Image-guided Radiotherapy

Image-guided Radiotherapy

Dirk Verellen
European Oncological Disease 2007
Published: October 2008
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Inaccurate knowledge of a patient's anatomy and position during the course of therapy has always been a major source of concern in radiation therapy, potentially compromising the clinical results by insufficient dose coverage of the target volume and/or overdose of normal tissues. The management of target localisation emanates from the concept of treatment margins to cope with the uncertainty of the true location of the target volume during irradiation (gross target volume (GTV); clinical target volume (CTV); set-up margin (SM); internal margin (IM); planning target volume (PTV); and planning risk volume (PRV)).1,2 It is generally accepted that two classes of these so-called set-up uncertainties can be identified: systematic and random. Systematic errors exist because the imaging performed for treatment planning is typically a snapshot and the target position determined at that moment may differ from the average target position at treatment time, or if a certain procedure introduces an error that is repeated systematically over time. The random error is the day-to-day deviation from the average target position introduced with internal organ motion and the repeated treatment set-up that occurs in fractionated radiation therapy.

The systematic error is generally considered more important, because if uncorrected it would propagate throughout the treatment course and lead to a deleterious effect on local control. Day-to-day variations may be substantial and require safety margins that limit the maximum dose administered to the tumour volume due to possible toxicity to surrounding healthy tissue. With the introduction of image-guided radiation therapy (IGRT), clinical confidence has grown and it is possible to examine whether the traditional fractionated radiation therapy at 2Gy per fraction is still the optimum strategy. This introduces treatment schedules using fewer fractions (so-called hypo-fractionation),3 and the day-to-day variation in target localisation may no longer be statistically random. Finally, motion management becomes an issue as tumour motion interacts with dose delivery, causing a dose spread along the path of motion in some delivery techniques.4

With the improved imaging modalities to define and delineate tumour volumes, identifying both morphological as well as functional and biologic information, and the introduction of treatment modalities that allow for shaped dose distributions (e.g. intensity-modulated radiation therapy (IMRT), stereotactic body radiotherapy (SBRT) and chargedparticle beams), the radiotherapy community is now capable of creating dose distributions that match the tumour volume tightly.3,5 Conformal radiation therapy (CRT) aims at shaping the dose distribution to the delineated target volume, whereas conformal avoidance aims at avoiding critical structures. These advances have been driven by the dual goals of maximising radiation dose to tumour volume while minimising the dose to surrounding healthy tissue. Accurate knowledge of the patient s anatomy during the radiation process is of utmost importance, and it can be argued that novel technologies such as IMRT and shaped-beam radiosurgery are useless without proper image guidance.

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