ESTRO 2021 Abstract Book

S474

ESTRO 2021

patient to all the Danish radiotherapy centers treating radiotherapy for brain tumours. Based on OAR delineations on this MRI the guidelines was finalized during a two day workshop. Following this, ten clinical oncologists with experience in neuro-oncology contoured the following OARs on a T1-weighted gadolinium enhanced MRI from 13 brain tumor patients: optic tract, optic nerve, chiasm, spinal cord, brainstem, pituitary gland and hippocampus, illustrated in Figure 1. Delineations was compared using customised MatLab scripts developed for the Danish Comprehensive Cancer Center for Radiotherapy (www.dcccrt.dk) workgroup 5 – treatment planning and automation. The metrics used for comparison was the dice similarity coefficient (Dice) and mean surface distance (MSD).

Results A total of 968 contours were delineated across the 13 patients. On average eight (range six to nine) individual contour sets were made per patient. The Dice and MSD are shown in Figure 2. Good agreement was found across all structures with a median MSD below 1 mm for most structures, with the chiasm performing the best with a median MSD of 0.45 mm. The Dice was primarily included to allow comparison to previously published studies and was also in this study found to be highly volume dependent. The brainstem (the largest structure) had the highest Dice value with a median of 0.89, although the median MSD with a value of 0.96 mm was slightly higher than the average value. The spinal cord was not contoured in its full length, many observers leaving out the most caudal end, hence metrics are not shown in the figure.