A necessary parameter in the treatment of cancer by medical x-ray is the rounded leaf offset, defined by the difference between the radiation field and visible light field sizes. Although often noted that manufacturer-recommended tables for the offset appear inaccurate on a machine-specific basis, there has been little treatment of the reasons why.

Expanding current methods for measuring the offset, this study delineates an improved approach using film-based techniques to improve precision. A mathematical model is derived describing the behavior of an offset curve, proper statistical analysis is applied to experimental design, and a new curve is validated. An experimental process is described for determination of offsets which reduce observed uncertainties in measurement from ±0.30 to ±0.042 cm. Finally, gathered data is found to match the predicted model with a 55.4% confidence interval, suggesting the process is plausible but in need of further definition of physical conditions.