Feasibility and advantages of using flattening filter-free mode for radiosurgery of multiple brain lesions

Feasibility and advantages of using flattening filter-free mode for radiosurgery of multiple brain lesions

Practical Radiation Oncology (2012) 2, e165–e171 www.practicalradonc.org Original Report Feasibility and advantages of using flattening filter-free...

866KB Sizes 0 Downloads 12 Views

Practical Radiation Oncology (2012) 2, e165–e171

www.practicalradonc.org

Original Report

Feasibility and advantages of using flattening filter-free mode for radiosurgery of multiple brain lesions Jia-Zhu Wang PhD a,⁎, Roger Rice PhD b , Arno J. Mundt MD b , Ajay Sandhu MD, DMRT b , Kevin T. Murphy MD b a

Radiation Oncology Center, St. Jude Medical Center, Fullerton, California Department of Radiation Oncology, Moores Cancer Center, University of California, San Diego, La Jolla, California

b

Received 17 December 2011; revised 2 March 2012; accepted 19 March 2012

Abstract Purpose: The 6-MV flattening filter-free mode (6F) of the Varian TrueBeam (Varian Medical Systems, Palo Alto, CA) enables faster dose delivery and shortens treatment time, which are especially beneficial for stereotactic radiosurgery. This study is to evaluate the feasibility and advantages of using 6F in stereotactic radiosurgery treatment of multiple brain lesions in comparison with regular 6-MV mode (6X). Materials and Methods: Ten patients having 2-12 brain metastases treated by intensity modulated stereotactic radiosurgery were selected for this study. For each patient, 2 RapidArc (RA; Varian Medical Systems) plans were generated: one using the 6F mode with a dose rate of 1400 monitor units (MU)/minute and another using the regular 6X mode of 600 MU/minute for a Varian TrueBeam linac. For each patient, both plans employed the same beam arrangement and optimization process. Results: The dosimetric parameters of homogeneity, conformity, and gradient indices were calculated and found to be comparable in the 6F and 6X plans for each patient. The mean dose to the normal brain and maximal doses to brainstem, chiasm, eyes, and optical nerves were also comparable in both RA plans using either 6F or 6X. The total number of MUs in the RA plans using 6F was 10%-20% more than that in the RA plan using 6X, but the beam-on-time was much less if 6F was used for planning and dose delivery (50% less). Conclusions: The fast delivery of the 6F beam is not only beneficial in stereotactic radiosurgery of a single brain lesion, but also for treating multiple brain lesions (2-12 lesions in this study group). Due to the beam falloff away from the central axis for large field sizes, more MUs are needed for 6F beams as compared with 6X. However, for the 6F mode with 1400 MU/minute, the delivery times are still much shorter compared with the 6X mode, thus greatly shortening the treatment time. © 2012 American Society for Radiation Oncology. Published by Elsevier Inc. All rights reserved.

Introduction Conflicts of interest: None. ⁎ Corresponding author. Radiation Oncology Center, St. Jude Medical Center, Fullerton, CA 92835. E-mail address: [email protected] (J.-Z. Wang).

Linac-based intracranial stereotactic radiosurgery (SRS) has traditionally used circular cones to provide beam collimation to deliver radiation using a few noncoplanar rotational arc beams. The cone-based arc

1879-8500/$ – see front matter © 2012 American Society for Radiation Oncology. Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.prro.2012.03.006

e166

J.-Z. Wang et al

technique provides excellent dose coverage for a small solitary lesion, but has the disadvantage of long treatment time for patients with multiple brain lesions, and for large or irregularly shaped lesions. It also produces large hot spots when multiple adjacent isocenters are used in the treatment of a large lesion. It typically requires 50-60 minutes for isocenter localization and multiple arc beam delivery to treat each lesion or isocenter. 1-4 Intensity modulated radiation treatment (IMRT) using multi-leaf collimators (MLC) has created a new technology for beam collimation and modulation, and it can provide an optimal tumor dose coverage while minimizing dose to adjacent critical organs. 5-7 IMRT-based radiosurgery (IMRS) on a conventional linear accelerator can be used to treat large or irregular lesions using a single isocenter. 8,9 An IMRS plan normally uses 9 noncoplanar IMRT fields to deliver the stereotactic radiosurgery dose. The whole treatment can be finished within 45-60 minutes including patient setup, independent of how many lesions are included in the treatment. Recent innovations further incorporate the intensity modulation technology with rotational beam delivery. RapidArc radiation therapy (RA) (Varian Medical Systems, Palo Alto, CA) is one such volumetric arc therapy technique that delivers modulated beams during gantry rotation with simultaneous adjustment of gantry rotation speed, MLC aperture, and dose delivery rate. 10-13 It has the capability of using less monitor units (MUs) and faster dose delivery than the regular IMRS treatment. The RA technique has been employed at University of California, San Diego since 2009 to treat various tumor sites, and has also been implemented for the intracranial radiosurgery of brain metastases. The Varian TrueBeam linear accelerator is equipped with a flattening filter-free mode for 6 MV and 10 MV. The 6F enables fast dose delivery at 1400 MU/minute by removing the flattening filter from the photon beam. The nonuniformity of the photon fluence can be compensated for through intensity modulation in the planning and dose delivery. 14-16 The fast dose delivery and therefore the shortening of treatment time, are especially beneficial for SRS. Aside from the fast delivery in treating a single lesion, the question arises whether the nonuniformity of photon fluence might hinder the optimal dosimetric outcome for cases of multiple brain lesions. The purpose of this study is to evaluate the feasibility of using 6F for the SRS treatment of multiple brain lesions and to determine if there is any benefit to using 6F mode over the flattened 6MV mode.

Methods and materials We have been using the cone-based arc beams for SRS to treat small solitary lesion(s), and the dynamic MLC-

Practical Radiation Oncology: October-December 2012

based fixed gantry beams for IMRS to treat multiple brain metastases or larger irregular brain tumor(s) since 2006. The prescribed doses ranged from 16 to 22 Gy for the single fraction cases and 6 Gy × 5 for larger lesions. The number of lesions treated with a single isocenter ranged from 1 to 14. Ten representative patients were selected for this study. They were selected from a pool of approximately 100 patients treated by IMRS in our institution from 2007 to 2009. They all received the same prescribed dose of 20 Gy delivered in a single fraction and had differing numbers of lesions ranging from 2 to 12, and volumetric RA plans had been created for these patients in our previous study. 17 The planning computed tomographic (CT) scans were done using 512 × 512 pixels and 1.25mm slice spacing. All patients also had a T1-weighted magnetic resonance (MR) scan with contrast, with a 26-cm field of view, 512 × 512 pixels, and 1-1.5 mm slice spacing. CT and MR image fusion was performed based on anatomy matching in terms of grayscales. The fusion was confirmed and the clinical target volume (CTV) was delineated on axial MR slices by a neurosurgeon. For multiple lesion cases where the prescribed dose was the same for all lesions, a single structure named CTV was used as a composite of all lesions. The average number of lesions in the group was 7 (range, 2-12) and the average CTV volume was 6.6 cc (range, 1.2-14.3 cc), as listed in Table 1. The planning target volume (PTV) was generated by adding a 2-mm geometric expansion to the CTV to account for setup and other uncertainties, based on our retrospective analyses of patient clinical records. 18 Critical organs, including the normal brain, brainstem, chiasm, right and left optical nerves, and right and left eyes, were also delineated. The normal brain was defined as the total brain volume excluding the PTV and brainstem. Critical organ expansions were created and used in the optimization process only to aid in achieving the optimization goals. The expansions were 5 mm, 3 mm, and 2 mm for the brainstem, chiasm, and eyes, respectively. In general, the prescribed dose in the SRS of brain metastases depends on the lesion size and the proximity to critical organs. In this group, the selected patients had the same prescribed dose of 20 Gy delivered in a single fraction. After optimization, the same normalization scheme was followed for all patients in the group where 100% of the prescribed dose covers 98% of the PTV. For each patient, 2 RA plans were created using 6F (1400 MU/minute) and 6X (600 MU/minute) modes in TrueBeam (Eclipse V8.9; Varian). Both RA plans used 2 coplanar arcs. The gantry arc spans, collimator angles, and field sizes differed across different patients, but were kept identical in the 2 plans for the same patient. In addition, the optimization objectives, priorities, and even the timing of priority changes during the optimization process were kept the same for both plans for each patient. The latter is important for the comparison study due to the adaptive nature of the current RA optimization process.

Practical Radiation Oncology: October-December 2012 Table 1 Patient no.

1 2 3 4 5 6 7 8 9 10 Average

6F vs 6X in SRS treatment of multiple brain lesions

e167

Total monitor units and the estimated beam-on-time used in RapidArc plans using 6F and 6X for 10 patients No. of lesions

CTV (cm )

6F

6X

2 4 4 5 6 7 8 8 9 12 7

12.1 3.4 1.2 3.1 5.3 1.6 14.3 8.0 9.7 7.5 6.6

398 3 4402 6092 4170 5373 5246 5332 4183 4472 4213 4747

3630 3988 5323 3709 4537 4485 4416 3543 3740 3616 4099

3

Total MUs

MU ratio

BOT (minutes)

BOT ratio

6F/6X

6F

6X

6F/6X

1.10 1.10 1.14 1.12 1.18 1.17 1.21 1.18 1.20 1.17 1.16

2.8 3.1 4.4 3.0 3.8 3.7 3.8 3.0 3.2 3.0 3.4

6.1 6.6 8.9 6.2 7.6 7.5 7.4 5.9 6.2 6.0 6.8

0.47 0.47 0.49 0.48 0.51 0.50 0.52 0.51 0.51 0.50 0.50

The prescribed dose is 20 Gy to cover 98% of planning target volume, which is generated from clinical target volume with 2-mm margin. The estimated BOTs are always slightly shorter than the actual values (see Discussion section). MUs, monitor units; BOT, beam-on-time; CTV, clinical target volume.

For comparison, doses to critical organs and dosimetric indices of homogeneity, conformity, and gradient were calculated in each plan. The homogeneity index (HI) is defined as the ratio of the maximum dose to the PTV (MD) to the prescribed dose (PD), which serves as a measure of dose homogeneity within the PTV volume: HI = MD/PD. The conformity index (CI) is defined as the ratio of the volume encompassed by 100% of the prescribed dose (V100) to the PTV volume: CI = V100/PTV. To evaluate dose gradient, the volume enclosed by the 50% dose cloud in each plan was also calculated (V50). The radii of the equivalent spheres of V50 and V100 were derived and denoted as R50 and R100, respectively. The gradient index (GI) is defined as the difference between R50 and R100: GI = R50 - R100 in centimeters. 19,20

Results It was found that isodose distributions and dose-volume histogram (DVH) curves of RA plans using either 6F or 6X mode showed close resemblance when the same arc beams are employed. As an example, the isodose distributions and their DVH curves of 2 RA plans of 2 co-planar arcs each using 6F and 6X beams, respectively, are shown in Fig 1, for a case with 5 brain lesions. The dose delivery using 6F mode will, in general, require more MUs when compared with plans using 6X mode. In cases of multiple brain lesions, with the field size often in the range of 15 cm, it will require more MUs for dose delivery due to the falloff of the dose profile beyond 2 cm from the central axis (Fig 2). Although the dose rate might change during normal RA delivery, it was found that nearly the full rate of 1400 MU/minute in 6F mode and 600 MU/minute in 6X mode was used in RA delivery due to the dose of 2000 cGy being delivered in 2 arcs.

Therefore, the ratio of MU and dose rate can be used as a close estimation of beam-on-time (BOT) in the RA delivery of high radiosurgery doses with an average deviation of 3% or less (see Discussion section). The estimated BOT of 6F and 6X plans for all patients in this group are listed in Table 1, together with the CTV volume and number of lesions for each patient. On average, the BOT using 6F was approximately 50% less than the BOT using the regular 6X mode, while the total monitor units in 6F plans averaged 16% higher (range, 10%-20%). The dosimetric characteristics were very similar for the RA plans using either 6F or 6X modes as shown in Table 2. The dosimetric indices of homogeneity, conformity, and gradient are listed for the 2 plans for each patient for comparison. The averages for the 10 patients used in this study, given in the order of 6F:6X for homogeneity index, conformity index, and gradient index are (1.14:1.14), (1.8: 1.8), and (2.3:2.4), respectively, as shown in Fig 3. Doses to critical organs for each patient were also found to be comparable in 6F and 6X plans, as shown in Table 3. The group average of the mean dose to normal brain and max dose to the other critical organs, in Gy, in the order of (6F:6X), are the following: normal brain (6.0:6.0); brainstem (10.0:10.1); chiasm (4.7:4.9); right eye (1.6:1.8); left eye (1.4:1.5); right optical nerve (2.4:2.5); and left optical nerve (2.4:2.7); as shown in Fig 4.

Discussion As the dose profile of 6F beams shows a sharp drop-off beyond 2 cm from the central axis (as shown in Fig 1) it is easy to envision the benefit of using 6F beams for stereotactic treatment for cases of a small single lesion in the brain, spine, or lung. On the other hand, static IMRS and rotational RA techniques have been routinely used to

e168

J.-Z. Wang et al

Practical Radiation Oncology: October-December 2012

Figure 1 Comparison of 2 RapidArc (RA) plans using the regular 6-MV mode (6X) and 6-V flattening filter-free mode (6F) for a patient having 5 brain lesions. The stereotactic radiosurgery prescription is 20 Gyx1. (A) Axial and coronal isodose distributions of 6X-RA plan are shown on the left; 6F-RA plan for the same patient on the right. The planning target volume (PTV) is shown in solid color and surrounded by the 100% isodose line. (B) The dose-volume histogram (DVH) of 6X- and 6F-RA plans are displayed together for comparison. Eyes and optical nerves are located at the left of the DVH curves as they received small doses in both plans.

Practical Radiation Oncology: October-December 2012

6F vs 6X in SRS treatment of multiple brain lesions

e169

Figure 2 Dose profiles at 10 cm depth for 6-MV flattening filter-free mode (6F) of Varian TrueBeam. The displayed profiles are of field sizes ranging from 2 × 2 to 40 × 40 cm. The grid size along the horizontal axis is of 1 cm.

treat multiple brain metastases, requiring larger fields, using the regular 6-MV (6X) beams. This study demonstrated the feasibility and the advantages of using 6F beams to treat multiple brain lesion cases. The patients in this study had RA plans done previously on Varian 23iX under ARIA V8.6 (Varian) using 6X. These plans were reoptimized and recalculated for TrueBeam, so the comparisons of 6F versus 6X in this study could be done with the same linear accelerator and the same software version, as TrueBeam requires ARIA V8.9 or higher. Gantry arc spans, collimator angles, and Table 2 Comparison of planning target volume, dose homogeneity index, conformity index, and gradient index in RapidArc plans using 6F and 6X beams Patient no. PTV HI (cm 3) 6F 1 2 3 4 5 6 7 8 9 10 Average SD

21.1 8.9 3.8 5.7 13.3 5.7 28.7 20.7 22.0 20.3 15.0 ±8.2

CI 6X

6F

GI (cm) 6X

6F

6X

1.13 1.15 1.2 1.2 1.4 1.4 1.09 1.09 1.6 1.6 1.7 1.7 1.15 1.15 2.5 2.5 1.5 1.7 1.10 1.12 1.7 1.8 2.2 2.2 1.15 1.16 1.6 1.6 2.3 2.4 1.12 1.14 1.7 1.8 2.3 2.3 1.17 1.13 1.7 1.4 2.9 3.1 1.15 1.15 1.8 1.9 2.4 2.4 1.15 1.17 2.3 2.1 2.9 2.8 1.14 1.16 2.2 1.9 3.4 3.6 1.14 1.14 1.8 1.8 2.3 2.4 ±0.02 ±0.02 ±0.4 ±0.3 ±0.6 ±0.6

The PTV volume of each patient is listed in column 2. PTV, planning target volume; HI, homogeneity index; CI, conformity index; GI, gradient index; SD, standard deviation.

field sizes are kept the same in both 6X and 6F plans for the same patient. On the other hand, due to the adaptive nature of the RA optimization process in the current software version, the optimization parameters and objectives were closely monitored and kept the same during optimization; the timing of priority changes during optimization were kept the same as much as possible when generating the 6X and 6F plans. The average of HIs is 1.14 for both 6F and 6X plans (in Table 2), and the variation of HI among patients is minimal. This is due to the fact that the weight and the value of the maximum PTV dose are controllable parameters during optimization and the objective of maximum dose to the PTV is, in general, set around 110% of the prescribed dose. This leads to the result of minimal variation in HI values across the patients. The GI is also comparable between the 2 plans but varies more from patient to patient. The patient to patient variation is expected as the achievable GI will be determined by the shape and size of the PTV, which varies considerably from patient to patient. The similarity in GI between the 2 plans is expected because the mechanics of the treatment deliveries are identical (MLC width and speed, fluence resolution) except for the rotational aspect of the RA delivery, which simulates more static beams. The RA plans of the 10 patients in our previous study (generated using Eclipse planning system V8.6 for Varian 23IX and 600 MU/minute dose rate) were delivered and the beam-on times were recorded based on the actual dose deliveries. 17 The BOT estimation method, mentioned above in the Results section, has been applied to the RA plans in our previous study and compared with the actual

e170

J.-Z. Wang et al

Practical Radiation Oncology: October-December 2012

Figure 3 Dosimetric comparison of 6F RapidArc and 6X RapidArc plans. The group average of the homogeneity index, conformity index, and gradient index over the 10 patient cases are plotted. All the error bars are of 1 standard deviation. The near-zero error bar in the homogeneity index display indicates a very small variation for different plans across the patients. See Discussion section for more details.

BOT. It was found that the mean deviation between estimated BOT and the delivered BOT is on the order of 2.8%, with a range of −2.1% to −3.1%.

Conclusions The dosimetric characteristics and doses to the critical organs in the RA plans using flattening filter-free mode were found to be comparable with the regular 6X plans in radiosurgery treatment of multiple brain lesions. Therefore, it is feasible to use the 6F mode to treat multiple brain lesions where the field sizes are on the order of 15 × 15 cm, even though the flattening filter-free mode has a dose fallTable 3 Patient no.

1 2 3 4 5 6 7 8 9 10 Average SD

off beyond 2 cm from the central axis. The total MUs used by 6F were about 16% higher compared with the 6X plans (range, 10%-20%) in the multiple lesion cases. The faster treatment delivery of the 6F mode (1400 MU/minute) has the advantage of reducing BOT by half when compared with the radiosurgery of multiple brain lesions with the regular 6X mode. This reduction results in 3.4 minutes versus 6.8 minutes to deliver 2000 cGy with 2 RA beams. The total treatment time, including patient setup and image guidance such as kilovoltage on-board imaging, conebeam CT (Varian Medical Systems), and the 3-dimensional surface imaging AlighRT (Vision RT, London, UK) prior to and during treatment delivery, is about 45 minutes for IMRS using multiple static beams, and 30 minutes for the RA treatment. The use of 6F mode in RA can further decrease

Mean doses to the normal brain and maximal doses to critical organs in RapidArc plans using 6F and 6X beams Brain (mean) (Gy)

Brainstem (Gy)

Chiasm (Gy)

Right eye (Gy)

Left eye (Gy)

6F

6X

6F

6X

6F

6X

6F

6X

6F

2.3 3.8 3.2 5.5 6.3 4.6 8.0 6.3 9.3 10.4 6.0 ±2.5

2.4 3.9 3.3 5.8 6.6 4.7 8.1 6.3 9.0 10.2 6.0 ±2.4

0.3 9.1 19.2 6.5 12.1 6.4 7.9 8.6 20.6 9.6 10.0 ±5.7

0.3 9.1 19.3 6.6 12.6 6.6 8.3 8.4 20.6 9.3 10.1 ±5.8

0.4 3.4 11.2 3.9 6.1 2.3 3.7 4.7 4.0 7.1 4.7 ±2.8

0.4 3.5 11.5 3.7 7.4 2.2 4.1 4.9 4.2 7.0 4.9 ±2.9

0.5 0.7 1.2 2.4 1.7 0.7 1.7 3.2 1.9 2.0 1.6 ±0.8

0.6 0.8 1.4 2.5 1.7 0.7 2.0 3.8 1.8 2.0 1.8 ±0.9

0.3 0.7 1.5 1.4 1.7 0.7 1.8 1.8 1.8 2.1 1.4 ±0.6

The group averages and standard deviations are listed in the last 2 rows. Prescribed dose is 20 Gy. SD, standard deviation.

Right optical nerve (Gy)

Left optical nerve (Gy)

6X

6F

6X

6F

6X

0.4 0.8 1.5 1.7 1.8 0.8 1.9 2.2 1.9 2.2 1.5 ±0.6

0.4 2.3 3.9 2.7 2.9 0.9 2.5 2.9 2.4 2.7 2.4 ±1.0

0.5 2.4 4.2 2.9 3.3 1.1 2.8 3.2 2.5 2.5 2.5 ±1.0

0.3 2.1 4.3 1.9 3.9 1.9 2.2 2.7 2.7 2.2 2.4 ±1.1

0.3 2.3 4.8 2.3 4.4 2.2 2.3 3.0 2.8 2.4 2.7 ±1.2

Practical Radiation Oncology: October-December 2012

6F vs 6X in SRS treatment of multiple brain lesions

e171

Figure 4 The mean dose to the normal brain and the max dose to organs at risk averaged over the entire patient group. The prescription dose is the same for all the patients in this group; 20 Gy delivered in 1 fraction. L, left; LT, left; r, right; RT, right.

the total treatment time. More important, the shortening of BOT reduces the potential for patient intrafraction motion and can greatly enhance the delivery accuracy. 21 This can be a significant benefit in stereotactic radiosurgery for large dose delivery in a single fraction.

References 1. Lutz WR, Winston KR, Maleki N. A system for stereotactic radiosurgery with a linear accelerator. Int J Radiat Oncol Biol Phys. 1988;14:373-381. 2. Betti OO, Munari C, Rosler R. Stereotactic radiosurgery with linear accelerator: treatment of arteriovenous malformations. Neurosurgery. 1989;24:311-321. 3. Schell MC, Bova FJ, Larson DA, et al. Stereotactic radiosurgery. Report of the American Association of Physicists in Medicine Task Group No.42. College Park: American Institute of Physics. 1995. 4. Winston KR, Lutz W. Linear accelerator as a neurosurgical tool for stereotactic radiosurgery. Neurosurgery. 1998;22:454-464. 5. LoSasso T, Chui C-S, Ling CC. Comprehensive quality assurance for the delivery of intensity modulated radiotherapy with a multileaf collimator used in the dynamic mode. Med Phys. 2001;28:2209-2219. 6. Low DA, Sohn JW, Klein EE, Markman J, Mutic S, Dempsey JF. Characterization of a commercial multileaf collimator used for intensity modulated radiation therapy. Med Phys. 2001;28:752-756. 7. Ezzell GA, Galvin JM, Low D, et al. Guidance document of delivery, treatment planning, and clinical implementation of IMRT: report of the IMRT Subcommittee of the AAPM Radiation Therapy Committee. Med Phys. 2003;30:2089-2115. 8. Lawson JD, Wang JZ, Nath SK, et al. Intracranial application of IMRT based radiosurgery to treat multiple or large irregular lesions and verification of infra-red frameless localization system. J Neurooncol. 2010;97:59-66. 9. Nath SK, Lawson JD, Wang JZ, et al. Optically-guided frameless linac-based radiosurgery for brain metastases: clinical experience. J Neurooncol. 2010;97:67-72.

10. Ling CC, Zhang P, Archambault Y, Bocanek J, Tang G, LoSasso T. Commissioning and quality assurance of RapidArc radiotherapy delivery system. Int J Radiat Oncol Biol Phys. 2008;72:575-581. 11. Lagerwaard FJ, van der Hoorn EA, Verbakel WF, Haasbeek CJ, Slotman BJ, Senan S. Whole-brain radiotherapy with simultaneous integrated boost to multiple brain metastases using volumetric modulated arc therapy. Int J Radiat Oncol Biol Phys. 2008;77:253-259. 12. Wu QJ, Yoo S, Kirkpatrick JP, Thongphiew D, Yin FF. Volumetric arc intensity–modulated therapy for spine body radiotherapy: comparison with static intensity-modulated treatment. Int J Radiat Oncol Biol Phys. 2009;75:1596-1604. 13. Tang G, Earl MA, Luan S, Wang C, Mohiuddin MM, Yu CX. Comparing radiation treatments using intensity-modulated beams, multiple arcs, and single arcs. Int J Radiat Oncol Biol Phys. 2010;76: 1554-1562. 14. Tsiamas P, Seco J, Han JZ, et al. A modification of flattening filter free linac for IMRT. Med Phys. 2011;38:2342-2352. 15. Hrbacek J, Lang S, Klöck S. Commissioning of photon beams of a flattening filter-free linear accelerator and the accuracy of beam modeling using an anisotropic analytical algorithm. Int J Radiat Oncol Biol Phys. 2011;80:1228-1237. 16. Kragl G, Baier F, Lutz S, et al. Flattening filter free beams in SBRT and IMRT: Dosimetric assessment of peripheral doses. Z Med Phys. 2011;21:91-101. 17. Wang JZ, Pawlicki T, Rice R, et al. Intensity-modulated radiosurgery with RapidArc for multiple brain metastases and comparison with static approach. Med Dosim. 2012;37:31-36. 18. Wang JZ, Rice R, Pawlicki T, et al. Evaluation of patient setup uncertainty of optical guided frameless system for intracranial stereotactic radiosurgery. J Appl Clin Med Phys. 2010;11:3181. 19. Shaw E, Scott C, Souhami L, et al. Single dose radiosurgical treatment of recurrent previously irradiated primary brain tumors and brain metastases: final report of RTOG protocol 90-05. Int J Radiat Oncol Biol Phys. 2000;47:291-298. 20. Feuvret L, Noël G, Mazeron JJ, Bey P. Conformity index: a review. Int J Radiat Oncol Biol Phys. 2006;64:333-342. 21. Kim S, Akpati HC, Kielbasa JE, et al. Evaluation of intrafraction patient movement for CNS and head & neck IMRT. Med Phys. 2004;31:500-506.