MR based workflow for external radiotherapy of prostate cancer.

• The requirements on spatial accuracy are high in external radiotherapy of prostate cancer. The technical achievements in image guided radiotherapy (IGRT) during the recent years have lead to a significant improvement of the daily positioning accuracy. Improved accuracy has made it possible to reduce the margin between the clinical target volume and the planning target volume, thus reduce the risk for normal tissue complications. MRI is to prefer to CT for delineation of the prostate target because of superior soft tissue contrast. Recent studies have shown that the dosimetric accuracy of dose calculations on MR material is acceptable. In the present work we analyze the spatial uncertainties that are connected to workflows where the CT has been excluded. We found that accuracy increase with a fully MR based workflow. The main reason is that the MR based workflow does not require any registration between MR and CT to enable target delineation on the MR series. Two different methodologies for patient positioning with MR as baseline were identified: A. Implanted fiducial markers with portal imaging at each treatment session. B. Imaging of the patient in the MR at every treatment session to localize the prostate. We found that the two positioning methods give equivalent spatial accuracy. The estimated required margins for the MR based workflows was around 8 mm, corresponding numbers for a CT based workflow using the same assessment methods was around 10 mm.

Nyckelord

External radiotherapy

Prostate

Margins

MRI

fiducial markers

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