The authors develop and investigate a user-friendly computational tool (FracMod) to quantify modulation complexity in planned IMRT fields. FracMod comprises a graphical user interface and variogram fractal dimension (FD) analysis tool developed by the authors using MATLAB(®), and made freely available at http://www.medphysfiles.com/index.php. FracMod is investigated for its ability to identify overly-modulated dynamic IMRT fields designed for prostatic carcinoma treatments. A set of 5 prostate alone plans and 5 prostate plus pelvic node plans were used to choose FD cut-points that ensure no false positives in distinguishing between moderate and high field modulation. IMRT quality control (QC) was performed on all the treatment fields using Varian(®) Portal Dosimetry and MapCHECK™. Receiver operating characteristic analysis was used to quantitatively compare the classification performance of FD and the number of monitor units (MUs). The effect of dose rate on the average leaf pair opening (ALPO) and the number of MUs delivered was also investigated. The variogram FD performed better than the number of MUs in identifying overly-modulated fields. FD thresholds >2.15 for prostate alone and >2.20 for prostate plus pelvic nodes correctly identified 75% and 100% of the high modulation fields, respectively, with no false positives. With appropriate cut-points, MapCHECK™ identified the most highly modulated IMRT fields, whereas Varian(®) Portal Dosimetry could not. As expected, ALPO decreases with increasing modulation and increasing dose rate. FracMod is a user-friendly tool that allows one to accurately quantify and identify overly-modulated IMRT fields at the treatment planning stage before they are sent for patient-specific QC.