To investigate the pattern of lymph node spread on magnetic resonance lymphography (MRL) in prostate cancer patients and compare this pattern to the clinical target volume for elective pelvis irradiation as defined by the radiation therapy oncology group (RTOG-CTV).

The charts of 60 intermediate and high risk prostate cancer patients with non-enlarged positive lymph nodes on MRL were reviewed. Positive lymph nodes were assigned to a lymph node region according to the guidelines for delineation of the RTOG-CTV. Five lymph node regions outside this RTOG-CTV were defined: the para-aortal, proximal common iliac, pararectal, paravesical and inguinal region.

Fifty-three percent of the patients had an MRL-positive lymph node in a lymph node region outside the RTOG-CTV. The most frequently involved aberrant sites were the proximal common iliac, the pararectal and para-aortal region, which were affected in 30%, 25% and 18% respectively.

More than half of the patients had an MRL-positive lymph node outside the RTOG-CTV. To reduce geographical miss while minimizing the toxicity of radiotherapy, image based definition of an individual target volume seems to be necessary.