Inter-Observer Variability in Target Volume Delineations of Benign and Metastatic Brain Tumours for Stereotactic Radiosurgery: Results of a National Quality Assurance Programme

Aims: To quantify inter-observer variation between all the intracranial stereotactic radiosurgery (SRS) providers in England in delineating the target volumes of four brain tumour cases.

Materials and methods: Twenty-two, cross-platform SRS providers in England were instructed during a national commissioning assessment to contour the gross tumour volume (GTV) of six brain metastases, one cavernous sinus meningioma, one vestibular schwannoma and one pituitary adenoma. An expert reference group provided feedback if submitted contours were considered to be outliers and those centres were instructed to resubmit their contours. All contours were analysed in Python. The target volume contour (observed volume; V_OBS), encompassing volume, 50% agreement volume (AV₅₀), 100% agreement volume (AV₁₀₀), concordance index (CCI) and discordance index (DCI) were calculated.

Results: Twenty-one centres participated using five different treatment platforms (CyberKnife, Gamma Knife, Varian Linac, Elekta Linac, Tomotherapy) and seven different treatment planning systems (GammaPlan, iPlan, Multiplan, Pinnacle, Eclipse, CMS Focal). The greatest variability was observed in the smallest brain metastases (GTV5 AV₅₀ 0.0 cm³, CCI 0.28-0.84, DCI 0.00-0.70) and pituitary case (AV₅₀ 1.1 cm³, CCI 0.42-0.82, DCI 0.01-0.40). The greatest agreement was observed with the vestibular schwannoma (AV₅₀ 2.8 cm³, CCI 0.77-0.94, DCI 0.00-0.17). There were four resubmissions for the cavernous sinus meningioma and three resubmissions for the pituitary adenoma.

Conclusions: Inter-observer variability was most evident with the smallest brain metastases and pituitary case. Several additional outliers and one acceptable contour were suggested using the metric-based analysis of AV₅₀, CCI and DCI. Comparing contours using these metrics is an effective way to identify whether individual contours are similar to the 'true' target and to flag potentially significant deviations.