Usefulness of Integrated PET/MRI in Oropharyngeal Squamous Cell Carcinoma Evaluation

Introduction and aim of study:

Head and neck cancer (HNC) continues to be a significant health care problem in Taiwan and oropharyngeal squamous cell carcinoma (SCC) is the common subtype. With the concern of organ preservation in recent years, concurrent chemoradiation is the major treatment modality for oropharyngeal SCC. Hence, accurate tumor anatomic and biologic evaluations are warranted for pretreatment planning, posttreatment monitoring and prognosis determination. Endoscopy with biopsy serve as the main diagnostic tools in patients with oropharyngeal SCC. While computed tomography (CT) and magnetic resonance imaging (MRI) are commonly used to evaluate the tumor extent of HNC, MRI is more preferred in the oropharyngeal area by virtue of its high contrast resolution, particularly in detecting perineural invasion. Whole body MRI is now feasible and thus distant site status of HNC can be evaluated in the single examination session. FDG- positron emission tomography/CT (PET /CT) is another common imaging modality to evaluate HNC.

According to the investigators' experience in patients with head and neck cancer, MRI and PET/CT have different advantages over each other and may complement to each other. In evaluating primary tumor status, the high spatial and contrast resolution of MRI can delineate the tumor extent from surrounding normal tissue in the complex head and neck anatomical region, while delineation of FDG-positive tissue by PET may help differentiation tumor growth from surrounding noncancerous tissue, particularly in irradiated bed. In evaluating regional nodal status, MRI is superior to PET/CT in detecting retropharyngeal or cystic necrotic metastatic adenopathy, whereas PET/CT excels MRI in detecting metastasis in sub-centimeter nodes. The combined use of MRI and PET can clearly demonstrate the patterns of nodal spread.[7]. Regards to the evaluation of the distant sites, whole body MRI is better in detecting metastatic lesion in high metabolic organs such as brain, liver or spleen, while PET/CT is superior in detecting those metastatic lesions in the curved, flat bones, and second primary bowel tumor such as esophagus cancer or colon cancer.

Currently, diffusion-weighted MR imaging (DWI) and dynamic contrast-enhanced perfusion weighted imaging (DCE-PWI) become clinically feasible to assess the functional aspects of HNC. The quantified parameter of DWI sequence is apparent diffusion coefficient (ADC) that relates to cellularity. DCE-PWI provides the volume transfer rate constant (Ktrans), relative extravascular extracellular space (Ve) and relative vascular plasma volume (Vp) as well as the efflux rate constant (Kep). These functional MRI techniques may provide biologic information, such as cellularity, angiogenesis and permeability. On the other hand, PET can provide its metabolic quantified parameters: standardized uptake value (SUV) that reflects glucose metabolism, metabolic tumor volume (MTV) that reflects tumor burden, and total lesion glycolysis (TLG) that integrates both glucose metabolism and tumor burden. These MRI and PET functional parameters appear promising for predicting chemoradiation response and prognosis of HNC. In our previous studies of oropharyngeal or hypopharyngeal SCC patients, Ktrans of the primary tumor was the only imaging parameter associated with local control, while ADC and Ve values of the neck metastatic nodes were independent prognostic factors for neck control. The maximal SUV of the regional lymph nodes was significantly associated with the occurrence of distant metastasis. TLG were significantly associated with overall survival.

Integrated PET/MRI is a novel imaging technology introduced into the clinical practice. The combination of PET and MRI in one single, hybrid scanner is promising. Initial data convey that PET/MRI performed favorably for HNC. It showed good diagnostic capability similar to PET/CT and can serve as a legitimate alternative to PET/CT in the clinical workup of patients with HNC. The metabolic ratios measured PET/MRI showed excellent agreement with those on standard PET/CT. PET/MRI incorporating the Dixon sequence for attenuation correction purposes yielded similar SUV values when compared to PET/CT.Nevertheless, most studies were limited in number of cases or no particular tumor entity was specifically analyzed, which somewhat weakened their power in terms of evidence-based medicine. In addition, the predictive value of PET/MRI in treatment outcome of HNC has not been reported. A prospective study of PET/MRI in a large cohort of patients with specific tumor origin and uniform treatment protocol would complement these pioneering efforts to fully validate the clinical usefulness of the novel integrated system.