Single isocenter treatment planning techniques for stereotactic radiosurgery of multiple cranial metastases

Insights into the dosimetric and geometric characteristics of stereotactic radiosurgery for multiple brain metastases: A systematic review

BACKGROUND

GammaKnife (GK) and CyberKnife (CK) have been the mainstay stereotactic radiosurgery (SRS) solution for multiple brain metastases (MBM) for several years. Recent technological advancement has seen an increase in single-isocentre C-arm linac-based SRS. This systematic review focuses on dosimetric and geometric insights into contemporary MBM SRS and thereby establish if linac-based SRS has matured to match the mainstay SRS delivery systems.

METHODS

The PubMed, Web of Science and Scopus databases were interrogated which yielded 891 relevant articles that narrowed to 20 articles after removing duplicates and applying the inclusion and exclusion criteria. Primary studies which reported the use of SRS for treatment of MBM SRS and reported the technical aspects including dosimetry were included. The review was limited to English language publications from January 2015 to August 2023. Only full-length papers were included in the final analysis. Opinion papers, commentary pieces, letters to the editor, abstracts, conference proceedings and editorials were excluded. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines were followed. The reporting of conformity indices (CI) and gradient indices, V12Gy, monitor units and the impact of translational and rotational shifts were extracted and analysed.

RESULTS

The single-isocentre technique for MBM dominated recent SRS studies and the most studied delivery platforms were Varian. The C-arm linac-based SRS plan quality and normal brain tissue sparing was comparable to GK and CK and in some cases better. The most used nominal beam energy was 6FFF, and optimised couch and collimator angles could reduce mean normal brain dose by 11.3%. Reduction in volume of the healthy brain receiving a certain dose was dependent on the number and size of the metastases and the relative geometric location. GK and CK required 4.5-8.4 times treatment time compared with linac-based SRS. Rotational shifts caused larger changes in CI in C-arm linac-based single-isocentre SRS.

CONCLUSION

C-arm linac-based SRS produced comparable MBM plan quality and the delivery is notably shorter compared to GK and CK SRS.

A novel weight optimized dynamic conformal arcs with TrueBeam™ Linac for very small tumors (≤1 cc) with single isocenter of multiple brain metastases (2≤, ≥4) in stereotactic radiosurgery: A comparison with volumetric modulated arc therapy

Introduction

We evaluated whether improved increase delivery efficiency of weight optimized dynamic conformal arc (WO-DCA) therapy in comparison to volumetric modulated arc therapy (VMAT) with single isocenter for SRS treatment of very small volume and multiple brain metastases (BMs).

Materials and Methods

20 patients having a less than 1 cc volume and 2≤, ≥4 of multiple BMs, redesigned for 20 Gy in 1 fraction using WO-DCA and VMAT techniques with double full coplanar and three partial noncoplanar arcs. Plan qualities were compared using tumor coverage, conformity index (CI), gradient index (GI), V4Gy, V10Gy, and V12Gy volumes of brain, monitor units (MUs), and percent of quality assurance pass rate (QA%).

Results

Both techniques satisfied clinical requirements in coverage and CI. VMAT had a significantly higher MU and mean GI than WO-DCA (for MUs; 2330 vs. 1991; P < 0.001, and for GI; 4.72 vs. 3.39; P < 0.001). WO-DCA was found significantly lower V4Gy (171.11 vs. 232.80 cm3, P < 0.001), V10Gy (25.82 vs. 29.71 cm3, P < 0.05), and V12Gy (14.35 vs. 17.28 cm3, P < 0.05) volumes than VMAT. WO-DCA was associated with markedly increase QA pass rates for all plans (97.65% vs. 92.64%, P < 0.001).

Conclusions

WO-DCA may be the first choice compared to the VMAT in reducing the dose in the brain and minimizing small-field dosimetric errors for very small SRS treatment of brain metastases in the range of ≤ 1 cc and 2≤, ≥4.

An Extremely Inhomogeneous Gross Tumor Dose is Suitable for Volumetric Modulated Arc-Based Radiosurgery with a 5-mm Leaf-Width Multileaf Collimator for Single Brain Metastasis

Introduction Single or multi-fraction (mf) stereotactic radiosurgery (SRS) is an indispensable treatment option for brain metastases (BMs). The integration of volumetric modulated arc therapy (VMAT) into linac-based SRS is expected to further enhance efficacy and safety and to expand the indications for the challenging type of BMs. However, the optimal treatment design and relevant optimization method for volumetric modulated arc-based radiosurgery (VMARS) remain unestablished with substantial inter-institutional differences. Therefore, this study was conducted to determine the optimal dose distribution suitable for VMARS of BMs, especially regarding dose inhomogeneity of the gross tumor volume (GTV). The GTV boundary, not margin-added planning target volume, was regarded as a basis for planning optimization and dose prescription. Materials and methods This was a planning study for the clinical scenario of a single BM. Eight sphere-shaped objects with diameters of 5-40 mm in 5-mm increments were assumed as GTVs. The treatment system included a 5-mm leaf width multileaf collimator (MLC) Agility® (Elekta AB, Stockholm, Sweden) and a dedicated planning system Monaco® (Elekta AB). The prescribed dose (PD) was uniformly assigned to just cover 98% of the GTV (D_98%). Three VMARS plans with different dose inhomogeneities of the GTV were generated for each GTV: the % isodose surfaces (IDSs) of GTV D_98%, normalized to 100% at the maximum dose (D_max), were ≤70% (extremely inhomogeneous dose, EIH); ≈80% (inhomogeneous dose, IH); and ≈90% (rather homogeneous dose, RH). VMARS plans were optimized using simple and similar cost functions. In particular, no dose constraint to the GTV D_max was assigned to the EIH plans. Results Intended VMARS plans fulfilling the prerequisites were generated without problems for all GTVs of ≥10 mm, whereas 86.4% was the lowest IDS for the D_98% for 5-mm GTV. Therefore, additional plans for 9- and 8-mm GTVs were generated, which resulted in 68.6% and 75.1% being the lowest IDSs for the D_98% values of 9- and 8-mm GTVs, respectively. The EIH plans were the best in terms of the following: 1) dose conformity, i.e., minimum spillage of PD outside the GTV; 2) moderate, not too excessive, dose attenuation outside the GTV, i.e., appropriate marginal dose 2-mm outside the GTV boundary as a function of GTV size; and 3) lowest dose of the surrounding normal tissue outside the GTV. In contrast, the RH plans were the worst based on all of the aforementioned measures. Conclusions On the assumption of uniform dose assignment to the GTV margin, a very inhomogeneous GTV dose is basically the most suitable for SRS of BMs in terms of 1) superior dose conformity; 2) minimizing the dose of the surrounding normal tissue outside the GTV; and 3) moderate dose spillage margin outside the GTV with a tumor volume-dependent rational increase, i.e., appropriate dose of the common PTV boundary. The concentrically laminated steep dose increase inside the GTV boundary for the EIH plan may also be advantageous for achieving superior tumor response, although early and excessive GTV shrinkage caused by the EIH plan during mfSRS can lead to surrounding brain injury.