Method to account for dose fractionation in analysis of IMRT plans: modified equivalent uniform dose.

PURPOSE To propose a modified equivalent uniform dose (mEUD) to account for dose fractionation using the biologically effective dose without losing the advantages of the generalized equivalent uniform dose (gEUD) and to report the calculated mEUD and gEUD in clinically used intensity-modulated radiotherapy (IMRT) plans. METHODS AND MATERIALS The proposed mEUD replaces the dose to each voxel in the gEUD formulation by a biologically effective dose with a normalization factor. We propose to use the term mEUD(D(o))(/n(o)) that includes the total dose (D(o)) and number of fractions (n(o)) and to use the term mEUD(o) that includes the same total dose but a standard fraction size of 2 Gy. A total of 41 IMRT plans for patients with nasopharyngeal cancer treated at our institution between October 1997 and March 2002 were selected for the study. The gEUD and mEUD were calculated for the planning gross tumor volume (pGTV), planning clinical tumor volume (pCTV), parotid glands, and spinal cord. The prescription dose for these patients was 70 Gy to >95% of the pGTV and 59.4 Gy to >95% of the pCTV in 33 fractions. RESULTS The calculated average gEUD was 72.2 +/- 2.4 Gy for the pGTV, 54.2 +/- 7.1 Gy for the pCTV, 26.7 +/- 4.2 Gy for the parotid glands, and 34.1 +/- 6.8 Gy for the spinal cord. The calculated average mEUD(D(o))(/n(o)) using 33 fractions was 71.7 +/- 3.5 Gy for mEUD(70/33) of the pGTV, 49.9 +/- 7.9 Gy for mEUD(59.5/33) of the pCTV, 27.6 +/- 4.8 Gy for mEUD(26/33) of the parotid glands, and 32.7 +/- 7.8 Gy for mEUD(45/33) of the spinal cord. CONCLUSION The proposed mEUD, combining the gEUD with the biologically effective dose, preserves all advantages of the gEUD while reflecting the fractionation effects and linear and quadratic survival characteristics.