Comparison of beam characteristics in intensity modulated radiation therapy (IMRT) and those under normal treatment condition

The goal in three dimensional conformal therapy is to delivered the prescribed dose conforming to the target and at the same time minimizing the dose to adjacent normal tissues. Intensity modulated radiation therapy (IMRT) is a irradiation technique designed to achieve this goal. The authors have studied the beam characteristics of IMRT fields characterized by small MUs. For 5x5 field size and low MU (1-5 MU), dose (ionization) is non-linear with MU. Above 5 MU, dose is linear with MU for 5x5. For 10x10 field size, dose varies linearly with MU for all MU studied. The beam profiles in IMRT are similar to normal treatment fields. For 2 MU, there is a large variation of profiles in the sub-fields and the first two sub-fields did not attain the maximum dose as expected, indicating that a continuous 'on' state of the accelerator does not guarantee beam 'readiness'. Above 10 MU, the profile variation with an IM group becomes acceptable. The leakage radiation associated with 'dark current' is at a rate of 0.01 nC/sec. Thus complex fields may result in larger spurious radiation in the IMRT mode when longer time is required to change the field parameters. It is important to establish a priori the lowest MU permitted in an IMRT group to reduce dose variation which may not be detected in pre-IMRT quality assurance checks.