Variations in radiotherapy delineation of the lateral compartments in patients with rectal cancer: results after an updated national guideline.

Background and purpose: In rectal cancer, accurate delineation is crucial for patients with enlarged lateral lymph nodes to minimize lateral local recurrence risk. This study aims to evaluate inter-physician variation in delineation of the lateral compartments, and the impact of training and implementation of standardized delineation protocols.

Methods: Twenty-three radiation oncologists from 15 institutes delineated the clinical target volume (CTV) on CT-scan in one example of a patient with rectal cancer. Parallel to this, the national consensus guideline was updated. Participating radiation oncologists completed an e-learning and online training session. Subsequently, 12 radiation oncologists re-delineated the same case. Variation was measured with the Dice-score and 95% Hausdorff distance.

Results: Considerable inter-physician variation was present before guideline distribution; and larger in the anterior compartment than the posterior compartment (Dice score 0.66 vs 0.80, p<0.01). After training, there was a significant improvement in 95% Hausdorff distance for the lateral compartments together (0.71 vs 1.02, p=0.02), but not in Dice score (0.76 vs 0.78, p=0.31), and neither for the anterior and posterior compartment separately. While delineation variation in the ventral and lateral sides decreased, the variation in the caudal side of the anterior compartment increased.

Conclusion: Substantial delineation variation in CTV of the lateral compartments in rectal cancer cases exists. This can be reduced by implementation of a delineation guideline with clear anatomical borders and subsequent training. Despite reduction in 95% Hausdorff distance, there is still need for further improvement in specific areas to assure adequate delineation.