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•Water-equivalent material (solid water, acrylic (PMMA), graphite)

•For collimator settings < 5cm: miniphantom made of high Z material

•Using a high-

Z

material build-up cap should not introduce any deviations

as long as the beam quality is constant...

Weber L, Nilsson P, Ahnesjö A (1997) Phys Med Biol 42, 1875-86

High-

Z

vs.

Low-

Z

Recommendation: Only use high-

Z

buildup caps for very small fields

and normalize these measurements

to those in low-

Z

caps in the

smallest possible overlapping field

size (roughly 4x4 cm

2

).

Factor-based dose calculations: basic dosimetric quantities

S

c

: mini-phantom/buildup cap design – material considerations