Independent calculations to validate monitor units from ADAC treatment planning system

Current standards of practice are based on the use of an independent calculation to validate the monitor units (MUs) derived from a treatment planning system. The ADAC PINNACLE treatment planning system has shown discrepancies of 10% or more compared to simple independent calculations for highly contoured areas such as tangential breast and chest wall irradiation. The ADAC treatment planning system generally requires more MUs to deliver the same prescribed dose. Independent MU calculation methods are based on full phantom conditions. On the other hand, the MUs from the ADAC treatment planning system are derived using realistic phantom scatter. As such, differences exist in TMR factors, off-axis wedge factors, and the phantom scatter factor. To systematically study the discrepancies due to phantom conditions, experimental measurements were performed with various percentages of tissue missing. The agreement between the experimental measurements and ADAC calculations was found to be within 2%. Using breast field geometry, a relationship between missing tissue and the dosimetric parameters used by ADAC was developed. This relationship, when applied, yielded independent MU calculations whose values closely matched those from the ADAC treatment planning system.