Rectal and bladder deformation and displacement during preoperative radiotherapy for rectal cancer: Are current margin guidelines adequate for conformal therapy?

PURPOSE

To evaluate rectal motion and estimate an appropriate target volume for preoperative radiotherapy (RT) for rectal cancer.

METHODS

Between January 2006 and December 2009, 17 rectal cancer patients undergoing preoperative RT underwent 39 cone-beam computed tomographic scans (CBCTs). CBCTs were fused to treatment planning CT scans by bony anatomy. The rectum and bladder were contoured on each scan. Margins of 2, 5, 10, and 15 mm were added to the rectum, and the volume and percent rectum on CBCT outside each of these margins were determined. The clinical target volume (CTV) was examined to determine the necessary margin beyond the posterior bladder edge to ensure coverage of the mesorectum at all time points.

RESULTS

Median percentage rectum on CBCT outside the planning rectum was 7.77% (range, 0.19%-42.91%). Two patients had 1 or more CBCT with 1% or greater rectum outside a 1.5 cm margin. Five patients had 1 or more CBCT with 1% or greater rectum outside a 1.0 cm margin. A CTV extending 1 cm into the posterior bladder edge (CTV1.0) was adequate at all time points for 79% of evaluable patients, and a CTV with a 1.5 cm anterior margin was adequate for 93% of patients. For 2 patients, the rectum extended outside the CTV1.0 on CBCT.

CONCLUSIONS

With a limited number of CBCT scans, we found that the rectum tended to remain within 1.5 cm of the initial location on treatment planning CT. However, an anterior margin of 1.5 cm beyond the posterior bladder edge provides better coverage of the mesorectum than 1 cm for the initial CTV.