Error in the parotid contour delineated using computed tomography images rather than magnetic resonance images during radiotherapy planning for nasopharyngeal carcinoma.

Purpose: To analyze the intra- and interobserver variations errors in the parotid contour delineated using computed tomography (CT) or magnetic resonance (MR) images of patients with nasopharyngeal carcinoma who underwent radiotherapy.

Materials And Methods: Forty-one nasopharyngeal cancer patients were selected. The patients underwent simulation with MR and CT scanning. The gross tumor volume and organs at risk were contoured on both contrasted CT and T1-MR images. For each patient, one radiotherapist delineated the parotid on CT and MR images ten times, and ten different radiotherapists were asked to delineate the parotid on CT and MR images once each. The inter- and intraobserver variations in volume and outline were compared.

Results: For the interobserver comparison, the volumes of the parotid as evaluated from the CT and MR images were 34.6 ± 12.1 cm(3) (left), 34.3 ± 9.0 cm(3) (right), and 24.6 ± 7.6 cm(3) (L), 23.2 ± 8.1 cm(3) (R), respectively. For the intraobserver comparison, the volumes evaluated from the CT and MR images were 28.2 ± 7.6 cm(3) (L), 29.4 ± 9.4 cm(3) (R), and 24.4 ± 7.6 cm(3) (L), 22.5 ± 7.4 cm(3) (R), respectively. The relative variations in volume when using MR images were 4.7 ± 0.7 % (L), 5.0 ± 0.6 % (R) for the interobserver variations and 2.3 ± 0.4 % (L), 2.1 ± 0.7 % (R) for the intraobserver variations. However, the inter- and intraobserver relative variations in volume when using CT images were 18.0 ± 4.8 % (L), 17.4 ± 4.6 % (R) and 6.3 ± 1.5 % (L), 6.8 ± 1.5 % (R), respectively.

Conclusion: The parotid contour is delineated more accurately and reproducibly on MR images than on CT images.