Figure 6.

The effect of shifting the isocenter of either the RPO field (a) or the LAO field (b); shifts are perpendicular to the match plane. The dose difference is measured using a Farmer chamber (accurate to within ± 0.5% and positioned to within $\pm 0.2\hspace{0.17em}\text{mm}$) for the OMF technique only.

The discrepancies between calculation and measurement are attributed to partial volume effects in the ion chamber. A reduction in sensitivity to target motion in the direction of the shift is realized when the OOMF or SJOMF techniques are employed.