Oropharynx, oral cavity, floor of the mouth: CT and MRI

Abstract

Pretherapeutic staging of tumors of the oropharynx, the oral cavity and the floor of the mouth is important and should be thorough and exact to ensure appropriate therapy. Particularly important is the assessment of infiltration of deeper compartments and the topographic relationship of tumor to vascular structures (lingual artery and vein, hypoglossal nerve), or the presence of spread of the tumor across the midline. As spread of tumor may occur to a large degree underneath normal appearing mucosa, clinical assessment of the true tumor extent is difficult.

In the last 20 years computed tomography (CT) has proved its value as a supplementary non-invasive method and established its role in modern diagnostic evaluation. Magnetic resonance imaging (MRI) is an non-invasive scanning method that offers excellent tissue contrast. Ultrasonography (US) is of secondary importance, but provides useful guidance due to its wide availability and its easy use.

This paper aims to depict the possibilities of modern CT and MRI to provide ‘one-stop-shopping’ information to the clinician as a basis for the right therapeutic approach and correct estimation of the individual patient’s prognosis. A clear problem oriented imaging strategy with standardized diagnostic criteria will lead to a cost effective evaluation.

Introduction

Malignant tumors of the oropharynx, the floor of the mouth, and the oral cavity represent about 2–5% of all malignancies. Ninety percent of these tumors are squamous cell carcinomas. Other histological types of tumors are rare: adenoid cystic carcinomas arise in the minor salivary glands and show infiltrative growth spreading preferably along nerves. This tumor is known for its tendency to recur, even years after apparently successful therapy. Adenocarcinoma and the extranodal manifestations of malignant lymphoma are the next most frequent malignant lesions of this region. Sarcoma can occur. In childhood rhabdomyosarcoma is the most frequently encountered malignant tumor of this area. Most of these tumors show a predominantly superficial spread. Depending on their location, size, and histologic differentiation they metastasize early to the regional submandibular and jugular digastric lymph nodes. The patients’ prognosis is ultimately determined by the degree of metastases. When these tumors progress they infiltrate the fatty connective tissue compartments of the parapharyngeal, sublingual, or submandibular spaces, preferably along groups of muscles and/or periosteal surfaces, while respecting muscle fascia in the early stages [1], [2].

The therapy depends on the histology and the degree of differentiation of the tumor, on its location and size, on the degree and direction of infiltration into neighbouring structures, and on the degree of regional lymph node metastases. Tumor of this area are therefore classified according to the TNM criteria of the UICC [3] to aid in establishing a modern and appropriate therapy.

• Stage T1 is a tumor of less than 2 cm (Fig. 1).

• Stage T2 is a tumor larger than 2 cm but less than 4 cm in maximum diameter (Fig. 2).

• Stage T3 is a tumor larger than 4 cm (Fig. 3).

• Stage T4 is a tumor with involvement of bony structures (Fig. 4).

Two general approaches are used in the therapy of primary tumors of this region: surgery and radiation/chemotherapy. For tumors of stages T1 and T2, primary surgical therapy with a curative goal should be aimed for. Here the tumor is resected with a margin of safety of at least 1.5 cm into the healthy surrounding tissue, while the first regional lymph node station is removed as well by suprahyoidal revision. In cases where radical excision of tumor with sufficient margin of safety is not possible (this is particularly the case for tumors of stage T3 and T4) post-operative radiation therapy with a dose of 56–70 Gy is performed. T3 tumors can also be preoperatively irradiated with 30 Gy, followed by resection of the ‘down-staged’ tumor. Areas of delayed wound healing, caused by radiotherapy, can be covered with a myocutaneous flap (skin–fat–muscle flap with intact vasculature, or using microsurgical vessel anastomoses). T4 tumors with involvement of bone are excised as well, often followed by radiotherapy; bone defects can be bridged with a metal plate. If there is no recurrence within 2 years, plastic reconstruction of the bone defect using a transplant from the iliac crest and the deep iliac circumflex artery can be attempted. If the tumor is inoperable, a combined radiotherapy and chemotherapy can provide a palliative approach [2], [4].

The goal of modern diagnostic imaging is to establish the location, the size, and the extent of tumor. Particularly important is the pretherapeutic determination of tumor margins with respect to anatomic landmark structures (lingual artery and vein, carotid artery, internal jugular vein; muscles, connective tissue spaces, and bone structures) as well as assessing for tumor spread across the mid-line. These findings play an important role in therapeutic planning [2], [4].