Apparent diffusion coefficients in GEC ESTRO target volumes for image guided adaptive brachytherapy of locally advanced cervical cancer

Abstract

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.

Clinical Applications for Diffusion Magnetic Resonance Imaging in Radiotherapy

“…Another study investigated the ADC values in the GEC ESTRO target volumes for image guided adaptive brachytherapy of locally advanced cervical cancer. 55 Using an ADC of 1.2x10 −3 mm 2 /s as the threshold to define low diffusion, they found that 37%, 20% and 11% of respective brachytherapy gross tumor volume, high-risk clinical target volume, and intermediate-risk target volume had low diffusion. These studies suggest that it is worthwhile to further investigate using ADC imaging as a means to define a boost target volume for advanced cervical cancers by either a highly conformal external beam radiation therapy boost plan or with brachytherapy.…”

Magnetic Resonance Image Guided Brachytherapy

“…However, it is currently not clear if low-perfusion regions represent radioresistant sub-volumes, and so far DCE MRI has not been incorporated into brachytherapy treatment planning. DWI at the time of cervix brachytherapy has been applied for evaluation of the extent of the residual GTV (39; 40) (Fig. 2).…”

Advances in radiotherapy: from 2D to 4D

“…Apparent diffusion coefficients (ADC) determined by DW-MRI correlate significantly with tumour cell density in cervical cancer and it is hypothesized that radiation dose should be intensified in volumes with high ADC values [ 8 ] . Dynamic contrast-enhanced MRI also seems promising in identification of cancer in the prostate, which may be useful for even further escalation of the radiation dose to focal tumour dense volumes [ 9 ] .…”