Multicenter multiplatform pattern-of-practice analysis of single-isocenter multitarget stereotactic radiosurgery

Abstract

Purpose: Single-isocenter multitarget stereotactic radiosurgery (SIMT SRS) offers enhanced clinical efficiency for treating multiple brain metastases. However, it introduces additional uncertainties, such as off-center dose and beam profile inaccuracies, as well as quality assurance (QA) challenges, complicating its implementation. This study aims to evaluate different SIMT SRS approaches.

Methods: We collected and analyzed SIMT SRS protocol and infrastructure parameters from 23 radiotherapy centers across Germany, Austria, and Switzerland, encompassing immobilization systems, computed tomography (CT) protocols, linear accelerators, treatment planning systems, beam configurations, imaging techniques, and QA practices. Consensus, deviations, and compliance with current guidelines were assessed. Subsequent studies will include on-site measurements, evaluation of treatment plan quality and delivery accuracy, and correlation of these findings with the analyzed protocols to identify potential links between protocol parameters and clinical outcomes.

Results: There is consensus (at least 80% agreement) for a CT slice thickness of ≤ 1 mm, the need for six-degree-of-freedom patient setup correction, and noncoplanar treatment. There is notable variability for intrafraction imaging (used by 70%), minimum accepted planning target volume diameter (ranging from 2-10 mm), SRS QA, and general plan parameters, such as photon energy and number of treatment fields. There is also high variability in employed linear accelerator models and treatment planning systems.

Conclusion: These findings highlight a lack of standardization in SIMT SRS practices. Combined with future measurements correlating protocols to treatment quality, our study will provide a foundation for recommendations to support the safe and standardized implementation of SIMT SRS.

Keywords: Anthropomorphic phantom; Brain metastases; End-to-end test; Ring trial; SIMT SRS.

Fig. 1

Computed tomographic (CT) representation of the anthropomorphic head phantom (RTsafe, Athens, Greece) with gel insert and reference structure case, including five target volumes. Target 1 is positioned near the center of mass of all targets, while other targets are progressively further away, with distances corresponding to their labels (e.g., target 2 is approximately 2 cm from target 1)

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

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)