Background. Diffusion weighted imaging (DWI) has gained interest as an imaging modality for assessment of tumor extension and response to cancer treatment. The purpose of this study is to assess the impact of the choice of b-values on the calculation of the Apparent Diffusion Coeffi cient (ADC) for locally advanced gynecological cancer and to estimate a stable interval of diffusion gradients that allows for best comparison of the ADC between patients and institutions. Material and methods. Six patients underwent a high resolution single shot EPI based DWI scan with 16 different diffusion gradients on a 3 Tesla Philips Achieva MR-scanner. Data analysis was performed by applying a monoexponential and a biexponential model to the acquired data. The biexponential function models the effect of both perfusion and diffusion. Results and conclusion. ADC changes of up to 40% were seen with the use of different b-values. Using a lower b-value Ն 150 s/mm 2 and an upper b-value Ն 700 s/mm 2 limited the variation to less that 10% from the reference ADC value. By eliminating the contribution of perfusion the uncertainty of quantitative ADC values were signifi cantly reduced.
Background and purpose. T2 weighted MRI is recommended for image guided adaptive brachytherapy (IGABT) in cervical cancer. Diffusion weighted imaging (DWI) and the derived apparent diffusion coeffi cient (ADC) may add additional biological information on tumour cell density. The purpose of this study was to evaluate the distribution of the ADC within target volumes as recommended by GEC-ESTRO: Gross Tumour Volume at BT (GTV BT), High-Risk Clinical Tumour Volume (HR-CTV) and Intermediate-Risk Clinical Target Volume (IR-CTV) and to evaluate the change of diffusion between fractions of IGABT. Material and methods. Fifteen patients with locally advanced cervical cancer were examined by MRI before their fi rst (BT1) and second (BT2) fraction of IGABT, resulting in a total of 30 MR examinations including both T2 weighted and DWI sequences. The Apparent Diffusion Coeffi cient (ADC) was calculated by use of three levels of b-values (0, 600, 1000 s/mm 2). ADC maps were constructed and fused with the GEC ESTRO target contours. The mean ADC value within each target volume was calculated. Furthermore, volumes of low diffusion (ADC low) were defi ned based on an ADC threshold of 1.2 ϫ 10 Ϫ3 mm 2 /s, and overlap with target volumes was evaluated. Change of ADC level in target volumes and change of ADC low volume from BT1 to BT2 was also evaluated. Results. The mean ADC was signifi cantly lower in GTV BT than in HR-CTV (p Ͻ 0.001) which again was signifi cantly lower than in IR-CTV (p Ͻ 0.001). There was no signifi cant change of the ADC low volume or ADC level within each target structure between BT1 and BT2 (p ϭ 0.242). All three GEC-ESTRO volumes contained volumes with low diffusion. The GTV BT contained 37.2% volume of low diffusion, HR-CTV 20.3% and IR-CTV 10.8%. Conclusion. With DWI we were able to fi nd a signifi cant difference in ADC-values for the three different GEC ESTRO targets. This supports the assumption that the target volumes used for dose prescription in IGABT contain tissues with different characteristics, with the tumour (GTV BT) being the volume with the lowest diffusion. No signifi cant changes were found from BT1 to BT2 indicating that changes of ADC level and volumes are stable at the time of BT. Further studies are needed to evaluate the role of DWI in target contouring and dose prescription for IGABT.
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