Early clinical experience with varian halcyon V2 linear accelerator: Dual‐isocenter IMRT planning and delivery with portal dosimetry for gynecological cancer treatments

Abstract Purpose Varian Halcyon linear accelerator version 2 (The Halcyon 2.0) was recently released with new upgraded features. The aim of this study was to report our clinical experience with Halcyon 2.0 for a dual‐isocenter intensity‐modulated radiation therapy (IMRT) planning and delivery for gynecological cancer patients and examine the feasibility of in vivo portal dosimetry. Methods Twelve gynecological cancer patients were treated with extended‐field IMRT technique using two isocenters on Halcyon 2.0 to treat pelvis and pelvic/or para‐aortic nodes region. The prescription dose was 45 Gy in 25 fractions (fxs) with simultaneous integrated boost (SIB) dose of 55 or 57.5 Gy in 25 fxs to involved nodes. All treatment plans, pretreatment patient‐specific QA and treatment delivery records including daily in vivo portal dosimetry were retrospectively reviewed. For in vivo daily portal dosimetry analysis, each fraction was compared to the reference baseline (1st fraction) using gamma analysis criteria of 4 %/4 mm with 90% of total pixels in the portal image planar dose. Results All 12 extended‐field IMRT plans met the planning criteria and delivered as planned (a total of 300 fractions). Conformity Index (CI) for the primary target was achieved with the range of 0.99–1.14. For organs at risks, most were well within the dose volume criteria. Treatment delivery time was from 5.0 to 6.5 min. Interfractional in vivo dose variation exceeded gamma analysis threshold for 8 fractions out of total 300 (2.7%). These eight fractions were found to have a relatively large difference in small bowel filling and SSD change at the isocenter compared to the baseline. Conclusion Halcyon 2.0 is effective to create complex extended‐field IMRT plans using two isocenters with efficient delivery. Also Halcyon in vivo dosimetry is feasible for daily treatment monitoring for organ motion, internal or external anatomy, and body weight which could further lead to adaptive radiation therapy.

Halcyon linear accelerator is mainly designed for intensity-modulated radiation therapy/volumetric modulated arc therapy (IMRT/ VMAT) delivery due to its unique features such as fast delivery via 4 RPM with a dose rate of 800 MU/s, FFF only beam, MLC characteristics, and automated daily IGRT workflow. The accuracy of IMRT/ VMAT planning and delivery depends on the quality of treatment planning system commissioning based on the beam data acquisition and modeling. [3][4][5][6][7] The Halcyon linac is preconfigured with a reference beam model built in the Eclipse treatment planning system which the users cannot modify. Thus, beam model parameters related to small fields and MLC dosimetry which typically are challenging for IMRT/VMAT commissioning can benefit in achieving good agreements between planning and delivery. Several studies have shown good agreements between measurements and calculated or reference values on Halcyon 1.0. 8,9 Halcyon 2.0 is capable of treating >28 cm treatment length using a dual isocenter (allowing maximum treatment length of 36 cm) which is limited to maximum field size of 28 × 28 cm 2 on Halcyon 1.0. To accomplish this, large fields are split into a few smaller fields to treat larger than the maximum field size (i.e., 28 × 28 cm 2 ) using two isocenters. For IMRT fields, the auto feathering technique has been introduced for splitting large fields. It involves splitting the beam into components with an overlap between them and with variable intensity in the overlap region. During an IMRT optimization, the dose objectives for dose gradients in the junction region are established, and dose is "feathered," where a dose gradient is generated in abutting fields to obtain a uniform dose in the target so that the junction target dose can be controlled uniformly without hot or/and cold spots. [10][11][12][13] Eclipse planning system 15.6 utilizes autofeathering technique for dual-isocenter IMRT planning with Halcyon 2.0.
Device-based pretreatment patient-specific quality assurance (QA) measurements have been widely practiced and is an accepted standard of care. 14 Portal dosimetry using EPID has been also used for pretreatment patient-specific QA. [15][16][17] EPID-based portal dosimetry checks need a separate portal dose image prediction algorithm to calculate portal doses using the fluence map for the field that requires additional calibration and commissioning. EPID-based portal image dosimetry is also utilized for performing in vivo dosimetry using separately available commercial systems or in-house developed software. 18 Portal image dosimetry in Halcyon is one of the many unique features that is integrated with EPID and Eclipse planning system using AAA (Analytical Anisotropic Algorithm) dose calculation algorithm. EPID-based portal dosimetry with Halcyon makes portal image dosimetry convenient, effective, and more efficient because it is performed by default unlike other conventional linac-based EPID portal dosimetry. Therefore in vivo EPID-based portal dosimetry on Halcyon can be used to assess daily treatment delivery efficiently.
Extended-field IMRT is commonly used for gynecological cancer patients with pelvic and/or para-aortic lymph node involvement. Traditionally, extended-field radiotherapy has been delivered with anterior-posterior opposed fields including the para-aortic lymph nodes.
With this technique, generous portions of the small bowel have been included in the treatment field, causing significantly increased toxicities. The use of IMRT has shown better pelvic and para-aortic region dose conformity while sparing critical organs like small bowel, kidneys, marrow, rectum, and bladder, resulting in decreased acute and late gastrointestinal morbidities. [19][20][21] Extended-field IMRT needs a dual isocenter with Halcyon due to its field size limit. As serving a high-volume center for gynecological cancer treatments, we report our clinical experience with Halcyon 2.0 focusing on a dual-isocenter IMRT planning and delivery for gynecological cancer patients and examine the feasibility of in vivo portal dosimetry. To the best of our knowledge, there are no reports available regarding Halcyon 2.0 clinical use experiences with the use of dual-isocenter IMRT treatments.

2.A | Patient characteristics and prescription
Three cervical and nine endometrium cancer patients were planned and treated with extended-field IMRT. Treatment was planned for pelvis and pelvic/or para-aortic node regions and the prescription dose was 45 Gy in 25 fractions (fxs) with a simultaneous integrated boost (SIB) dose of 55 or 57.5 Gy in 25 fxs to the involved nodes (Table 2). Planning CT scan was done with full bladder and empty rectum, and the same filling status was instructed for daily treatment as well. Only multibeam (fixed gantry) static IMRT technique was used for the present study cohort. For each patient, nine gantry angles per isocenter were set in equal distance with 40°apart, 180°, 140°, 100°, 60°, 20°, 340°, 300°, 260°, 220°, with a total of 18 beams for 2 isocenters ( 2.E | Review of daily treatment exit dose using in vivo portal image dosimetry

2.B | Planning process
For each fraction, each treatment field portal image was reviewed, and all fields were combined to make a composite field. These were compared to the reference baseline (1st fraction) by the image planar dose using gamma analysis with the criteria of 4%/4 mm and 90% of total pixels for the composite field.

| RESULTS
Extended-field IMRT plans for all 12 patients met the planning criteria and were approved by the treating physician. A total of 300 fractions were delivered as planned. Homogeneity index (HI) 22 for PTV1 was between 0.05 and 0.33 (HI = 0 as the perfect uniformity of dose in the target indicated by the squareness of the DVH defined by ICRU 83). The summary of plan quality for target doses and OARs is also presented in Tables 4   and 5. For OARs, most were well within the dose volume criteria.

3.A | Evaluation of treatment plans
The largest deviation was the maximum point dose for rectum for one patient and it was 20% higher than the maximum planning goal due to its relatively large overlap with PTV1 (Table 4).

3.B | Pretreatment patient-specific QA results
Pretreatment QA results are given in Table 6. Gamma dose evaluation analysis for both MatriXX ion chamber array device and portal dosimetry yielded a mean value of 99.0 % (ranges: 97.3%-99.9 %) and 100% passing rates, respectively. An automatic shift between the two isocenters was verified using the MatriXX device simulating the planned shift.

3.C | Treatment delivery and in vivo portal dosimetry results
Prior to each treatment fraction, 3D CBCT for setup verification was acquired. We used iterative CBCT (iCBCT) pelvis mode for all patients for each fraction and it took 36.7 s for a full 360°rotation ( All 12 patients were treated with 2 isocenters using 18 treatment fields with 9 gantry angles for each isocenter (multibeam or fixed gantry IMRT technique). Treatment delivery time was between 5 and 6.5 min (depending on total MUs; total MU ranges were between 1995 and 3808) with a constant dose rate of 800 MU/min (Table 6). After the first isocenter was treated, treatment couch was shifted automatically to the second isocenter for beam delivery.
Eight out of total 300 fractions (2.7%) had a gamma passing < 90% of total pixels with 4 %/4 mm criteria ( Table 6, Fig. 1). As can be seen in the Fig. 1   AAPM TG 218 recommendations for IMRT QA methodologies and tolerance limits have been published recently. 14 According to its recommendations, the true composite (TC) method, which consists of delivering all beams to a measurement device using the actual treatment beam geometry for the patient most closely simulates the treatment delivery to the patient. In addition, field-by-field (FF) analysis is useful to evaluate some subtle delivery errors for each treatment field which is highly modulated. EPID-based portal dosimetry for patient-specific QA is very convenient and efficient to measure the TC method and evaluate FF analyses compared to other devicebased measurement methods.
In vivo dosimetry using the integrated EPID showed a good consistency in daily treatment delivery overall. The interfractional in vivo exit dose change during the treatment course based on gamma analyses showed that 97.3% of total fractions (292 of 300 fractions) passed using the criteria of 4%/4 mm with 90% of total pixels in the dose plane (Table 6 and Fig. 1). The present study demonstrated that daily in vivo portal dosimetry based on exit dose is feasible with Halcyon 2.0. Halcyon EPID-based in vivo dosimetry has the potential to monitor patients during complex treatments and possible (or significant) changes of organ motion, internal or external anatomy and body weight which could further lead to adaptive radiation therapy.
In vivo interfractional dose variation based on gamma analyses was found to be relatively higher for two patients only (Fig. 1). This suggests that these changes were not caused by systematic treatment delivery errors or dosimetric failures because the variation was found as a random pattern. Patient no. 8 had <90 % of total pixels T A B L E 6 Results of pretreatment quality assurance (QA) and in vivo dosimetry.   Upper left side image represents fraction 10 and right side fraction 1 (baseline). The center image was the result of gamma evaluation and 100% total pixels were passed (representing "area gamma < 1.0") using gamma analysis of 4%/4 mm with 90% of total pixels criteria.

Patient
The lower left image shows a planar dose profile comparison between the two fractions along the planes with x axis (left-right direction) and y axis (superior-inferior direction) area at the isocenters.
The recent study by Nailon et al. 18  Halcyon VMAT while maintaining comparable plan quality obtained with C-arm linac-based treatment. [23][24][25] There are a few limitations in the present study. First, the in vivo portal dosimetry on the first fraction treatment was set as the reference for comparison with the consecutive fractions because it was not possible to calculate the predicted portal image dose from the planning system. If predicted portal dose could be calculated as a reference, the tolerance level criteria may be validated as in pretreatment patient-specific QA. Therefore, calculated predicted portal dose would improve the quantitative evaluation of daily treatment verification with EPID on Halcyon. However, the present study was to assess interfractional in vivo dose variation and our approach using the first fraction as the baseline worked well for checking a consistency in the delivery during the treatment course. Second, the cohort of patients for the present study is relatively small, only 12 patients. Further investigation will be needed with a larger cohort for validation. In addition, we did not investigate the intrafractional motion during the treatment. However, several studies demonstrated that intrafractional motion did not cause substantial dosimetric change when treatment time was <10 min for pelvis treatments. [26][27][28] We believe that intrafractional motion remains minimal for our study cohort due to relatively short treatment time. Next, we did not quantitate the correlation in detail between the in vivo portal dose and the anatomy change.
The correlation between in vivo portal dose and the anatomy change could be quantitated; however, anatomy change can not only affect the daily in vivo portal dose but also setup uncertainty and other external factors may contribute to the interfractional dose variation. This subject needs a further research.
Lastly, the present study focused only on pelvic IMRT cases reflecting the high volume of gynecological cancer treatments in our clinic. Other disease sites may cause different results. However, based on the current study, two-isocenter technique with Halcyon 2.0 was found to work well dosimetrically in this complex disease configuration and it is likely to work well for other disease sites.

| CONCLUSION
Early clinical experience with Halcyon 2.0 linear accelerator was described in the present study, focusing on assessment of dual isocenter IMRT delivery for gynecological cancer patients. Halcyon 2.0 is not only effective to create complex extended-field IMRT plans using dual isocenters but also performs an efficient and fast delivery. In vivo dosimetry integrated with EPID in Halcyon was evaluated and demonstrated feasibility for daily treatment monitoring. Some interfractional changes were detected in random pattern depending on internal organ motion (small bowel in this study) and SSD change. Halcyon EPIDbased in vivo dosimetry has the potential to function as an additional daily monitoring system for complex IMRT and adaptive radiotherapy.
If predicted portal dose for treatment exit dose could be calculated, daily treatment in vivo verification will be accurately quantitated.

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