Impact of interobserver variability in contouring of AVMs

Abstract

Delineation of the nidus of brain AVMs on digital subtraction angiography (DSA) forms the basis of target definition for radiosurgery (RS). However, interclinician discrepancy in the assessment of nidus position and volume exists. In this study, the magnitude of the interobserver variability as well as its dosimetric and clinical impact are assessed. DSAs from 31 patients who underwent RS for a brain AVM were selected for nidus contouring by six experienced clinicians (2 neurosurgeons, 2 radiation oncologists, 2 neuroradiologists) using BrainScan v5.2 software (BrainLAB AG, Heimstetten, Germany). All patients had a minimum follow-up of 3 years or earlier complete obliteration. The interobserver variability was assessed by 1. the ratio between the volume of agreement (VOA6) of all observers and the corresponding encompassing volume (ECV); 2. the same ratio for all possible pairs of observers (VOA2/ECV2); 3. translation of the center of mass of all individual volumes. Dosimetric consequences were calculated for both the use of standard collimators and for dynamic conformal arcs. The original treatment plan was evaluated for dosimetric coverage of the VOA6. In one patient no VOA6 could be determined due to a large contouring variation. The mean ratio between VOA6 and ECV6 was 0.19 ± 0.14. For all possible pairs of clinicians, the VOA6/ECV6 was 0.45 ± 0.14. The mean translation between the center of mass of the individual volumes was 2.6 mm. The mean coverage of the individual volumes by the 80% ( = prescription isodose)and 75% isodoses was 88.1% and 90.2%, respectively, when standard round collimators were used. Using dynamic arc technique, this was significantly (p = 0.002) lower (78.9% and 82,6%, respectively). Complete obliteration was seen in 24/31 (77%) cases at a mean of 33 months. There was no significant relationship with nidus volume (p = 0.09) and marginal SRS dose (p = 0.09). The VOA6 was incompletely covered by the original treatment plan in 3/31 patients (10%); In 1/24 with complete obliteration (4%) and 2/7 with incomplete obliteration (29%). Failure to obliterate did not correlate with VOA6/ECV6 (p = 0.69), VOA2/ECV2 (p = 0.56), but was significantly correlated with the mean coverage of individual volumes by the 80% isodose (p = 0.004). There is a significant interobserver variability in the contouring of the nidus on DSA and a better definition is required. This variability may result in underdosage. The clinical impact of this will increase with the use of more conformal stereotactic techniques