Planning and deliverability limits of photon beams Lattice SBRT for large tumours using clinical radiotherapy systems

Purpose

Lattice radiotherapy (LRT) with regular geometric lattice layouts, has gained clinical and research attention due to better compatibility with automation in lattice creation and inverse planning [1, 2]. No definitive conclusions regarding optimum evaluation metrics, distribution pattern, and lattice geometries in LRT have been established, warranting further studies. This in silico study aims to evaluate heterogenous dose distributions in Lattice SBRT, using four different geometric lattice layouts applied to the same GTV across ten clinical cases.

Method

Four geometric lattice layouts with vertices’ diameter of 0.7, 1.0, 1.5, and 2.0 cm have been investigated for large GTVs (677-1984cc) across ten cases of various types of tumours, following the planning and evaluation methodology of the LITE SABR M1 trial [1, 4]. The prescribed doses were 66.70Gy to the high dose peaks-vertices (PTV_6670) and 20Gy to the rest of the GTV&PTV, in 5 fractions. Treatments plans were created with VARIAN Eclipse v15.6, HDMLC, RapidArc and Acuros (Dmedium) algorithm, using 4 – 5 full or partial arcs. The dosimetry plan evaluation was conducted using existing and novel dosimetry metrics. Plan deliverability was evaluated using standard clinical SBRT measurement -based with Delta4 verification systems.

Results

Heterogenous dose distribution of total of eighty Lattice SBRT plans was evaluated using existing dosimetry metrics such as PVDR = Dp/Dmean (100-95)%, DR = DmeanPTV_6670/ Dmean, PTV_Avoid, D10/D90, Gy, DR_1.5, Gy, A novel metric, Lattice Rating Index, LRI = PVDR/ (D50 GTV/Prescription dose GTV) was introduced and explored. This metric integrates the PVDR and the GTV D50 quantifying the desired steep dose gradient within GTV. Higher LRI values reflect a dose in “GTV – PTV_ 6670″ closer to the prescribed valley dose, along with high PVDRs. The local gamma function pass rates (3%, 2mm) measured with Delta 4 system were in the range of >90%.

Conclusions

Four regular geometric lattice layouts with vertex diameters as small as 0.7 and 1.0 cm were successfully planned using RapidArc for ten large tumours cases. The newly proposed “Lattice Rating Index” offers a promising metric for comprehensive evaluation of heterogeneous “peaks and valleys” dose distributions within GTVs in LRT.

Outputs

Staykova, V., Eaton, D. J., Misson, S., & Kirby, M. (2025). Plan evaluation of four lattice SBRT layouts using existing and novel dosimetry metrics. Radiotherapy and Oncology, 206(Suppl. 1), S2743–S2745. https://doi.org/10.1016/S0167-8140(25)04434-2

Staykova, Vanya, Kirby, Michael , Misson-Yates, Sarah and Eaton, David (2023) Spatially fractionated radiation therapy with photon beams literature review, BIR 3rd Annual Radiotherapy and Oncology Meeting, London, 30/31 March 2023 DOI:10.13140/RG.2.2.29252.19844