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.

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.

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.

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.