Rotational setup errors remain a challenge in radiotherapy centers without six-degree-of-freedom (6DoF) couch correction systems. Small angular deviations may compromise target coverage and organ-at-risk sparing. Although image-guided radiotherapy (IGRT) has improved setup verification, many centers continue to rely on three-degree-of-freedom (3DoF) couches.

This technical report evaluates an approximation in which small rotational setup deviations are represented as equivalent translational displacements using the vector cross-product formulation (Δ = θ × r). Head-and-neck, breast, lung, pelvis, and thorax scenarios were modeled using an Elekta Infinity linear accelerator and Monaco TPS (version 6.1.4.0) with Monte Carlo dose calculation. Simulated rotational perturbations were assessed with and without translation-based correction using dose–volume histogram endpoints Dmean, D95, and V95.

Translation-based correction improved D95 across all treatment sites. The largest improvement was observed in head-and-neck geometry (approximately 2%), while pelvic and thoracic scenarios demonstrated intermediate gains. Breast and lung cases showed measurable improvements. Dmean increased modestly, whereas V95 remained essentially unchanged, suggesting improved target coverage without compromising volumetric conformity.

Translation-based correction is not a substitute for 6DoF couch technology but may provide a low-cost mitigation strategy for centers limited to 3DoF correction capability. Future studies should include phantom validation and automated correction approaches.