Image Guided Radiotherapy Verification is the use of imaging modalities in the radiotherapy pathway, with the aim of improving techniques through increased treatment accuracy. This greater accuracy then opens the door, for the possibility of, dose escalation in order to try to gain better tumour control. IGRV can be split into two broad categories:
1. Planning - Using functional data / dynamic data (Staging, Localisation and planning)
2. Treatment - Using 3D / soft tissue / 4D (Verification)
The use of IGRV during the radiotherapy planning stage of treatment, has been shown to have a number of significant benefits. The first is in the accurate staging of the disease. Accurate staging is part of the diagnosis stage, before treatment delivery, and is essential in determining the correct treatment modality; not only in terms of optimum effective treatment, but also in cost efficiency terms. For example, if a more accurate imaging technique was to show spread from the tumour, that was not yet apparent on other imaging techniques, then the oncologist may opt for a more systematic first line treatment over localised radiotherapy alone. Accurate disease staging, also has an essential role to play in the treatment of palliative cancer patients. Images showing the absolute true extent of a patient's disease may mean they are not put through unnecessary radical treatment that will not extend their life or improve their symptoms. Through the use of imaging techniques such as photon emission tomography (PET), we are able to rationalise our margins with the aim to ultimately reduce them, without compromising in the ultimate tumour control, during the localisation and planning stages respectively.
The use of PET imaging allows for the assessment of the biological markers that show characteristics of a cancerous cell or the area surrounding the lesion. As a diagnostic tool PET is very effective at helping to stage cancers. PET data can even help with determining prognostic outcome in head and neck patients. They looked at the up take of [18F]FDG and found lower uptake levels before and in the first 2 weeks of treatment, directly correlated with better local tumour control. However, the study did not have enough data to show how this information would affect overall disease free survival or even remission.
Some studies have shown very significant findings, with over half of all the patients in their study having their initial staging altered following a PET scan. They also reported that found 14 out of the 16 patients had a resulting marked gross tumour volume (GTV) significantly different to that, when using computerised tomography (CT) alone. Their study was carried out only using head and neck patients, which is a limitation, as it is notoriously difficult to outline an accurate GTV in such patients, especially post very intrusive surgery, due to the most complicated anatomy found in this area and the scar tissue formed from such invasive surgery. A similar study of PET-CT fusion in other anatomical areas, may not show such clear cut results as Wang et al found. Schinagl et al (2006) went as far as saying radiotherapy should adopt a whole new form of tumour volume definement, in the shape of a biological tumour volume (BTV) technique, achieved solely with the information from PET, functional MRI or CT.
One main limitation with PET is that the image specificity is very poor. Altering the windowing can make the apparent area of biochemical function appear larger or smaller. This obviously has a great impact if this information is being used to determine the GTV, and could potentially lead to under or over exaggerating the true treatment area. Some studies have been carried out to determine the optimum windowing level, but further research is required to specifically look into the outcomes of those patients who have had PET responsible for their treatment outlining comparatively to conventional GTV outlining.
Lisa Wright @ http://www.myradiotherapy.com
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