Poster - Thur Eve - 64: Preliminary investigation of arc configurations for optimal sparing of normal tissue in hypofractionated stereotactic radiotherapy (HF-SRT) of multiple brain metastases using a 5mm interdigitating micro-multileaf collimator

Abstract

Purpose: To evaluate normal tissue sparing in intra-cranial HF-SRT, comparing various arc configurations with the Synergy Beam Modulator (SynBM) and Agility linacs, the latter incorporating leaf interdigitation and backup jaws. Methods: Five patients with multiple brain metastases (BMs), (5 BMs (n=2), 3 BMs (n=3)) treated with HF-SRT using 25 Gy (n=2) or 30 Gy (n=3) in 5 fractions, were investigated. Clinical treatment plans used the SynBM. Each patient was retrospectively re-planned on Agility, employing three planning strategies: (A) one isocenter and dedicated arc for each BM; (B) a single isocenter, centrally placed with respect to BMs; (C) the isocenter and arc configuration used in the SynBM plan, where closely spaced (<5cm) BMs used a dedicated isocenter and arcs. Agility plans were normalized for PTV coverage and heterogeneity. Results and Conclusion: Strategy A obtained the greatest improvements over the SynBM plan, where the maximum OAR dose, and mean dose to normal brain (averaged for all patients) were reduced by 55cGy and 25cGy, respectively. Strategy B was limited by having a single isocenter, hence less jaw shielding and increased MLC leakage. The maximum OAR dose was reduced by 13cGy, however mean dose to normal brain increased by 84cGy. Strategy C reduced the maximum OAR dose and mean dose to normal brain by 32cGy and 9cGy, respectively. The results from this study indicate that, for intra-cranial HF-SRT of multiple BMs, Agility plans are equal or better than SynBM plans. Further planning is needed to investigate dose sparing using Strategy A and the SynBM.