Abstract W P422: ABC/2 Does Not Accurately Predict Volume of Arteriovenous Malformations

Abstract

Introduction: Stereotactic radiosurgery (SRS) represents an important method of curing arteriovenous malformations (AVMs) and preventing associated hemorrhages. The decision to proceed with SRS is commonly based on calculated nidal volume, as smaller AVMs are more likely to obliterate. When counseling patients for SRS, physicians commonly use the ‘ABC/2’ method. We aim to understand whether such an approximation is accurate when compared to the exact volume calculated from thin-cut axial sections used for SRS planning. Hypothesis: AVM volume calculated by the ‘ABC/2’ method on digital subtraction angiography (DSA) is significantly different than the volume calculated using SRS planning techniques. Methods: 85 AVMs treated with SRS were identified from our database. Maximum nidal diameters in orthogonal planes on DSA images were recorded to determine volume by the ‘ABC/2’ formula. The nidal target volume was extracted from operative reports of SRS. The volumes were then compared using descriptive statistics and the paired t-test. Results: The average volume was 7.1 cm 3 (SD 7.94) when using SRS planning methods, and 11.2 cm 3 (SD 15.6) when using the ABC/2 methodology. Moderate correlation was seen between ABC/2 and SRS (r = 0.657; p <0.001). Paired samples T-test revealed significant differences between the SRS volume and ABC/2 (t = -3.2; p = 0.002). When the AVMs were stratified based on an ABC/2 volume, the significant differences remained (t = -2.1; p = 0.046 for ABC/2 volume less than 7 cm 3 and t= -4.2; p <0.001 for ABC/2 volume greater than 7 cm 3 ). Comparisons of AVM volume using SRS planning techniques with analysis utilizing equations for the volumes of various shapes (cones, cylinders, and spheres) demonstrated significant differences with cones and solids (p < 0.001 in both) while spherical volume was statistically indistinguishable from SRS planning volumes (p = 0.388). Conclusions: ‘ABC/2’ on DSA is not an accurate method of calculating AVM nidal volumes as it overestimates the volume by 57.7%. As a result, practitioners may choose not to offer SRS - incorrectly concluding that a particular AVM is too large to have an appropriate response to SRS. We propose that SRS planning techniques should be utilized before counseling patients on treatment.