THE ACCURACY OF BRAIN METASTASES CONTOURING FOR STEREOTACTIC RADIOSURGERY (SRS) BY NEURO-ONCOLOGISTS; WHAT DO THE NEURO-RADIOLOGIST AND NEURO-SURGEON ADD?

Variability exists between clinical oncologists when contouring gross tumour volume (GTV) and normal tissue organs at risk (OAR) volumes. This variability is the ‘weakest link’ in the context of the highly conformal and highly accurate treatment delivery used for SRS. In 2016-17NHS England commissioned 17 SRS centres. The service specification mandates “treatment protocols will ensure that target definition is performed by either a sub-specialised neuro-surgeon and / or neuro-oncologist (clinical oncologist) with input from a neuro-radiologist before a treatment plan is created.”. To evaluate the additional contribution by the neuro radiologist we analysed contouring conformality for all 47 patients treated in our centre between June 2017 and February 2018. GTV margins were contoured first by the clinical oncologist, and then copied and amended with input from the neuroradiologist and sometimes neurosurgeon. 1 mm margin was added to the GTV for the planning target volume (PTV). 47 patients, each with 1–4 metastases/resection cavities, resulted in treatment plans for 67 metastases/cavities. Doses used were: 15-24Gy in 1 fraction, 24Gy in 3 fractions or 25-30Gy in 5 fractions (if close to critical optic structures or brainstem). We used Eclipse TPS (v13.7) for Varian (Palo Alto, CA) Clinac iX with millennium MLC (5 mm) and Exactrac imaging system (Brainlab, Munich DE). All plans were created either using dynamic conformal arc (DCA) or VMAT RapidArc (RA) techniques with 6 MV photons and calculated using AAA (v10) on a 1 mm dose grid. Values for the final treated GTV and PTV (A) were compared with the GTV and PTV that were generated by the Clinical oncologist alone (B) will be compared using the Conformity analysis consisted of Jaccard coefficient, Dice coefficient, Geographical Miss and Discordance index as defined below. Results will be presented.