Collected parameters and data for the 23 RT centers are summarized in Fig. 2, whereby the following subsections refer to this figure.

Fig. 2

Single-isocenter multitarget stereotactic radiosurgery (SIMT SRS) infrastructure and protocol parameters across 23 radiotherapy (RT) centers (13 experienced and 10 benchmarking). The absolute number of centers using each technique is indicated in parentheses. Asterisks denote the number of included CyberKnife (CK) centers. Vendor names, system names, and abbreviations are detailed in the text

Radiotherapy system

There are 5 TrueBeams, 6 TrueBeams STx, 1 Clinac and 1 Novalis TX (Varian Medical Systems, Palo Alto, CA, USA), 7 Versa HD (Elekta, Stockholm, Sweden) and 3 CK systems. Apart from potential system-specific uncertainties, the type of treatment machine might be relevant for SIMT SRS application due to its specific multileaf collimator (MLC) system. The Varian TrueBeam STx and Novalis TX (NTX) have an MLC with a leaf width of 2.5 mm for the innermost leaf pairs and 5 mm for the outer leaves. All other C‑arm linear accelerators are equipped with MLC systems with 5 mm leaves throughout the entire field.

Couch degrees of freedom

There is consensus in both groups to use a 6-degrees of freedom (DoF) patient couch. Since this parameter is primarily used to assess the capability for rotational alignment correction, and as the CK enables this through its robotic arm, the CK centers have been included in this group. Only two benchmarking centers use couch systems with 4 DoF.

Immobilization system

There is consensus among the experienced C‑arm centers to use double-layer mask systems. Double-layer masks are mandatory when using Brainlab ExacTrac or Varian HyperArc, which are predominantly used by this group.

CT slice thickness

There is consensus in both groups to use a slice thickness of 1 mm or lower for the primary planning CT. Only one benchmarking center uses 2 mm. Interestingly, all experienced centers use 1 mm exactly, while five benchmarking centers use a thickness below 1 mm.

Treatment planning system and technique

There is a large variety of treatment planning systems (TPS): 9 centers use Eclipse by Varian (including 3 HyperArc), 5 Raystation (Raysearch, Stockholm, Sweden), 2 Monaco by Elekta, 4 Elements (Brainlab, Munich, Germany) and 3 Precision (for CK by Accuray). Systems with dedicated automated planning capability for SIMT SRS (HyperArc, Elements and Precision) are predominantly used by the experienced group.

Photon energy

The majority of centers use flattening-filter-free (FFF) beams, with X6FFF being the most commonly used option among both experienced and benchmarking centers.

Number of treatment beams

Except for CK with over 100 and HyperArc with fixed 4, the number of treatment beams across the other centers is rather diverse. On average, centers plan on using 1.7 beams per couch angle (not counting CK and not discriminating between half and full arcs). The number of beams per couch angle is higher for centers with fewer couch angles and lower for centers with more couch angles.

Number of couch angles

There is consensus among all centers to use noncoplanar treatment with at least two different couch angles. For CK centers, noncoplanarity is achieved through the use of the robotic arm, which is why they are included in the group using more than 4 angles.

Dose calculation and reporting

A total of 14 centers use a calculation grid size of 1 mm or lower, while 7 centers use 1.25 mm, 1 uses 1.5 mm, and 1 uses 2 mm.

Calculation algorithms are often categorized into type A, type B, and type C, depending mainly on their capability to account for lateral particle transport [15,16,17,18]. Due to the variety of employed algorithms and their different versions in our study group, we adhere to this terminology. The 3 CK centers use type A (PencilBeam), 8 centers use type B (PencilBeam convolution/superposition, e.g., AAA by Varian or PB X by Brainlab, and collapsed cone algorithms), and 12 use type C algorithms (advanced modelling, e.g., AcurosXB by Varian and full Monte Carlo algorithms).

Six centers calculate and report dose to water, 17 to medium.

For all these parameters, there is no significant difference between experienced and benchmarking centers.

Intrafraction imaging

There is consensus in the experienced group to use intrafraction imaging, whereas no consensus exists in the benchmarking group. Overall, 11 centers use Exactrac by Brainlab with X‑ray setup verification either at fixed Gantry angles and/or after couch rotation. One center uses MV imaging (Gantry 0°) after each couch rotation. One center uses surface tracking with C‑Rad (CRad, Uppsala, Sweden). The 3 CK centers use X‑ray image guidance. Seven centers do not use intrafraction imaging but rely on initial setup imaging.

Average number of SIMT SRS per year

This parameter is used to distinguish between experienced and benchmarking centers. Experienced centers have clinically used SIMT SRS prior to this study and treat at least one patient per year with SIMT SRS. Only five C‑arm centers treat more than 10 patients per year.

Average number of stereotactic head treatments per year

The annual number of stereotactic head treatments per center ranges from 0 to over 200. The majority of centers (20) perform at least 50 per year, indicating that our study group is well experienced in this field. On average, SIMT-SRS-experienced centers also perform more stereotactic treatments overall.

Minimum accepted PTV diameter for SRS

Values range from 2–10 mm. Four subgroups were defined that could influence the average minimum accepted planning target volume (PTV) diameter d, and each was compared with the respective remaining group of C‑arm centers [13, 14]. CK centers (dmean = 3 mm) accept significantly smaller diameters (p = 0.030) than the C‑arm centers (dmean = 6.7 mm). Among C‑arm centers, there is no significant difference between 2.5 mm and 5 mm MLC systems. However, C‑arm centers with more SRS experience (≥ 2 SRS per week, dmean = 5.3 mm) accept significantly smaller diameters (p = 0.043) than the remaining C‑arm centers (dmean = 7.8 mm). C‑arm centers using intrafraction imaging (dmean = 5.3 mm) accept significantly smaller diameters (p = 0.006) than those without it (dmean = 8.3 mm). The comparison is illustrated in Fig. 3.

Fig. 3

Comparison of minimum accepted planning target volume (PTV) diameters between subgroups: C‑arm MLC 2.5 mm referring to the employed multileaf collimator (MLC) system, SRS-exp indicating increased SRS experience (at least 2 per week) and if-img indicating the use of intrafraction imaging. C‑arm (other) is the reference group of all C‑arm centers, excluding the corresponding subgroups. Asterisks below the box indicate a significantly smaller diameter (p < 0.05)

Distance-adaptive PTV margin

A subgroup of 3 experienced centers uses distance-adaptive margins, where margins increase with increasing distance between target and isocenter. While 1 center only distinguishes between isocenter and off-isocenter (using 1 mm and 2 mm margin, respectively), the other 2 centers apply multiple incremental steps, resulting in a margin range from 0.5 mm and 1.0 mm at the isocenter, increasing to 1.5 mm and 2.0 mm for distances up to 7 cm.

Daily imaging isocenter check

There is consensus in the experienced group to verify the consistency between treatment and imaging isocenter every day stereotactic treatment is scheduled (this refers to any stereotactic treatment, not necessarily SIMT SRS). Among the centers not checking this daily, the test frequency ranges from 2 days to 3 months.