Measurement of the absolute tumor extension into IAC. On FIESTA-C, the slice with the most lateral extension of tumor is selected for measurement. An auxiliary line (yellow) is drawn between the anterior and posterior lip of the porus acusticus. The length of absolute tumor extension (labeled as L) is defined as the distance between the midpoint of the yellow line and the most lateral point of tumor extension. Defining the absolute extension of tumor in this way is different from assuming the extension of tumor as its projection on the posterior wall of IAC. The latter method appears ideal to pre-operatively define the length of the drilled posterior wall of IAC. However, the presence of tumor may erode the bony structure of the posterior wall of IAC, making delineation of the projection line difficult on pre-operative imaging studies. FIESTA-C, heavily T2-weighted fast imaging employing steady-state acquisition with cycle phase.