Magnetic resonance imaging in postprostatectomy radiotherapy planning.

PURPOSE: To investigate whether the use of magnetic resonance imaging (MRI) in prostate bed treatment planning could influence definition of the clinical target volume (CTV) and organs at risk.

METHODS AND MATERIALS: A total of 21 consecutive patients referred for prostate bed radiotherapy were included in the present retrospective study. The CTV was delineated according to the European Organization for Research and Treatment of Cancer recommendations on computed tomography (CT) and T(1)-weighted (T(1)w) and T(2)-weighted (T(2)w) MRI. The CTV magnitude, agreement, and spatial differences were evaluated on the planning CT scan after registration with the MRI scans.

RESULTS: The CTV was significantly reduced on the T(1)w and T(2)w MRI scans (13% and 9%, respectively) compared with the CT scans. The urinary bladder was drawn smaller on the CT scans and the rectum was smaller on the MRI scans. On T(1)w MRI, the rectum and urinary bladder were delineated larger than on T(2)w MRI. Minimal agreement was observed between the CT and T(2)w images. The main spatial differences were measured in the superior and superolateral directions in which the CTV on the MRI scans was 1.8-2.9 mm smaller. In the posterior and inferior border, no difference was seen between the CT and T(1)w MRI scans. On the T(2)w MRI scans, the CTV was larger in these directions (by 1.3 and 1.7 mm, respectively).

CONCLUSIONS: The use of MRI in postprostatectomy radiotherapy planning resulted in a reduction of the CTV. The main differences were found in the superior part of the prostate bed. We believe T(2)w MRI enables more precise definition of prostate bed CTV than conventional planning CT.