Delineation of the larynx as organ at risk in radiotherapy: a contouring course within "Rete Oncologica Piemonte-Valle d'Aosta" network to reduce inter- and intraobserver variability.

Aims: To evaluate the usefulness of a contouring course in reducing inter- and intraobserver variability in the definition of the larynx as organ at risk (OAR).

Methods: Within the "Rete Oncologica Piemonte-Valle d'Aosta" network, a contouring course focusing on larynx delineation was proposed. Twenty-six radiotherapist technicians (RTTs) experienced in delineating OARs were asked to contour larynx before and after the training. An expert radiation oncologist defined the reference volume for educational purpose. The contoured volumes obtained before and after the course were compared using descriptive statistics (mean value, standard deviation-SD, and coefficient of variation-COV) of volumes and maximum diameters. Conformity index (CI), dice coefficient (DC), and percentage of overlap were used to evaluate the spatial accuracy of the different volumes compared to the reference. Further analysis regarding the variation in the centre of mass (COM) displacement was performed.

Results: The mean volume was 40.4 cm before and 65.9 cm after the course, approaching the reference value. Mean anteroposterior, laterolateral, and craniocaudal diameters improved, getting each closer to the reference. Moreover, the COM moved approaching reference coordinates. Mean percentage of intersection and DC strongly increased after the course, rising from 57.76 to 93.83 % and from 0.68 to 0.89, respectively. CI enhanced from 0.06 to 0.31.

Conclusions: This study shows an improvement in larynx definition after the contouring course with lower interobserver variability and major consistency compared to the reference volume. Other specific educational activities may further increase the quality of radiation therapy contouring in this setting.