Dosimetric effects of magnetic resonance imaging-assisted radiotherapy planning: Dose optimization for target volumes at high risk and analytic radiobiological dose evaluation

Based on the assumption that apparent diffusion coefficients (ADCs) define high-risk clinical target volume (aCTV_HR) in high-grade glioma in a cellularity-dependent manner, the dosimetric effects of aCTV_HR-targeted dose optimization were evaluated in two intensity-modulated radiation therapy (IMRT) plans. Diffusion-weighted magnetic resonance (MR) images and ADC maps were analyzed qualitatively and quantitatively to determine aCTV_HR in a high-grade glioma with high cellularity. After confirming tumor malignancy using the average and minimum ADCs and ADC ratios, the aCTV_HR with double- or triple-restricted water diffusion was defined on computed tomography images through image registration. Doses to the aCTV_HR and CTV defined on T1-weighted MR images were optimized using a simultaneous integrated boost technique. The dosimetric benefits for CTVs and organs at risk (OARs) were compared using dose volume histograms and various biophysical indices in an ADC map-based IMRT (IMRT_ADC) plan and a conventional IMRT (IMRT_conv) plan. The IMRTADC plan improved dose conformity up to 15 times, compared to the IMRT_conv plan. It reduced the equivalent uniform doses in the visual system and brain stem by more than 10% and 16%, respectively. The ADC-based target differentiation and dose optimization may facilitate conformal dose distribution to the aCTV_HR and OAR sparing in an IMRT plan.