A simple, yet novel hybrid‐dynamic conformal arc therapy planning via flattening filter‐free beam for lung stereotactic body radiotherapy

Abstract Purpose Due to multiple beamlets in the delivery of highly modulated volumetric arc therapy (VMAT) plans, dose delivery uncertainties associated with small‐field dosimetry and interplay effects can be concerns in the treatment of mobile lung lesions using a single‐dose of stereotactic body radiotherapy (SBRT). Herein, we describe and compare a simple, yet clinically useful, hybrid 3D‐dynamic conformal arc (h‐DCA) planning technique using flattening filter‐free (FFF) beams to minimize these effects. Materials and Methods Fifteen consecutive solitary early‐stage I‐II non‐small‐cell lung cancer (NSCLC) patients who underwent a single‐dose of 30 Gy using 3–6 non‐coplanar VMAT arcs with 6X‐FFF beams in our clinic. These patients’ plans were re‐planned using a non‐coplanar hybrid technique with 2–3 differentially‐weighted partial dynamic conformal arcs (DCA) plus 4–6 static beams. About 60–70% of the total beam weight was given to the DCA and the rest was distributed among the static beams to maximize the tumor coverage and spare the organs‐at‐risk (OAR). The clinical VMAT and h‐DCA plans were compared via RTOG‐0915 protocol for conformity and dose to OAR. Additionally, delivery efficiency, accuracy, and overall h‐DCA planning time were recorded. Results All plans met RTOG‐0915 requirements. Comparison with clinical VMAT plans h‐DAC gave better target coverage with a higher dose to the tumor and exhibited statistically insignificance differences in gradient index, D2cm, gradient distance and OAR doses with the exception of maximal dose to skin (P = 0.015). For h‐DCA plans, higher values of tumor heterogeneity and tumor maximum, minimum and mean doses were observed and were 10%, 2.8, 1.0, and 2.0 Gy, on average, respectively, compared to the clinical VMAT plans. Average beam on time was reduced by a factor of 1.51. Overall treatment planning time for h‐DCA was about an hour. Conclusion Due to no beam modulation through the target, h‐DCA plans avoid small‐field dosimetry and MLC interplay effects and resulting in enhanced target coverage by improving tumor dose (characteristic of FFF‐beam). The h‐DCA simplifies treatment planning and beam on time significantly compared to clinical VMAT plans. Additionally, h‐DCA allows for the real time target verification and eliminates patient‐specific VMAT quality assurance; potentially offering cost‐effective, same or next day SBRT treatments. Moreover, this technique can be easily adopted to other disease sites and small clinics with less extensive physics or machine support.

next day SBRT treatments. Moreover, this technique can be easily adopted to other disease sites and small clinics with less extensive physics or machine support.

K E Y W O R D S
FFF-beam, hybrid-DCA, lung SBRT, VMAT

| INTRODUCTION
With the development of more precise and accurate treatment delivery, stereotactic body radiation therapy (SBRT) treatment of medically inoperable early-stage non-small-cell lung cancer (NSCLC) patients shows higher tumor local-control rate and minimal treatment-related toxicity. [1][2][3][4][5] For the selected peripherally located NSCLC patients; single-dose of SBRT has become a curative treatment option as shown by the randomized trials. [6][7][8][9][10][11][12][13] For instance, Videtic and colleagues 7 compared 2 single-fraction SBRT dosing schemes of 30 and 34 Gy for 80 medically inoperable early stage-I NSCLC patients. Both treatment schedules provided similar tumor local-control and overall survival rates with minimal pulmonary toxicity. Thus, a stereotactic, single-dose of 30 Gy is an equally effective treatment for the selected NSCLC patients and it was radially adopted for patients treatment in our clinic. Recently, there has been growing interest in the clinical use of flattening filter free (FFF) beams to deliver lung SBRT treatment. 14-18 FFF-beams have much higher dose rates compared to traditional flattened-beams that use flattening filters (FF). FFF beams can reduce beam on time (specifically beneficial for single large dose treatment), resulting in better patient comfort and reducing dose delivery uncertainty due to less intrafraction motion error and can potentially reduce out-of-field dose with less head scatter and electron contamination. 16 Combining FFF-beams with volumetric modulated arc therapy (VMAT) 18,19 resulted in greater treatment efficiency for complex lung SBRT plans compared to historically used plans with 8-15 noncoplanar fixed fields or several coplanar dynamic conformal arcs (DCA) with flattened-beams. [19][20][21] The same results were observed when compared to linac-based intensity-modulated radiation therapy (IMRT), VMAT plans, helical TomoTherapy or optimized robotic CyberKnife plans (showing significant increases in SBRT treatment times). [22][23][24][25] However, for single-dose lung SBRT treatments, highly modulated VMAT plans are highly susceptible to delivery uncertainties due to small-field dosimetry errors 26 and interplay effects 27 due to multileaf collimator (MLC) modulation of multiple beamlets as a function of lung tumor motion.
Coupled with DCA, FFF-beams allow for faster delivery of lung SBRT treatments with a steep dose fall-off outside the target. Many researchers have studied the use of VMAT with FFF beams, but not much has been written yet on the use of DCA and FFF beams. Currently, in our clinic we use non-coplanar VMAT lung SBRT plans with 3-6 partial arcs and 6 MV-FFF (1400 MU/min) beams. However, often times there is concern about treating a moving target with a highly modulated beam as mentioned before. Additionally, delivering VMAT plan requires additional commissioning effort, potentially a higher degree of quality assurance (QA) and testing of Linac chain and tighter Linac tolerances due to smaller fields and variation in dose rate with simultaneous gantry and MLC movement. Because of that, sometimes-delivering VMAT plan would be difficult with the older Linac. Furthermore, depending on the dose algorithm used there may be concerns over the accuracy of the calculation for small-field dosimetry in areas of tumor-tissue interfaces. DCA with FFF beams allow the user to take full advantage of the high dose rate with a decrease in the overall monitor units (MU) to deliver SBRT treatments in under a few minutes.
To address these issues we have designed a novel, yet simple hybrid-DCA (h-DCA) therapy planning technique that can reproduce lung SBRT plans similar to clinical VMAT plans in compliance with the RTOG-0915 requirements. 6 Our h-DCA plans used a non-coplanar hybrid technique with 2-3 differentially weighted partial DCAs (similar to those used by VMAT plans) plus 4-6 static beams depending upon tumor size and location on a per-patient basis.
About 60-70% of the beam-weight was given to the DCA and the rest was distributed among the static beams to maximize the target coverage and minimize the dose to the organs-at-risk (OAR). Our h-DCA provides highly conformal dose distributions by delivering doses with MLC dynamically conforming to the beam's-eye-view (BEV) projections of the target and steers isodose distributions by using a few static-beams. In contrast, VMAT delivers the optimized dose distribution by using many small beamlet-based intensity modulations using a combination of several separated MLC segments per arc. Our h-DCA plans are quicker to plan and deliver the lung SBRT treatment. Even though h-DCA does not use intensity-modulated beams, it still generates highly conformal radiosurgical dose distributions and satisfies the conformity and OAR requirements of the lung SBRT protocol. The h-DCA plans could potentially minimize smallfield dosimetry errors and MLC interplay effects.
In this report, we describe a novel method and compare this simple, yet clinically useful 3D-hybrid planning technique for single-dose (30 Gy) SBRT treatments of the peripheral lung lesions. In addition, the delivery efficiency and overall planning time of the h-DCA were estimated.

2.A | Patient characteristics
After obtaining an institutional review board approval from our institution, fifteen consecutive Stage I-II NSCLC patients with peripherally located tumors who underwent single-dose lung SBRT treatments (30 Gy) were included in this study. In our clinic, only The tumor characteristics are summarized in Table 1

2.C | Clinical VMAT plans
For the fifteen consecutive patients, clinically optimal VMAT-SBRT plans were generated in Eclipse TPS using 3-6 (mean, 4) partial noncoplanar arcs (with ±5-10°couch kicks) for a Truebeam linear accelerator (Varian Palo Alto, CA, USA) consisting of standard millennium MLC and 6MV-FFF (1400MU/min) beams. The isocenter was placed at the geometric center of the PTV. These partial non-coplanar arcs had an arc length of approximately 200-220°, and collimator angles (between 30°and 135°) were manually optimized to reduce the MLC tongue-and-groove dose leakage throughout the arc rotation on a per-patient basis. Jaw tracking option was used during plan optimization to further minimize out-of-field dose leakage. The prescription dose was 30 Gy in 1 fraction to the PTV while covering at least 95% of the PTV with prescription dose and ensuring that all hot spots (between 120% and 130%) fall within the ITV. All clinical treatment plans were calculated using Eclipse TPS with the advanced Acuros-XB (Version 13.6) algorithm 28-32 on the 3D-CT images for heterogeneity corrections with 2.5 mm × 2.5 mm × 2.5 mm calculation dose grid-size (CGS) and the photon optimizer (PO) MLC algorithm. The dose to medium reporting mode was used, and the planning objectives followed RTOG-0915 requirements (Arm 1). 6

2.D | Quality assurance and treatment delivery
Before delivering each VMAT-SBRT plan, a daily QA check on kilovoltage to megavoltage imaging isocenter coincidence was performed, including IsoCal measurement for the precise and accurate target localization. Our IsoCal localization accuracy for Truebeam was <0.5 mm. All the QA procedures including patient-specific QA were in compliance for SBRT treatment delivery. 5 Our Octavius 4D (PTW, Freiburg, Germany) phantom (with an Octavius 1500 detector array insert) QA pass rate was 97.6% ± 2.7%, on average, for 3%/ 3 mm criteria. All patients were treated in our clinic with cone-beam CT-guided procedure on our Truebeam. On the treatment day, patient set up prior to single-dose lung SBRT was performed using an in-house SBRT/IGRT protocol; 6 by co-registering the pretreatment conebeam CT with the planning CT scan. Image registration was performed automatically based on the region of interest bony landmark, followed by manual refining performed by the treating physician to ensure that the tumor was registered within the ITV contoured on the planning CT. The patient position was re-positioned for 6 • Gradient index, GI: ratio of 50% prescription isodose volume to the PTV. GI has to be smaller than 3-6, depending on the PTV.
• Maximum dose at any point 2 cm away from the PTV margin in any direction, D 2cm : D 2cm has to be smaller than 50-70%, depending on the PTV.
• Percentage of normal lung receiving dose equal to 20 Gy or more, V 20 : V 20 should be less than 10% per protocol, V 20 less than 15% is acceptable with minor deviations.
• Heterogeneity index, HI: HI = Dmax/prescribed dose was used to evaluate the dose heterogeneity within the PTV.
• Gradient distance, GD: GD is the average distance from 100%

3.B | Dose to OAR and delivery efficiency
The dosimetric differences (mean and standard deviation) between clinical VMAT and h-DCA plans for the OAR (spinal cord, heart, esophagus, ribs, skin, and normal lung) and delivery parameters including the phantom QA results are listed in Table 3. One case whose tumor was abutting rib had slightly higher than protocol suggested rib dose on both plans. Although, statistically insignificant differences (P-value >0.05) were found for all the evaluated dosimetric parameters except for maximum dose to skin, which increased slightly with h-DCA plans compared to clinical VMAT plans (P = 0.015, highlighted in bold). However, for the maximal dose to skin, the absolute differences were typically less than 1.0 Gy and well below SBRT protocol guidelines. Therefore, that difference is not expected to be clinically significant. If needed, skin dose can also be managed by adding one more static field in the h-DCA plan. Maximum dose to 1000 cc of lung was also similar for both plans (not shown here). However, comparison of treatment delivery parameters (total MU and BOT) significantly favored the h-DCA plans (see Table 3); the average BOT was improved by a factor of 1.51.
The improvement of treatment delivery efficiency is directly associated with the h-DCA planning technique (forward planning approach) with no beam modulation through the PTV as shown in dose. There is no beam modulation across the target volume with h-DCA plan; therefore, we did not calculate MF for h-DCA plans (see Table 3). Although, for the clinical VMAT plans the MF factor was up to 4.9 (average 3.0 ± 0.63) suggesting highly modulated treatment deliveries. The dose delivery accuracy of these clinical VMAT plans, the corresponding h-DCA plans were 90.5% ± 7.7% and 98.6% ± 1.4%, on average respectively with 2%/2 mm gamma passing rate criteria while using an Octavius QA phantom measurement -suggesting that significant dose deviation can be seen with highly modulated clinical VMAT plans compared to h-DCA plans.
Comparison of BOT on a per-patient basis is shown in Fig. 3. It has been observed that BOT was systematically lower for all patients and improved by a factor of 1.51, on average, when utilizing the h-DCA plans. The lower BOT will reduce the time the patient is on the  and achieved similar target coverage (see Table 2) compared to clinical VMAT plans. For all patients, the h-DCA plans met RTOG dosimetric compliance criteria including normal lung V 20Gy , V 10Gy , V 5Gy and were similar to clinical VMAT plans. The other OAR (spinal cord, heart, esophagus, ribs, and skin, see Table 3) were well below protocol dose guidelines. The h-DCA plans required less MU to deliver the same total prescribed dose due to no beam modulation across the target. Therefore the BOT was reduced (average BOT The change in respiratory patterns between the CT simulation and the time of treatment has been studied in the past. [34][35][36][37] Although, it has been reported that there were only small changes (within ±3 mm) due to intrafractional and interfractional motion in lung SBRT treatments, the mean patient set up time from tumor localization to the end of treatment cone beam CT scan was about 40 min. 36 It was suggested that an isotropic 5 mm PTV margin around the ITV was sufficient to address these potential motion errors. Furthermore, the interplay effect between the MLC as demonstrated in phantom QA measurement (see Table 3). Moreover, h-DCA could allow for real-time target verification (with no MLC modulation through the target, it allows for imaging treatment fields during treatment, if desired) and also eliminates patient-specific VMAT QA-potentially offering cost-effective, same or next day SBRT treatments to lung lesions. This technique can be easily adopted to other diseases sites (including hypofractionated centrally located lung lesions, stereotactic treatment of brain or abdominal/ pelvis lesions including liver SBRT) and small radiotherapy clinics with less extensive physics or machine support for SBRT treatments.
However, larger lung lesions seated near the critical structures or reirradiation patients can potentially benefit with highly optimized IMRT/VMAT plans. [44][45][46] Future work includes adding a few field-infield control points into those static beams to further improve our h-DCA plan quality. Due to decreased MU and BOT with h-DCA planning, deep inspiration breath-hold lung SBRT planning may be of value in future investigations. can potentially allow for cost-effective same or next day SBRT treatments to lung lesion. Another major advantage of the h-DCA technique is that it can be easily adopted to small community sites with less extensive physics and machine resources; potentially expanding SBRT programs to satellite clinics. The h-DCA planning can be easily adopted to other disease sites such as stereotactic treatment of brain or any abdominal/pelvis lesions such as liver, pancreas, or adrenal glands SBRT.

CONFLI CT OF INTERESTS
The author have no conflict of interests to disclose.