Quantitative assessment of inter-observer variability in target volume delineation on stereotactic radiotherapy treatment for pituitary adenoma and meningioma near optic tract

Background: To assess inter-observer variability in delineating target volume and organs at risk in benign tumor adjacent to optic tract as a quality assurance exercise. Methods: We quantitatively analyzed 21 plans made by 11 clinicians in seven CyberKnife centers. The clinicians were provided with a raw data set (pituitary adenoma and meningioma) including clinical information, and were asked to delineate the lesions and create a treatment plan. Their contouring and plans (10 adenoma and 11 meningioma plans), were then compared. In addition, we estimated the influence of differences in contouring by superimposing the respective contours onto a default plan. Results: The median planning target volume (PTV) and the ratio of the largest to the smallest contoured volume were 9.22 cm(3) (range, 7.17 - 14.3 cm(3)) and 1.99 for pituitary adenoma, and 6.86 cm(3) (range 6.05 - 14.6 cm(3)) and 2.41 for meningioma. PTV volume was 10.1 +/- 1.74 cm(3) for group 1 with a margin of 1 -2 mm around the CTV (n = 3) and 9.28 +/- 1.8 cm(3)(p = 0.51) for group 2 with no margin (n = 7) in pituitary adenoma. In meningioma, group 1 showed larger PTV volume (10.1 +/- 3.26 cm(3)) than group 2 (6.91 +/- 0.7 cm(3), p = 0.03). All submitted plan keep the irradiated dose to optic tract within the range of 50 Gy (equivalent total doses in 2 Gy fractionation). However, contours superimposed onto the dose distribution of the default plan indicated that an excessive dose 23.64 Gy (up to 268% of the default plan) in pituitary adenoma and 24.84 Gy (131% of the default plan) in meningioma to the optic nerve in the contours from different contouring. Conclusion: Quality assurance revealed inter-observer variability in contour delineation and their influences on planning for pituitary adenoma and meningioma near optic tract.