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Dose per MU formalisms:
factor-based dose calculations
Modulation with non-physical wedges
Non-physical wedges are delivered with one of the Y-jaws moving in or out during beam at
variable dose rate and at variable speed.
Wedge factors:
• do not depend on depth (no beam hardening)
• vary with beam energy and off axis position
Varian EDW
• Y-jaw motion into field (0.5
.
cm from opposing) jaw; in
variable speed and dose rate
• Jaw positions per MU: stored lookup (GSTT) tables;
one per beam energy
• 7 wedge angles in total (as combination of
60
°
wedged and open fields)
• WF depends on energy, wedge angle, field size and
Siemens VW
• Y-jaw motion out of field, starting from 1
.
cm of
opposing jaw
• Jaw positions per MU: calculated using a
mathematical algorithm with energy dependent
parameters.
• Multiple wedge angles between 10
°
and 60
°
off-axis position (along Y direction; asymmetric fields)
• WF
≈
0.4 - 1
• WF depends on energy, wedge angle, field size and
off-axis position (along Y direction; asymmetric
fields)
• WF
≈
1 from calculation points on CAX
In MU formalisms WF for non-physical wedges are either derived from
measurements or are calculated (from GSTT date or using the algorithm of
creating the wedge