Non-coplanar inverse planning IMRT using the MIMiC system: clinical significance in choice of 2-cm/1-cm mode and single couch vs. multiple couch angles.
The IMRT delivery process using MIMiC is extremely effective. In the hands of an experienced practitioner, MIMiC can provide excellent conformity to tumor volume with minimal damage to adjacent critical structures. However, the process is not completely automatic, even though it is an inverse planning process. The choice of machine treatment mode, 1- or 2-cm mode, and the choice of single couch angle vs. multiple, will yield quite different end results, with all prescription parameters being equal. The choice of mode and number of couch angles that yield excellent results of good coverage of the PTV with minimal hot spots and appropriate degree of normal tissue sparing still depends on the clinical experience of the planner. The size, shape, and total length of the PTV and those of the organs of interest dictate the choice. The authors demonstrated some clinical examples with comparative studies of the results of different choice of these parameters. In general, smaller beamlets with multiple couch angles give better PTV conformity, and critical organ avoidance. The down side of these choices are: (a) relatively more scattered hot spots within PTV, (b) longer time required to deliver the treatment, and (c) increased complexity of the process that could increase the chance of human error in treatment delivery.