Use of Normal Tissue Complication Probability Models in the Clinic

A three-dimensional dose distribution is reduced to a two-dimensional (2D) dose–volume histogram (DVH) by discarding all spatial, anatomic and physiologic data. The 2D graph is then further reduced to a single value of merit, such as the mean dose, the percent of the organ receiving ≥20 Gy (V20), or a model-based normal tissue complication probability (NTCP).

As the (idealized) irradiated organ fraction decreases, the tolerance dose (D) increases, more so for larger values of n or smaller values of a (=1/n). represents the reference volume (usually the full organ volume), and represents the volume irradiated.

Volume–effect parameter. The effect of changing the n parameter (= 1/a) in the Lyman model with the generalized equivalent uniform dose equation to compute normal tissue complication probability (NTCP) is shown. Starting with a (real) rectal dose–volume histogram (DVH) computed for an intensity-modulated radiation therapy (IMRT) prostate patient plan (upper left), the DVH is first transformed into a single number by the generalized equivalent uniform dose (gEUD) equation that weights dose values exponentially. Lower figure shows the cumulative contribution of each part of the DVH to the overall gEUD for all bins below the given dose value. As one can see, if a is set to 1 (rightmost curve), gEUD would equal the mean dose (e.g., for parallel organs), and many voxels with doses as low as 20 to 30 Gy contribute significantly to the gEUD and therefore may increase the final NTCP value (although contributions are proportional to dose, so higher dose still does contribute more for the same volume). As n decreases, the value of gEUD is a determined mainly by the highest dose voxels (e.g., for series organs). Typical clinical values for late rectal bleeding are n ≈ 0.1. Unfortunately, investigators sometimes report a (especially when discussing the gEUD) and other-times use n, where n =1/a.