Traditionally patients are positioned for external beam radiotherapy treatment using ink marks on the skin. The megavoltage treatment beam has then been used to produce planar images, on film or digital detectors, to image the bony anatomy and verify the position of the treatment fields. This method assumes the position and shape of the tumour and critical surrounding normal tissues are fixed with respect to the bony anatomy, which is often not the case. It also relies on planar megavoltage images, which are not very clear.
Both of these problems have been solved by the advent of Image Guided Radiotherapy (IGRT) in which kilovoltage imaging equipment, as used in diagnostic radiology, has been attached to the LINAC to produce superior 2D planar images of the bony anatomy at the time of treatment. IGRT can also visualise the tumour and surrounding healthy tissue in 3D, identifying changes in shape or position of both, using cone-beam CT (CBCT) images.
The use of Image Guided Radiotherapy, including CBCT, enables the repositioning of patients to improve the accuracy of their treatment, immediately before the radiation dose is delivered.
3D vs 2D image guided radiotherapy
3D IGRT shows soft tissue detail and position while 2D does not. However, there is much discussion about the effects of extra kV doses being given to patients with 3D cone-beam CT. GenesisCare's IGRT protocols seek to comply with IRMER by using 3D IGRT when it is clearly justifiable and 2D IGRT when it is clinically sufficient.
When the soft tissue containing the tumour does not move in relation to the bone, and therefore the position of the bone will establish the position of the tumour, 2D IGRT is used. This is the case with the breast, for example.
When the position of soft tissue is not 'tied' to the bone and is potentially different everyday, 3D Image Guided Radiotherapy is used. Tumors in the pelvis, prostate and chest move depending on the size and shape of organs such as the rectum, bladder and lungs, requiring 3D IGRT to ensure accurate dose delivery. 3D Image Guided Radiotherapy may also be used at the beginning of treatment and at intervals to ensure 2D delivery remains on target.
Above content is courtesy of Elekta