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Verification of monitor unit calculations for non-IMRT clinical radiotherapy:
Report of AAPM Task Group 114
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The requirement of an independent verification of the monitor units (MU) or time
calculated to deliver the prescribed dose to a patient has been a mainstay of radiation
assurance. The need for and value of such a verification was obvious when calculations
were performed by hand using look-up tables, and the verification was achieved by a second
person independently repeating the calculation. However, in a modern clinic using
CT/MR/PET simulation, computerized 3D treatment planning, heterogeneity corrections, and complex
calculation algorithms such as convolution/superposition and Monte Carlo, the purpose of
and methodology for the MU verification have come into question. In addition, since the
verification is often performed using a simpler geometrical model and calculation
algorithm than the primary calculation, exact or almost exact agreement between the two
can no longer be expected. Guidelines are needed to help the physicist set clinically
reasonable action levels for agreement. This report addresses the following charges of the
task group: (1) To re-evaluate the purpose and methods of the “independent second check”
for monitor unit calculations for non-IMRT radiation treatment in light of the complexities of
planning. (2) To present recommendations on how to perform verification
of monitor unit calculations in a modern clinic. (3) To provide recommendations on
establishing action levels for agreement between primary calculations and verification,
and to provide guidance in addressing discrepancies outside the action levels. These
recommendations are to be used as guidelines only and shall not be interpreted as
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