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Distortion-corrected T₂ weighted MRI: a novel approach to prostate radiotherapy planning

¹ Academic Department of Radiotherapy and Oncology, ² CRUK Magnetic Resonance Imaging Group, ³ Department of Radiology, ⁴ Joint Department of Physics, Royal Marsden Hospital and Institute of Cancer Research, Downs Rd, Sutton SM2 5PT, UK

Correspondence: Dr A S N Jackson, Academic Department of Radiation Oncology, Christie Hospital NHS Trust, Wilmslow Rd, Manchester M20 4BX, UK. E-mail: Andrew.Jackson{at}manchester.ac.uk

The purpose of this study was to evaluate distortion-corrected MRI as a radiotherapy planning tool for prostate cancer and the resultant implications for dose sparing of organs at risk. 11 men who were to be treated with radical conformal radiotherapy for localized prostate cancer had an MRI scan under radiotherapy planning conditions, which was corrected for geometric distortion. Radiotherapy plans were created for planning target volumes derived from the MRI- and CT-defined prostate. Dose volume histograms were produced for the rectum, bladder and penile bulb. The mean volume of the prostate as defined on CT and MRI was 41 cm³ and 36 cm³, respectively (p = 0.009). The predicted percentage of the rectum treated to dose levels of 45–65 Gy was significantly lower for plans delineating the prostate with MRI than for those with CT. The rectal-sparing effect was confined to the lowermost 4 cm of the rectum (anal canal). There were no differences between the predicted doses to bladder or penile bulb (as defined using MRI) between plans. In conclusion, prostate radiotherapy planning based on distortion-corrected MRI is feasible and results in a smaller target volume than does CT. This leads to a lower predicted proportion of the rectum, in particular the lower rectum (anal canal), treated to a given dose than with CT.