To increase geometric precision volumetric imaging has been developed to visualize PTV and OAR in-session. CT scans have been implemented both with MV and kV x-rays. Helical MV CT is a novel treatment method that consists in rotating the MV treatment beam around the patient with couch moving through the bore. The images are of a quality that can be utilized for dose recalculation.
Cone-beam CT system is also innovative, instead of using the MV beam a kV tube with respective detector is mounted on the linear accelerator (LINAC). CT scans acquired have been used for image guidance and verification.
MV cone-beam CT is also available, the advantage of this scan is the less modification required to a conventional LINAC compared with the previous methods.
The importance of these improvements is not only the enhanced contrast for detection but also the high geometric precision and accuracy for guiding the positioning of the patient and eventually adapting the treatment plan.
In fact adjustments can be divided in two groups:
-on-line correction: imaging before each treatment, alignment with reference image and patient repositioning;
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-off-line correction: images acquired frequently along treatment, after a few sessions systematic geometric error can be calculated and correction applied with repositioning of the patient;
The ultimate method deriving from these improvements is the adaptive radiation therapy. During treatment a variation of tumor volume and of healthy tissues is common, through the use of new anatomic information the radiation plan and fraction doses can be adapted to the changes.
Image registration is also an important tool that allows automatic registration of bone anatomy with reference via software.