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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
Factor-based dose calculations: basic dosimetric quantities
S
c
: mini-phantom/buildup cap design – material considerations
as long as the beam quality is constant...
4 MV
18 MV
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
).