Abstract

Manon Spaniol¹, Sabine Mai¹, Tonja Zakrzewski¹, Michael Ehmann¹, Florian Stieler¹

¹University medical center Mannheim, university of Heidelberg, Department of radiation oncology, Mannheim, Germany

Intracranial radiotherapy using Leksell Gamma Knife® (LGK, Elekta Instrument AB, Sweden) is a well-established treatment option in radiation therapy. With the introduction of LGK Lightning (Elekta Instrument AB, Sweden), the treatment planning options in Leksell Gamma Plan® (LGP, Elekta Instrument AB, Sweden) allow real inverse planning instead of optimized forward planning. In this study, we investigated the benefits of inverse planning in single session and fractionated Gamma Knife radiotherapy in terms of plan quality and efficiency for meningiomas (men) and vestibular schwannomas (vs). The prescription doses ranged from 11 Gy to 50 Gy for vs (1–27 fractions) and 12 Gy to 52.2 Gy for men (1–26 fractions).

Twenty-three patient treatment plans (n=12 men, n=11 vs) that were created using forward planning with automated shot optimization in LGP and treated with LGK Icon™ at our institution were selected retrospectively as patient cohort. For each forward plan, an inverse (inv) and inverse full coverage (fc) treatment plan was generated using LGK Lightning. Plan quality (coverage, selectivity, gradient index, maximum dose to the target “D_max target” and organs at risk “D_max OAR”) and efficiency (beam-on-time “BOT”) characteristics were compared between the clinical forward and inverse (inv and fc) plans. Additionally, the medians were reported and the significance of the results was assessed using a paired-sample Wilcoxon test (significance level <0.05).

For the inv plans, a significant dose reduction has been achieved for 87% of the OARs, while maintaining the target coverage. In the forward plans, the median D_max OAR was 14.9 Gy (men) and 14.1 Gy (vs) and was reduced to 12.25 Gy (men, p=0.002) and 12.95 Gy (vs, p=0.007) in the inv plans. For the fc plans, a reduction was observed in 53% of the OARs where the median D_max OAR was lowered to 14.4 Gy (men, p=0.192) and 13.35 Gy (vs, p=0.985). Additionally, the fc plans resulted in significantly higher target coverage. The median coverage rose from 0.98 (men) and 0.97 (vs) to 1.0 (men, p=0.005) and 0.99 (vs, p=0.006).

The BOT was significantly shortened for 87% of the inv and fc plans. The median BOT was reduced for men from 22.5 min to 18.5 min (inv, p=0.007) and 19 min (fc, p=0.017) and for vs from 19.3 min to 15 min (inv, p=0.008) and 9.8 min (fc, p=0.01). 

The results of this study show that inverse planning with LGK Lightning can reduce the BOT and the dose to the OARs, while maintaining the target coverage for meningiomas and vestibular schwannomas.