Dosimetric evaluation of proportion-based hybrid IMRT/RapidArc planning for neoadjuvant radiotherapy in locally advanced rectal cancer

Optimizing radiotherapy (RT) techniques for neoadjuvant treatment of locally advanced rectal cancer (LARC) requires a balance among target dose quality, organ-at-risk (OAR) sparing, and delivery efficiency. Although hybrid IMRT/VMAT strategies have been explored in several disease sites, evidence regarding proportion-based hybrid planning in LARC, particularly on the Halcyon platform, remains limited. Seventeen patients with LARC treated with neoadjuvant chemoradiotherapy were retrospectively analyzed. For each patient, five plans were generated: IMRT, RapidArc (a VMAT technique), and three hybrid IMRT/RapidArc plans with IMRT/RapidArc dose weightings of 30%/70%, 50%/50%, and 70%/30%. Target coverage, homogeneity index (HI), conformity index (CI), treatment delivery time, monitor units (MUs), gamma passing rates, and OAR dosimetric parameters were compared using the paired Wilcoxon signed-rank test with Bonferroni correction. All plans achieved adequate planning target volume coverage. Varying the IMRT/RapidArc weighting in the hybrid plans resulted in systematic trade-offs among dose homogeneity, OAR sparing, and delivery efficiency. For example, the 70%-IMRT/30%-RapidArc plan achieved bladder and small-intestine sparing comparable to standalone IMRT while providing shorter delivery times and lower MUs. Increasing the RapidArc proportion further improved delivery efficiency but was associated with higher OAR exposure. Proportion-based hybrid IMRT/RapidArc planning on the Halcyon platform effectively integrates the dosimetric advantages of IMRT with the delivery efficiency of RapidArc. Hybrid strategies allow flexible trade-offs between target quality, OAR protection, and workflow efficiency, which may support personalized RT planning in LARC. Further validation in larger cohorts is warranted.