Dosimetric impact of flattening filter-free photon beams on bolus volumetric modulated arc therapy planning for left chest wall irradiation using active-breathing coordinate techniques

Background: Deep inspiration breath hold (DIBH) reduces cardiac doses, but evidence comparing flattening filter (FF) and flattening filter-free (FFF) beams in breast volumetric modulated arc therapy (VMAT) is limited. FFF beams are established in stereotactic and hypo fractionated treatments, yet their benefit in breast VMAT remains unclear. Bolus (BOL) use in breast VMAT is controversial, and multi-machine data are scarce. This study aims to evaluate the effects of respiratory management [free breathing (FB) vs. active breathing coordinator (ABC)], BOL use, and beam modality (FF vs. FFF) on left-breast VMAT planning across different linear accelerators. Methods: Twenty-five left breast cancer patients treated with the ABC technique at Fortis Hospital, Punjab (January 2022 to August 2023) were initially considered. Ten patients with involvement of the chest wall, axilla, level III lymph nodes, supraclavicular fossa (SCF), and internal mammary nodes (IMNs), receiving 5,000 cGy in 25 fractions using VMAT, were included. While other breast planning scenarios were excluded. Plans were generated using Monaco TM Treatment Planning System (TPS) with 6 MV and 6 MV FFF beams for Elekta VersaHD TM (Group 1) and Eclipse TM TPS for Varian TrueBeam TM (Group 2) and Halcyon TM (Group 3). For each patient, plans were created for BOL, no bolus (NOBOL), and 13 fractions BOL plus 12 fractions NOBOL [combination of bolus and no bolus (CBNB)]. Target coverage, organ at risk (OAR) doses, monitor units (MUs), modulation index (MI), and integral dose were compared. statistical analysis was performed one-way analysis of variance (ANOVA) in SPSS software (Version 31.0). Results: FB and ABC showed no notable differences with BOL-FF, NOBOL-FF, BOL-FFF, NOBOL-FFF, and CBNB-FFF plans across all groups in target coverage (P<0.001), integral dose (P=0.009), MU (P<0.001), and MI (P<0.001). Heart V25 was reduced by 2% with ABC (P<0.001) plans in all groups. Oesophagus mean dose was slightly reduced (P=0.009) in Group 1. The V95 showed 6% less variation in NOBOL plans compared to BOL (P<0.001) plans in Group 1 for both FF and FFF. CBNB-FF and CBNB-FFF plans showed less coverage (P<0.001) than BOL-FF and BOL-FFF but more than NOBOL-FF and NOBOL-FFF. MU and MI were higher (P<0.001) in FFF plans in Group 1 and slightly higher (P<0.001) in Group 2. Mean dose to ipsilateral (P=0.10) and contralateral lung (P=0.43) was slightly higher, and ipsilateral lung V20 was 6% higher (P=0.50) in Group 1 than in Groups 2 and 3. Spinal cord (P=0.06), liver (P=0.08), and oesophagus doses (P=0.49), as well as MU (P<0.001) and MI (P<0.001), were lower in Group 2. Opposite breast dose was lower (P=0.95) in Group 3. Conclusions: The study results indicate that all three plan groups are clinically acceptable. For left-sided breast cancer, the ABC plan with CBNB using both FF and FFF beams provides optimal target coverage and OAR sparing; however, treatment with FFF beams requires a higher number of MU.