Table 4.

Estimation of margins.

	Random σ and systematic Σ error [mm]						Margin [mm]
Direction	LR		SI		AP		LR	SI	AP
	Σ	σ	Σ	σ	Σ	σ
No correction	2.0	2.9	2.7	3.9	2.6	4.3	7.0	9.5	9.5
Correction 1×/week	1.9	2.8	2.3	3.5	2.4	3.9	6.7	8.2	8.7
Correction 3×/week	1.6	2.5	1.8	3.0	1.7	3.3	5.8	6.6	7.7
Correction 5×/week	1.4	2.0	1.4	2.3	1.3	2.2	4.9	5.1	4.8

For our patient group the standard deviations of systematic errors (Σ) and random errors (σ) are listed in LR, SI and AP directions. Then the margins from CTV to PTV are calculated in all three directions according to the prescription 2.5 × Σ + 0.7 × σ [14] in order to consider these errors.

The errors are summarized in the first line as derived from the weekly portal images without any correction. In the following lines the variation is reduced by controling and correcting the position with increasing frequency per week (once, three times, every day). Correction is conducted if the threshold of 5 mm is crossed in any direction. After correction we assume a residual error of 3 mm in the respective direction. Under these circumstances the margins can be diminished from approximately 10 mm to 5 mm.