their plans for the patient treatment. In this report, we evaluate consecutive ten low-and intermediate-risk prostate cancer patient's SBRT treatment plans who were recently treated on our Halcyon Linac.
These plans were evaluated for plan quality, treatment delivery efficiency, and accuracy as part of commissioning and clinical implementation of our prostate SBRT program on this novel Linac.
Moreover, this report provides the benchmark study for other clin-   18 years old with a prostate cancer clinically staged T1c-T2a. All patients were grade group 1-3 prostate adenocarcinoma and had a PSA ≤ 10 ng/Ml. Exclusion criterion included androgen deprivation therapy, prior prostate surgery, prior pelvic radiation therapy, crohn's diseases, ulcerative colitis and international prostate symptom score (IPSS) ≥15. All patients had no evidence of enlarged lymph nodes seen on CT/MRI images and bone scans revealed no evidence of distance bony metastases.

2.2B | Imaging and target delineation
All patients were immobilized using the Body Pro-Lok TM platform  SBRT board were included in the final dose calculation. Dose to medium reporting mode was enabled and the planning objectives followed the RTOG-0938 requirements for prescription isodose surface coverage, target dose homogeneity, high and low dose spillages and dose to limiting OAR. 12 These patients were treated every other day using a Halcyon kV-CBCT imaging protocol.

2.4D | Truebeam VMAT plans
For comparison, all patients' plans were retrospectively reoptimized in the Eclipse TPS using the same numbers of full arcs, identical collimator rotations, and arc geometry on our SBRT-dedicated C-

2.5E | Plan comparison
The clinical Halcyon VMAT and Truebeam VMAT plans were com-

| DISCUSSION
In this report, we have presented the plan quality, treatment delivery efficacy and accuracy of prostate SBRT treatments using the O-ring Halcyon Linac. We found that all VMAT prostate SBRT plans generated using Halcyon Linac had similar or better dosimetric plan quality compared to SBRT-dedicated Truebeam VMAT plans. This included: target conformity, tumor dose homogeneity, and intermediate dose fall-off around the prostate. Additionally, all highly conformal clinical Halcyon VMAT plans met RTOG-0938 requirements and achieved similar target coverage (see Table 1) compared to Truebeam VMAT plans. All Halcyon VMAT plans provided similar OAR (rectum, bladder, penial bulb, femoral heads, skin, and urethra) sparing and met SBRT protocol requirements (see Table 2

CONFLI CT OF INTEREST
None.