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Randomized three-dimensional conformal radiotherapy dose escalation studies have consistently shown improved biochemical control rates for prostate cancer.

This benefit was associated with the problem of increased rectal toxicity. The increasing implementation of image-guided radiotherapy (IGRT) allows a reduction of safety margins around the clinical target volume (CTV). Thus, the same or improved local tumour control with lower rectal toxicity due to reduced rectal volume within the target volume can be expected.

Better dose conformality can be reached with intensity-modulated radiotherapy (IMRT) in comparison to three-dimensional conformal radiotherapy techniques - as used in dose escalation studies - offering an additional opportunity for healthy tissue protection.

The opportunity for "dose-painting" is another advantage associated with IMRT - areas with a higher risk of tumour recurrence or a higher tumour load can be covered with larger doses in the same fraction. In view of an often multifocal micrsoscopic spread of prostate cancer - not possible to delineate with the presently available imaging modalities - the whole prostate must still be covered by an effective dose.

The aim of this study was to compare the dose distribution in IMRT treatment plans with and without a simultaneous integrated boost (IB) after performing a PET-CT with 18F-choline for treatment planning. Dose-volume histograms and equivalent uniform doses (EUD) were evaluated and normal tissue complication probability (NTCP) for the bladder and rectum compared.

Treatment planning with a simultaneous integrated boost allows an individually adapted dose escalation. The boost volume, as defined by a tumour-to-background uptake ratio >2 in a PET-CT with choline, was found to correlate with several well established prognostic parameters. The therapeutic ratio can be improved by a considerable dose escalation to the macroscopic tumour site without considerably increasing the NTCP for the rectum and bladder.

In contrast to a minor impact of the prescription dose to the integrated boost, EUD and NTCP for the organs at risk are predominantly dependent on the dose to the prostate. However, depending on the prescription dose to the integrated boost, bladder or rectum Dmax might exceed possible planning objectives in case of lesion locations in direct vicinity to the organs at risk - an individually decided compromise is the necessary consequence.

Source: UroToday

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