Variations in Radiation Therapy Delineation of the Lateral Compartments in Patients with Rectal Cancer: results after an updated national guideline

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 interphysician variation in delineation of the lateral compartments, and the impact of training and implementation of standardized delineation protocols. Methods and Materials: Twenty-three radiation oncologists from 15 institutes delineated the clinical target volume (CTV) on computed tomography scan in 1 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 redelineated the same case. Variation was measured with the Dice score and 95% Hausdorff distance. Results: Considerable interphysician variation was present before guideline distribution; and larger in the anterior compartment than the posterior compartment (Dice score 0.66 vs 0.80, P < .01). After training, there was a significant improvement in 95% Hausdorff distance for the lateral compartments together (0.71 vs 1.02, P = .02), but not in Dice score (0.76 vs 0.78, P = .31), and neither for the anterior and posterior compartment separately. Whereas delineation variation in the ventral and lateral sides decreased, the variation in the caudal side of the anterior compartment increased. Conclusions: 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 anatomic borders and subsequent training. Despite reduction in 95% Hausdorff distance, there is still need for further improvement in specific areas to assure adequate delineation.