All radiotherapy departments routinely use a technical check to determine any geometric errors. These errors can be divided into two groups; random and systematic errors. Systematic errors arise from a failure of a combination of equipment and patient stability in a particular setup or technique. Random errors occur due to daily setup errors, poor patient restraint equipment, and organ movement.
IGRV in the treatment part of the radiotherapy pathway seeks to correct them. There is no one form of IGRV that can correct for all the factors that contribute to the different forms of geometric error. Each system has its own strengths and weaknesses and the ‘ideal’ system is likely to be a combined approach.
Currently these geometric uncertainties are accounted for by adding margins to the GTV. This results in having to treat a larger area than necessary to ensure that the PTV is treated at a high enough dose to achieve the desired result. The objective of the IGRV in the verification of the treatment is to be able to reduce margins, increase doses and therefore increase the probability of tumor control. The slight irony is that the more data we collect through IGRV, the more we realize that some of our margins may be too small to account for organ movement in many areas we treat, meaning our GTV can actually increase with the use of IGRV.
Many departments have been using some form of IGRV for several years and have developed imaging protocols to correct systematic errors. Megavoltage electronic portal imaging (MV EPI) primarily uses bony anatomy to determine the mismatch between the treatment area that was planned and the area that is actually being treated.
By averaging this variance/misfit over a few days, the systematic error can be determined and thus corrected. MV EPI does not address organ movement when used alone. Some soft tissue can be seen on the images, but in most cases it is not adequate or of sufficient quality to be used for treatment verification.