| Abstract: |
Most head and neck cancers are squamous cell carcinomas (HNSCC) arising in the oral cavity, oropharynx, larynx, and hypopharynx. The incidence of HNSCC is rising in the industrialized world, and there is increasing recognition of the role of human papillomavirus (HPV) infection as an etiologic factor in the development of this disease. CT or MRI is essential to define the extent of the primary tumor, assess lymph node size, and detect bone and bone marrow disease. [18F]FDG PET/CT imaging from the skull base to the floor of the pelvis is essential in patients with locally advanced disease and in all patients who are candidates for definitive chemoradiotherapy to define N and M staging. Interpretation of head and neck PET/CT requires expertise in head and neck anatomy and an understanding of variations of physiologic [18F]FDG uptake in normal tissues. Scan interpretation is primarily based on visual assessment, although SUV measurements may be helpful. However, SUV is related to lesion size and may be underestimated in small primary tumors. Most primary tumors originate in the palatine tonsil, base of tongue, pyriform sinus, nasopharynx, and larynx. HNSCC tends to spread locally with invasion of adjacent structures. T staging is routinely done by either CT or MRI, not PET. However, occasionally [18F]FDG PET may visualize small submucosal primary tumors that are difficult to distinguish from adjacent normal tissues with anatomic imaging studies. The presence of nodal metastases is an independent prognostic factor for survival in patients with head and neck cancer and may affect the choice of surgical or radiation therapy. Early and complete removal of neck node metastases is a prerequisite for cure. PET provides added value in the initial nodal staging. However, false-positive [18F]FDG uptake can occur in inflamed, reactive lymph nodes. False-negative [18F]FDG PET studies may occur when nodes have a small tumor burden, cystic degeneration of metastatic nodes with a small rim of viable tumor tissue, or low tracer uptake in the metastatic node. Nodal metastases in close proximity to the primary tumor may not be detectable as separate hypermetabolic focus when the primary tumor shows very intense tracer uptake. Distant metastases are rare in patients with head and neck cancers, but the frequency increases with higher T stage and size, number of tumor-involved lymph nodes, jugular vein invasion, and in primary tumors in the hypo-and oropharynx. The most common location is the lung, followed by bone and liver. [18F]FDG PET/CT is sensitive for the detection of distant metastases. In patients treated with definitive chemoradiotherapy, [18F]FDG PET/CT should be done approximately 12 weeks after the end of therapy to assess response to treatment. [18F]FDG PET/CT imaging is particularly useful to detect residual disease in neck lymph nodes. Clinical parameters and structural imaging cannot reliably predict the presence of residual metastatic neck disease; "planned neck dissection" was usually performed in the past after the completion of chemoradiotherapy, especially in patients with initial N2-N3 disease. Recently, in light of the high negative predictive value of posttreatment [18F]FDG PET, this approach seems no longer justified. In patients with suspected recurrence, [18F]FDG PET/CT has the highest sensitivity and specificity regardless of the primary treatment modality. However, post-therapy inflammation remains a potential source for false-positive interpretation. Carcinoma of unknown primary tumors accounts for approximately 1-2% of head and neck cancers. Occurrence of nodal metastases in neck levels I-III increases the likelihood for a primary HNSCC. However, in 5-40% of cases a primary malignancy is not identified during diagnostic evaluation, and these patients undergo extensive workup, including panendoscopy of the upper aerodigestive tract and [18F]FDG PET/CT. The use of [18F]FDG PET/CT may reduce the number of panendoscopies. Other imaging studies are usually not needed, because it is extremely unlikely that they m y identify a primary tumor that cannot be detected by [18F]FDG imaging. In view of its unique epidemiology, tumor biology, and prognosis, nasopharyngeal carcinoma (NPC) should be considered separately. The highest incidence is noted in Southern Chinese (20 times more common than in Caucasians). NPC tends to spread submucosally, in parapharyngeal tissues, along the maxillary and the mandibular nerves. Neck lymph node metastases are seen in the vast majority of patients. The lateral retropharyngeal nodes are traditionally considered first echelon nodes. The rate of distant metastases is high, involving bone, lung, and liver. Local disease extent is best defined by MRI, in particular disease involvement of the skull base and intracranial spread. Neck lymph node involvement is probably best characterized by a combination of MRI and [18F]FDG PET/CT. [18F]FDG PET/CT is useful for detection of disease outside the neck. The combination of MRI and [18F]FDG PET/CT also appears most appropriate for chemoradiotherapy response and recurrent disease assessment. Similar to other head and neck cancers, the intensity of [18F]FDG uptake in NPC primary tumors provides prognostic information. The utility of PET for delineating the gross tumor volume (GTV) in radiotherapy has been investigated. Most studies agree that the incorporation of PET in the radiotherapy planning process clearly improves target design over CT-based planning. © Springer Nature Switzerland AG 2022. |
