Table 2.

Average transit dose rates along the x‐ and z‐axes. The average transit dose rate is also represented as a percentage of the focus dose on the day of measurement rate (2.22 Gy per minute for the 18 mm helmet). The difference between the doses measured at 0 cm along the x‐ and z‐axes results from the nonisotropic isodose distribution at isocenter; dose measured will depend on the orientation of the ionization chamber.

Average Transit Dose Rates Along the x‐axis
Position (cm)	Measured Transit Dose Rate (cGy/min)	Percent of Focus Dose Rate (%)
$-6.2$	$0.36\pm 0.06$	$0.16\pm 0.03$
$-3.2$	$0.70\pm 0.04$	$0.32\pm 0.02$
$-1.2$	$6.11\pm 1.85$	$2.75\pm 0.83$
0	$7.72\pm 0.08$	$3.48\pm 0.04$
1.2	$5.28\pm 2.45$	$2.38\pm 1.10$
3.2	$0.59\pm 0.02$	$0.27\pm 0.01$
6.2	$0.36\pm 0.04$	$0.16\pm 0.02$
Average Transit Dose Rates Along the z‐axis
Position (cm)	Measured Transit Dose Rate (cGy/min)	Percent of Focus Dose Rate (%)
$-6.2$	$0.00\pm 0.00$	$0.00\pm 0.00$
$-3.2$	$0.10\pm 0.10$	$0.05\pm 0.05$
0	$8.66\pm 0.19$	$3.90\pm 0.08$
1.2	$1.37\pm 0.30$	$0.62\pm 0.14$
3.2	$1.88\pm 0.14$	$0.85\pm 0.06$
6.2	$3.34\pm 0.04$	$1.51\pm 0.02$