Table 2.

Beam attenuation due to PMMA base plate (12 mm) + air gap simulating head and neck RT geometry [Figures 2, 3]

Gantry angle	Path length (mm) PMMA	PMMA base plate + air gap % attenuation (degree)
0	12.00	5.82
10	12.185	5.84
20	12.77	6.27
30	13.86	6.97
40	15.66	7.36
50	18.67	7.42
60	0.0	1.26
70	0.0	0.00
80	0.0	0.00
90	0.0	0.00

At gantry angle 60, part of the beam is passing through the base plate