Table 4.

The overall uncertainty of the calibration curves. There were 6 points of measurements, which are shown in the “Relative location” column. For the averaged computed tomography (CT) calibration evaluation, the averaged CT calibration curve was applied for all 5 CT images to obtain the water-equivalent thickness (WET) values from the treatment planning system (TPS). For the CT-specific calibration evaluation, the CT-specific calibration curves were applied to a given CT scan to obtain the WET values from the TPS.

Relative location (cm)	Averaged CT calibration (%)					CT-specific calibration (%)
	CT1	CT2	CT3	CT4	CT5	CT1	CT2	CT3	CT4	CT5
X = 0	−2.8	−0.6	−2.3	−0.1	−1.2	−3.0	−0.5	−2.5	−1.3	−1.6
X = −4.1	−3.7	−3.0	−3.7	−3.3	−3.5	−4.3	−4.0	−3.3	−3.4	−3.7
X = −8.4	−2.2	−0.7	−0.8	1.2	0.1	−2.2	−0.8	−1.7	−0.1	−0.2
X = −13.4	−2.1	1.2	0.5	1.6	1.2	−1.5	0.9	0.6	1.8	0.9
X = −17.9	−0.2	0.1	−0.4	0.3	−0.6	−1.9	−0.7	−0.6	−0.7	−0.8
Z = 2	2.4	2.6	2.6	3.4	2.8	1.9	2.0	2.3	2.4	2.3
RMSE	2.5	1.7	2.1	2.1	2.0	2.7	1.9	2.1	2.0	2.0

Abbreviation: RMSE, root mean square error.