Linac-based VMAT radiosurgery for multiple brain lesions: comparison between a conventional multi-isocenter approach and a new dedicated mono-isocenter technique

Abstract

BackgroundLinac-based stereotactic radiosurgery or fractionated stereotactic radiotherapy (SRS/FSRT) of multiple brain lesions using volumetric modulated arc therapy (VMAT) is typically performed by a multiple-isocenter approach, i.e. one isocenter per lesion, which is time-demanding for the need of independent setup verifications of each isocenter. Here, we present our initial experience with a new dedicated mono-isocenter technique with multiple non-coplanar arcs (HyperArc™, Varian Inc.) in terms of a plan comparison with a multiple-isocenter VMAT approach.MethodsFrom August 2017 to October 2017, 20 patients with multiple brain metastases (mean 5, range 2–10) have been treated by HyperArc in 1–3 fractions. The prescribed doses (Dp) were 18–25 Gy in single-fraction, and 21–27 Gy in three-fractions. Planning Target Volume (PTV), defined by a 2 mm isotropic margin from each lesion, had mean dimension of 9.6 cm3 (range 0.5–27.9 cm3). Mono-isocenter HyperArc VMAT plans (HA) with 5 non-coplanar 180°-arcs (couch at 0°, ±45°, ±90°) were generated and compared to multiple-isocenter VMAT plans (RA) with 2 coplanar 360°-arcs per isocenter. A dose normalization of 100%Dp at 98%PTV was adopted, while D2%(PTV) < 150%Dp was accepted. All plans had to respect the constraints on maximum dose to the brainstem (D0.5cm3 < 18 Gy) as well as to the optical nerves/chiasm, eyes and lenses (D0.5cm3 < 15 Gy). HA and RA plans were compared in terms of dose-volume metrics, by Paddick conformity (CI) and gradient (GI) index and by V12 and mean dose to the brain-minus-PTV, and in terms of MU and overall treatment time (OTT) per fraction. OTT was measured for HA treatments, whereas for RA plans OTT was estimated by assuming 3 min. For initial patient setup plus 5 min. For each CBCT-guided setup correction per isocenter.ResultsSignificant variations in favour of HA plans were computed for both target dose indexes, CI (p < .01) and GI (p < .01). The lower GI in HA plans was the likely cause of the significant reduction in V12 to the brain-minus-PTV (p = .023). Although at low doses, below 2–5 Gy, the sparing of the brain-minus-PTV was in favour of RA plans, no significant difference in terms of mean doses to the brain-minus-PTV was observed between the two groups (p = .31). Finally, both MU (p < .01) and OTT (p < .01) were significantly reduced by HyperArc plans.ConclusionsFor linac-based SRS/FSRT of multiple brain lesions, HyperArc plans assured a higher CI and a lower GI than standard multiple-isocenter VMAT plans. This is consistent with the computed reduction in V12 to the brain-minus-PTV. Finally, HyperArc treatments were completed within a typical 20 min. time slot, with a significant time reduction with respect to the expected duration of multiple-isocenters VMAT.

Dosimetric impact of rotational errors on the quality of VMAT‐SRS for multiple brain metastases: Comparison between single‐ and two‐isocenter treatment planning techniques

“…Several studies have demonstrated that multiarc noncoplanar VMAT can deliver highly conformal plans to the target(s) and spare adjacent critical structures . More recently, single‐isocenter VMAT‐SRS treatment techniques were introduced for dose delivery to multiple intracranial targets/lesions concurrently, with the latter being an attractive approach since treatment duration can be further reduced without necessarily compromising plan quality . A single isocenter has been found sufficient for VMAT‐SRS of multiple intracranial metastases, whereas minor improvements in plan quality can be achieved when additional isocenter(s) are used …”

“…The main drawback of a single‐isocenter VMAT‐SRS technique is that it exhibits increased sensitivity to geometric uncertainties (compared to other approaches) and, therefore, its efficacy partly relies on the overall spatial accuracy . Patient positioning and immobilization is a typical source of translational and rotational uncertainties .…”

“…[9][10][11][12][13][14][15][16] More recently, single-isocenter VMAT-SRS treatment techniques were introduced for dose delivery to multiple intracranial targets/lesions concurrently, with the latter being an attractive approach since treatment duration can be further reduced without necessarily compromising plan quality. [17][18][19] A single isocenter has been found sufficient for VMAT-SRS of multiple intracranial metastases, whereas minor improvements in plan quality can be achieved when additional isocenter(s) are used. 20 The main drawback of a single-isocenter VMAT-SRS technique is that it exhibits increased sensitivity to geometric uncertainties (compared to other approaches) and, therefore, its efficacy partly relies on the overall spatial accuracy.…”

“…20 The main drawback of a single-isocenter VMAT-SRS technique is that it exhibits increased sensitivity to geometric uncertainties (compared to other approaches) and, therefore, its efficacy partly relies on the overall spatial accuracy. 9,12,17,18,[21][22][23][24] Patient positioning and immobilization is a typical source of translational and rotational uncertainties. 25 Thermoplastic masks are commonly used in intracranial frameless VMAT-SRS applications, and residual patient setup errors can be detected using appropriate image-guided techniques.…”

A hybrid volumetric dose verification method for single‐isocenter multiple‐target cranial SRS

“…Interestingly, while dynamic conformal arcs were the mainstay of linac‐based radiosurgery for years, the feasibility of single‐isocenter multiple‐target (SIMT) approach was first demonstrated with a relatively new volumetric modulated arc therapy (VMAT) technique 8, 9. The most recent commercial implementation (HyperArc, Varian Medical Systems) offers refinements in terms of planning automation and collision prevention 10. Alternatively, Huang et al11 proposed a concept of single isocenter dynamic conformal arcs (SIDCA), whereby each lesion is treated by a dedicated group of dynamic conformal arcs but all groups share the same isocenter positioned between all targets.…”