Background: We reviewed the relationship between 4 conformation indices (CIs) and acoustic neuroma volumes to determine how CIs change with tumor volume (TV). Methods: Gamma Knife® (GK) stereotactic radiosurgery (SRS) plans for 96 acoustic neuroma patients were examined and the prescription isodose volume (PIV), T V, and TV receiving the prescription isodose (TV PIV) were recorded. The 4 CIs calculated were the following ratios: PIV/TV, TVPIV/TV, TVPIV/PIV, and TVPIV 2/(TV × PIV). CIs were stratified based upon TV as < 2.0 cm3 (n = 49) or > 2.0 cm3 (n = 47). Results: For TVs < 2.0 cm3, the mean PIV/TV, TVPIV/ TV, TV PIV/PIV, and TVPIV2/(TV × PIV) ratios were 1.984, 0.982, 0.588, and 0.577 respectively. For TVs > 2.0 cm 3, the mean PIV/TV, TVPIV/TV, TVPIV/PIV, and TVPIV2/(TV × PIV) ratios were 1.272, 0.971, 0.777, and 0.754 respectively. Conclusions: The PIV/TV, TVPIV/ PIV, and TVPIV2/(TV × PIV) ratios improved as TV increased whereas the TVPIV/TV ratio decreased. The greatest variance in the CIs was present at very small TVs. We conclude that CIs can be used to compare GK SRS plans for target volumes > 2.0 cm3.