Room: Track 3
Purpose: To develop a simultaneous integrated boost volumetric modulated arc therapy (SIB-VMAT) technique to replace sequential boost for left-breast cancer patients treated with deep inspiratory breathe hold (DIBH) treatment on Halcyon® via 6MV-flattening filter free (FFF) beam. A new SIB hypofractionated left-breast technique is introduced providing equivalent BED while shortening treatment by 4-5 days.
Methods: Eight left-breast DIBH patients previously treated on Truebeam® with 3DCRT to 42.56 Gy in 16 fractions to the whole breast (6-18MV) and 10 Gy boost in 4-5 fractions to the tumor bed (6-18MV) were replanned on Halcyon (6MV-FFF) using tangential-VMAT technique. The new SIB prescription was 3.13 Gy/fraction to the tumor bed and 2.66 Gy/fraction to the whole breast for total 16 fractions. Plans included added flash and avoidance sectors to minimize dose to organs-at-risk (OAR).
Results: SIB-VMAT plans displayed similar target coverage to 3DCRT plans. Mean heart and LAD doses increased for SIB-VMAT plans but were below acceptable protocol limits. Ipsilateral lung (p=0.08) and contralateral breast (p=0.100) doses were equivalent. SIB-VMAT technique significantly decreased skin D5cc (p=<0.001). SIB-VMAT plans also exhibited lower volume of irradiated normal breast tissue outside of the tumor bed to the boost prescription dose (23.9% 3DCRT vs. 1.4% SIB-VMAT, p=0.002).
Conclusion: SIB-VMAT display similar target coverage to 3DCRT plans, while maintaining clinically acceptable OAR doses. SIB-VMAT reduces the volume of irradiated normal breast and skin dose which may improve overall cosmetic outcome. SIB-VMAT is a viable option with Halcyon where an electron beam is unavailable, there are dosimetry challenges with 3DCRT due to FFF-beam profile, treatment time is a limiting factor, or patients can access only Halcyon Linac clinic. Clinical validation of this novel treatment technique is underway.