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Foundations of Radiation Physics: Monitor Unit Calculation and Verification


John Gibbons

J Gibbons1*, R Stern2*, (1) Mary Bird Perkins Cancer Center, Baton Rouge, LA, (2) UC Davis Cancer Center, Sacramento, CA

SU-A-Salon EF-1 Sunday 7:30:00 AM - 9:30:00 AM Room: Salon EF

AAPM Task Group 71 has defined the nomenclature and methodology for performing monitor unit (MU) calculations for photon and electron beams. Calculations within this protocol are made using the dose per MU under normalization conditions that is determined for each user’s beams. For both photon and electron beams, this normalized dose per MU and associated dosimetric functions are determined using flat, water phantom data.
The requirement of an independent verification of the monitor units calculated todeliver the prescribed dose to a patient has been a mainstay of radiation oncology quality assurance.However, in a modern clinic using CT/MR/PET simulation, computerized3D treatment planning, heterogeneity corrections, and complex calculation algorithms such asconvolution/superposition and Monte Carlo, the purpose of and methodology for the MU verificationhave come into question. In addition, since the verification is often performed using a simplergeometrical model and calculation algorithm than the primary calculation, guidelines are needed to help the physicistset clinically reasonable action levels for agreement. The AAPM Task Group 114 report re-evaluates the purpose and methods of the “independent second check” formonitor unit calculations for non-IMRT radiation treatment in light of the complexities of modernday treatment planning.The report provides recommendations on how to perform verification of MU calculations in a modern clinicand on the establishment and implementation ofaction levelsfor agreement between primary calculations and verification.

Learning Objectives:
1. Understand the current-recommended nomenclature, methodology, and measurements required to perform MU calculations within the TG71 protocol.
2. Know the components of a good monitor unit verification program.
3. Be able to apply the action level guidelines for agreement between primary and verification monitor unit calculations presented in the TG114 report.


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