Diffusion-weighted magnetic resonance imaging of head and neck squamous cell carcinomas

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Abstract

Objective

To evaluate whether diffusion-weighted imaging (DWI) is a reliable technique to quantify microstructural differences between head and neck squamous cell carcinomas (SCC) and tumour-free soft tissue.

Materials and methods

DWI was obtained from 20 patients with histologically proven, untreated head and neck SCC. DWI was acquired using a diffusion-weighted, navigated echo-planar imaging sequence with a maximum b-value of 800 s/mm2. For an objective assessment of image quality, the signal-to-noise ratio (SNR) was calculated. Microstructural differences between vital tumour tissue and tumour-free soft tissue were quantified by calculating the apparent-diffusion-coefficients (ADC) on a pixel by pixel method.

Results

Echo-planar DWI provided good image quality in all patients (mean SNR 18.4). The mean ADC of SCC, (0.64 ± 0.28 × 10−3 mm2/s), was significantly (P < 0.0001) lower than that of the tumour-free soft tissue, (2.51 ± 0.82 × 10−3 mm2/s).

Conclusion

DWI is a reliable diagnostic tool to quantify the microstructural differences between vital tumour tissue and tumour-free soft tissue in patients with head and neck SCC.

Introduction

Head and neck cancer is a significant cause of morbidity and mortality worldwide, with about 400,000 new cases per year [1]. At the time of diagnosis the majority of these patients present with loco regionally advanced disease requiring an amplified and aggressive treatment regime consisting of neoadjuvant therapy and extended surgery [2]. Patients treated for advanced head and neck cancer may suffer from severe sequelae such as disfigurement, speech disorders, dry mouth, constriction of local tissues, chewing and swallowing dysfunction. Furthermore, head and neck cancer causes high health care costs and is therefore a considerable challenge to health care providers. Both sequelae and health care costs can be diminished with optimized treatment, which requires accurate estimation of tumour size and tumour extent. For such estimation accurate discrimination of vital tumour tissue and tumour-free soft tissue is mandatory.

Currently computed tomography and magnetic resonance imaging (MRI) are the two imaging modalities of choice for establishing the diagnosis and for staging head and neck cancer. Although both diagnostic tools yield high accuracy in detecting and staging such cancers, the discrimination of benign versus malignant tissues is sometimes difficult [3]. This shortcoming is mainly due to the fact that both of these techniques may only evaluate the macroscopic morphology of biologic tissues without reflecting the microstructural differences. The application of contrast agent is also lacking specificity in this respect, because tumours, therapy induced apoptosis and tumour necrosis will show substantial contrast enhancement, which may lead to false positive diagnosis in up to 46% of cases [3].

Diffusion-weighted imaging (DWI) is a non-invasive technique which analyzes the structure of a biologic tissue at a microscopic level and may therefore be a potential tool for evaluating the microstructural differences between vital tumour tissue, and tumour-free tissue [4], [5]. The principle of DWI is to exploit the translational motion of water protons in biologic tissues, which is caused by the Brownian motion [6]. On diffusion-weighted MRI translational motion causes phase dispersion of excited water protons, which consecutively leads to a signal loss on DWI [5], [7]. Signal loss can be quantified by calculating the “apparent-diffusion-coefficient” (ADC), which refers to the specific diffusion capacity of biologic tissue [8].

In squamous cell cancer (SCC) the translational motion of water protons is substantially diminished because of its hypercellularity and the increased amount of diffusion hindering obstacles present in this tumour resulting in a restricted diffusion capacity of the tumour tissue compared to benign tissue. This concept of restricted diffusivity has been derived from theoretical and biological diffusion models, from in vitro studies and from animal models [4], [7], [9], [10]. On DWI restricted diffusion in tumour tissue results in high signal intensities whereas not restricted diffusion in benign tissue leads to a significant signal loss [4]. Thus, the differences in the microstructure of benign and neoplastic tissues result in substantially diverse diffusion capacities, which in turn lead to the specific contrast of benign and malignant tissues on DWI [10], [11]. It is therefore conceivable that quantitative analysis [12], [13] of DWI will provide an objective parameter to differentiate benign from neoplastic tissues.

The purpose of this study was to evaluate whether DWI is a feasible and reliable technique for differentiating head and neck SCC from tumour-free tissue based on the differences in their diffusion capacities.

Section snippets

Materials and methods

Between October 2003 and August 2005 we prospectively investigated all patients with known or clinically suspected head and neck SCC with DWI, who agreed to volunteer for this study. Written informed consent was obtained from all these patients. Patients with either previous therapy or those without biopsy for histological correlation within 2 months of scanning were excluded. Twenty patients with histologically proven SCC were included into the study cohort.

MRI was performed using a 1.0 T MRI

Results

DWI showed good image quality in all patients with a mean SNR of 18.4 (minimal SNR 2.2).

The mean ADC of the SCC was 0.64 ± 0.28 × 10−3 mm2/s, which was significantly (P < 0.001) lower than the mean ADC of the tumour-free soft tissue (2.51 ± 0.82 × 10−3 mm2/s) (Fig. 1, Fig. 2). The 95% CI of SCC and tumour-free soft tissue was 0.51–0.77 × 10−3 mm2/s and 2.13–2.90 × 10−3 mm2/s, respectively. The 95% confidence intervals of these groups showed no overlap.

The intra-individual difference between the ADCs of the SCC

Discussion

DWI has attracted attention as a non-invasive imaging modality for tissue characterization in oncologic radiology [5], [16], [17], [18], [19]. In several studies a specific contrast between benign tissue and tumours has been reported to be present on DWI, which makes this imaging technique a powerful tool in the evaluation and in staging of neoplasia [4], [10].

First reports of the feasibility of DWI in onco-radiology were published on tumours of the musculoskeletal system [16], [17], [20].

Conclusions

DWI provides good image quality for reliable diffusion analysis of head and neck SCC. The microstructural differences between head and neck SCC and tumour-free tissue result in a specific contrast on DWI and the calculation of the tissue specific ADC allows quantitative analysis of this specific contrast. Hence, the significant difference between the ADCs of SCC and tumour-free tissue provides an objective and un-biased parameter for tissue characterization.

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