Setup reproducibility of the semi-Fowler’s position in radiotherapy: a practical feasibility study

Objectives: To evaluate the setup reproducibility of the semi-Fowler’s position as a practical positioning strategy for radiotherapy patients who are unable to tolerate standard supine or prone positioning. Materials and Methods: Fifteen patients with respiratory or musculoskeletal limitations preventing standard positioning were retrospectively analysed. All patients were treated in the semi-Fowler’s position using routinely available immobilisation components. Setup reproducibility was assessed using orthogonal kilovoltage (kV) imaging with two-dimensional image matching. Translational displacements along the lateral (X), longitudinal (Y) and vertical (Z) axes were evaluated using the intraclass correlation coefficient, Lin’s concordance correlation coefficient and Bland–Altman analysis. Results: Median setup errors across all translational directions were close to zero, with the majority of displacements remaining within ±0.5 cm. Bland–Altman analysis demonstrated acceptable agreement across repeated fractions. Slightly increased variability was observed along the longitudinal and vertical axes, consistent with gravitational effects associated with semi-inclined positioning. Conclusions: The semi-Fowler’s position can be implemented as a practical and reproducible positioning option for selected radiotherapy patients when standard setups are not tolerated, without compromising geometric accuracy under routine image guidance.