Table 1.

Offsets and slopes of Hounsfield unit (HU) calibration

Machine installed 2007			Machine installed 2009
From	Offset	Slope	From	Offset	Slope
31/10/2007	25 ± 15	1000 ± 55	14/01/2010	35 ± 9	982 ± 27
29/07/2011	12 ± 15	992 ± 41	01/04/2011	15 ± 14	962 ± 33
01/07/2012	8 ± 4	970 ± 12	01/07/2012	10 ± 5	940 ± 10

Uncertainties of ±1 standard deviation are shown on the offsets and slopes, based on the variation of the mean HU value of water and 1.694 inserts over the relevant time period. The offset and slope used in the dose recalculation is chosen from this table on the basis of the date of the image. The values given have been used for all dates from the date given in the “from” column to the date given in the row below. The values in the final row are still valid in November 2015.