Management of Cancer of the Head and Neck
Imaging: General Guidelines

Clinical Evaluation:

Sectional imaging provided by computed tomography (CT) and magnetic resonance imaging (MRI) today allow superior anatomic mapping that allows the surgeon to make the most informed treatment plan. When such imaging should be utilized, which modality is most useful for a particular patient, and how the imaging studies should be performed will be summarized. In addition, an imaging approach to following the post operative patient and when to use ultrasound or CT-guided biopsies will be discussed.

Primary Tumor:

Today, the excellent resolution and image quality available on the newest CT and MRI scanners makes them modalities that can provide precise anatomic tumor mapping not previously available by any technique. Imaging is probably not necessary in patients with small, superficial masses such as neoplasms of the skin, mobile subcutaneous masses, and mobile masses in the superficial parotid gland. Based on several large surgical practices that see head and neck cancer patients, about 20% of cancer patients will fall into this category.

However, such imaging is the most accurate methodology to evaluate deep disease in the remaining 80% of patients with head and neck cancers. Most often, imaging is utilized for accurate tumor mapping by demonstrating tumor margins that may extend beyond those clinically identified. This usually occurs with pharyngeal, palatal, base of tongue, and laryngeal tumors, but also applies to tumor extension into the floor of the mouth. Tumor extension to bone (skull base, pterygoid plates, pterygopalatine fossa, mandible, and vertebra) and tumor spread to the internal carotid artery, cavernous sinus, nerves, orbital apex, and retropharyngeal muscles are also best demonstrated by CT and MRI and such tumor spread strongly impacts on treatment decisions.

Less often in a patient who is difficult to examine clinically, imaging can demonstrated areas of the pharynx, larynx, and trachea that are not possible to assess directly or can not be visualized well enough by direct observation to confidently rule out pathology.

Lymphadenopathy:

Compared to palpation, CT and MRI can show a palpable mass thought to be a solitary node, to actually be a conglomerate nodal mass. Level I adenopathy can be better differentiated from a submandibular gland mass by imaging than by palpation. In addition, imaging can visualize central nodal necrosis and thus occasionally identify a clinically suspected reactive node as containing metastatic disease. The detection of retropharyngeal adenopathy also remains almost solely in the realm of CT and MRI and the presence of such modal metastasis requires modification of both surgical and radiation fields. It is also estimated that about 5% of clinically silent metastatic nodes will be identified on CT and MRI and thus the most thorough assessment of the neck is by combined clinical and imaging evaluation. Imaging also best demonstrates extranodal (extracapsular) tumor extension and if that disease has involved the great vessels or bone.

The neck is normally not imaged just to evaluate adenopathy. Rather, imaging should be performed to evaluate adenopathy and a primary tumor site. This concept is most important in those patients that present with metastatic adenopathy and an unknown primary. Thus, an imaging study of the neck should include all structures from the skull base to the manubrium. In cases of thyroid carcinoma or tumor recurrences about a tracheotomy site, the imaging should be extended down to the level of the carina. An imaging examination that is solely limited to a palpable mass should be acceptable to the referring physician.

It is important for clinicians to recognize that the radiologist can better refine an imaging study and more succinctly evaluate it if there is good communication between the clinician and the radiologist. Thus, a brief statement about physical findings and pertinent history should be communicated to the radiologist prior to the CT or MRI study.

Choice of Imaging Modality:

CT is usually considered the basic initial study of the neck. It should be performed as a post-contrast examination with the patient supine and their head in a neutral position. Contrast is necessary to best differentiate pathological borders from normal tissue and to best separate adenopathy from vessels. Thus, CT examination should be ordered as a contrast study unless there is a medical reason not to give iodinated contrast. The scan should start above the skull base and extend caudally to at least the top of the manubrium. The scan should be performed as continuous 3 mm thick scans or as spiral scans with reconstructions at 2 mm. If skull base invasion is clinically suspected, a coronal scan of this area should be included as part of the initial examination.

One of the reasons CT is considered the basic imaging modality is because it has been shown that overall it evaluates lymphadenopathy better than MRI. That is, although MRI can also identify such disease, it is generally not as accurate as CT, it is more expensive than CT, and it takes longer both to perform the study and interpret the examination than CT. In the neck below the hard palate, CT is also generally considered the modality of choice due, in part, to its faster scan time than MRI and thus less degradation artefact due to swallowing and vascular pulsations.

In general, MRI is most often utilized as the primary imaging modality when evaluating tumor spread in the paranasal sinuses, cavernous sinuses, dura, brain, nasopharynx, oropharynx, palate, base of tongue, and floor of mouth. That is, the closer to the skull base, MRI offers more advantages over CT. MRI is also the modality of choice for evaluating perineural tumor spread. The MRI studies should include non-contrast and post contrast sequences. Although sequences can vary from one imaging center to another, most such studies should have at least T1-weighted, T2-weighted (spin echo or fast spin echo) and T1-weight post contrast fat suppressed series. Most of these sequences should be obtained in the axial and coronal projections. If applicable, sagittal images can also be obtained.

If one follows this imaging approach, there still will be cases that are initially imaged with one modality that then require a second study using the other imaging modality. Thus, if the initial examination is an MRI and there is skull base or facial bones invasion, CT often better shows the bone and often provides additional information helpful in surgical planning. Conversely, if CT is the initial study and retropharyngeal or perineural disease is suspected, MRI often adds complimentary information. This is also true for tongue base and floor of mouth cancers. In general, both examination are necessary on only about 10%-20% of cancer patients, however, in these cases, the surgeon can make the most informed treatment decisions possible.

It is best to discuss these difficult cases with the radiologist ahead of time to get the imaging input concerning the modality(s) of choice.

With the development of multidetector CT scanners, excellent reconstructions can be now be achieved in any plane with resolution indistinguishable from the that of the primary scan plane. In addition, with the more universal use of power contrast injectors, better perspicuity of pathology can be obtained. As a result, some of the advantage of MRI over CT may be diminishing in such areas such as the base of tongue, skull base, nasopharynx and parapharyngeal spaces.

Once a suspected deeply situated tumor mass is identified on a conventional CT or MRI study, CT guided biopsy should be considered. This technique is relatively painless to the patient and, if performed with a cytologist present, usually allows a tumor diagnosis to be confirmed. This approach often obviates the need for open surgical biopsy, especially in cases where curative surgery may not be feasible.

Ultrasound examination of superficial masses is recommended in children over CT or MRI. The ultrasound study of superficial lymph nodes can also distinguish metastatic nodes from reactive nodes by assessing nodal morphology and hilar vasculature, among other criteria. Ultrasound biopsy is also a valuable technique for masses in the thyroid gland and for primarily superficial masses. Deeply situated masses are probably more often biopsied by the CT guided technique.

The present generation of PET scanners have superior resolution to the prior units. In the untreated patient, a positive PET study allows identification of tumor, especially in difficult cases where no obvious mass was seen clinically or on either CT or MRI. However, the presence of inflammation in the post operative and/or post irradiated patient can also cause a positive PET study. As a result, PET is now often used in the difficult post treatment patient for its negative predicative value. Thus, a negative PET study has a very high correlation with the absence of tumor.