Sci-Thur AM: Planning - 09: Assessing dynamic IMRT field modulation in prostate plans

In a previous study, the variogram fractal dimension (FD) method was found to be very accurate at identifying planned head and neck IMRT fields that are overly-modulated. In the current study, the authors used MATLAB® to develop FracMod, a graphical user interface (GUI) and variogram FD analysis tool to assess modulation complexity of dynamic IMRT fields designed for treatments of the prostate alone and prostate plus pelvic nodes. A set of 5 prostate plans (25 fields) and 5 prostate plus pelvic node plans (35 fields) were used to choose FD cut-points that ensure no false positives (100% specificity) in distinguishing between moderate field modulation (typical modulation used clinically at the authors' institution) and high modulation. Field modulation was controlled by adjusting fluence smoothing parameters in the Eclipse™ treatment planning system. The area under the curve (AUC) from receiver operating characteristic (ROC) analysis was used to quantitatively compare the ability of FD and the number of monitor units (MUs) for distinguishing between the moderate and high modulation fields. The variogram FD method gave AUCs of 0.96 (almost perfect classification) and 1.00 (perfect classification) for the prostate alone and the prostate plus pelvic node fields, respectively. The variogram FD method is an accurate metric; performing better than the number of MUs at identifying high modulation IMRT fields planned for the treatment of prostatic carcinoma. Hence, FracMod will enable Radiotherapy Physicists to easily and accurately quantify the degree of modulation of IMRT fields and adjust overly-modulated fields at the treatment planning stage.