Table 3

Middle position of rails
	6 MVphotons		20 MVphotons
	Carbon couch top	Rails	Carbon couch top	Rails
Set Default	much too high	good	much too high	good
Set 1	much too high	good	much too high	good
Set 2	too high	too low	too low	good
Set 3	too high	good	good	good
Set 4	too high	good	too low	too high
Set 5	too low	too low	good	good
Set 6	good	good	good	good
External position of rails
	6 MVphotons		20 MVphotons
	Carbon couch top	Rails	Carbon couch top	Rails
Set Default	too high	good	much too high	good
Set 1	too high	good	much too high	good
Set 2	good	too low	too high	good
Set 3	good	good	good	good
Set 4	good	too high	good	good
Set 5	too low	too low	too low	too low
Set 6	good	good	good	good

Table 3: Results of the comparison between measured and calculated attenuation for the subsumed gantry angles. Aberration of <2.2% indicates a good agreement between calculated and measured attenuation, a positive aberration>2.2% too low HU values and a negative attenuation>2.2% too high HU values.