Posttreatment CT and MR Imaging in Head and Neck Cancer: What the Radiologist Needs to Know

©RSNA, 2012 • radiographics.rsna.org

ByDr. Naglaa Mahmoud

Registrar of Clinical RadiologyKCCC

In patients with head and neck cancer, posttreatment imaging can be complicated and difficult to interpret because of the complexity of the surgical procedures performed and the postirradiation changes, but such imaging is critical for the evaluation of

(a)the response to therapy and (b)tumor control.

Posttreatment changes are affected by the type of surgery performed, reconstruction, neck dissection and radiation therapy.

Treatment Methods and Expected Posttreatment Imaging Findings

In general, the management of early-stage head and neck cancer consists of single-modality treatment with either radiation therapy or surgery.

For locally advanced head and neck cancer (stage III or IV) without distant metastases, multimodality treatment consisting of a combination of curative surgery followed by adjuvant radiation therapy, with or without chemotherapy.

Surgery with or without Reconstruction

Curative resection requires a wide local excision to obtain negative surgical margins.

Reconstructive techniques are mainly classified into three types of flap reconstruction.

1- Local flap reconstruction involves a geometric repositioning of adjacent tissue.

2- Pedicle flap reconstruction involves rotation of donor tissue to cover a defect, with preservation of the original arterial and venous systems.

3- Free flap reconstructive technique involves the transfer of tissue that is vascularized by local vessels, with anastomosis to the tissue defect by using microvascular techniques.

Depending on their tissue components, flaps are divided into:

1. Simple flap consists of one tissue type, such as skin or subcutaneous tissues.

2. Composite flap consists of more than one tissue, for example, myocutaneous, fasciocutaneous, free jejunal interposition and osseous flaps.

The more common types of free flaps used are the rectus abdominis myocutaneous free flap, radial forearm free flap, lateral arm flap, anterior lateral thigh flap, iliac crest flap, fibula free flap and jejunal free flap.

Myocutaneous flaps are initially depicted as a mass with soft-tissue attenuation representing muscle.

These flaps will gradually show denervation atrophy, which causes volume loss and fatty replacement of the muscle.

Sharp boundaries exist between the flap and the adjacent normal structures, which is an important sign indicating benignity.

It is therefore important to assess the superior and inferior margins of the flap, where local recurrence most commonly occurs.

Myocutaneous flaps show a wide spectrum of enhancement ranging from almost no contrast enhancement to diffuse intense enhancement.

Neck Dissection

The three major types of neck dissection are; 1- Radical neck dissection involves the removal en bloc of all of the ipsilateral lymph nodes (levels I–V), including the sternocleidomastoid muscle, IJV, SMG and spinal accessory nerve.

2- Modified radical neck dissection is the same as radical neck dissection but with preservation of one or more of the following structures: the sternocleidomastoid muscle, IJV, SMG or spinal accessory nerve.

3- Selective neck dissection has four common subtypes: the supraomohyoid type (levels I–III), the lateral type (levels II–IV), the posterolateral type (levels II–V) and the anterior compartment type (levels VI and VII).

The main CT and MR finding after neck dissection is the absence of the tissues resected with the cervical lymph nodes.

Another CT finding is an area of soft-tissue attenuation surrounding the carotid sheath completely.

At MR imaging, the postoperative area shows low to intermediate signal intensity on T1- and T2- weighted MR images, a finding that represents fibrosis or scar.

Fat planes are obliterated, which makes the identification of nodal recurrence more difficult.

Normal appearance after neck dissection for right cervical lymph node metastases of unknown primary cancer. A right-sided radical neck dissection with a pectoralis major pedicle flap was performed.Axial CT shows an area of soft-tissue attenuation surrounding the right neurovascular bundle. Note that the fat planes are obliterated.

Radiation Therapy

Radiation therapy for head and neck cancer is classified into two types:

1.External beam radiation therapy which uses photon, electron beam or proton beam radiation delivered from a source external to the patient.

2.Brachytherapy which uses radioactive sources, such as iodine seeds, iridium or cesium, that are implanted in the patient permanently or temporarily.

Changes after radiation therapy are divided into :

1. Early reactions or complications are observed during the course of RT or within 90 days after treatment.

2. Late complications, appear more than 90 days after the completion of RT, may take months to years to emerge and are often irreversible.

Concurrent chemo-radiation approaches, neoadjuvant chemotherapy, and altered fractionation regimens have improved treatment outcomes, although with increased early toxic effects.

CT and MR imaging findings of early reactions to RT are thickening of the skin and platysma, reticulation of the subcutaneous fat, edema and fluid in the retropharyngeal space, increased enhancement of the major salivary glands, thickening and increased enhancement of the pharyngeal walls, and thickening of the laryngeal structures .

SCC of the hypopharynx. Axial contrast-enhanced CT 3 months after RT shows thickening of the skin and platysma, reticulation of the subcutaneous fat, edema and fluid attenuation in the retropharyngeal space, increased enhancement of the submandibular glands.

On the other hand, late reactions to radiation therapy include atrophy of the salivary glands and thickening of the pharyngeal constrictor muscle, platysma and skin.

SCC of the hypopharynx, 1 year after completion of RT.Contrast-enhanced fat-saturated T1-weighted MR images show atrophy of the parotid and submandibular glands and thickening of the pharyngeal constrictor muscle and skin.

Posttreatment Imaging

Tumor Recurrence

Tumors typically recur within the first 2 years after treatment.

The most common locations for tumor recurrence are in the operative bed and at the margins of the surgical site.

CT demonstrates recurrence as an infiltrating slightly hyperattenuating mass with enhancement, with or without bone destruction.

Tumor recurrence has attenuation similar to that of muscle, therefore, if a suspected mass has lower attenuation than that of muscle, it is unlikely to be a malignancy and often is related to edema.

Tumor recurrence in a 47-year-old man with SCC of the left maxillary sinus. A left total maxillectomy and midface reconstruction with a flap had been performed. An axial contrast-enhanced CT image shows a heterogeneously enhancing mass adjacent to the flap.

MR imaging demonstrates tumor recurrence as an infiltrative mass with intermediate T1-weighted signal intensity, intermediate to high T2-weighted signal intensity and enhancement.

The differential diagnosis for tumor recurrence includes a vascularized scar, which represents early fibrosis.

Differentiation of a vascularized scar from tumor recurrence is difficult because such a scar appears as a soft-tissue mass with ill-defined margins and enhancement, which is similar to the findings for tumor recurrence at both CT and MR imaging.

Retraction and decreased signal intensity on T2-weighted MR images at the follow-up examination are suggestive of fibrosis.

In addition, diffusion-weighted MR imaging is a useful tool to differentiate tumor recurrence from normal postoperative changes and fibrosis.

High signal intensity on diffusion-weighted MR images with a decreased value for the (ADC) is suspicious for recurrence.

Tumor recurrence in a patient with SCC of the oropharynx.

T1-weighted (a) DWI (b) and ADC map (c) obtained at 1 year 7 months after chemoradiation therapy show a mass in the right tonsil.

The mass has low signal intensity on the T1-weighted image (a) and high signal intensity on the diffusion-weighted image (b), with a reduced value for the ADC (c).

Perineural tumor spread is a unique form of tumor recurrence.

Most commonly seen with SCC, followed by adenoid cystic carcinoma, malignant lymphoma and malignant schwannoma.

The imaging findings of perineural tumor spread are nerve enlargement with enhancement, foraminal enlargement, obliteration of fat planes and replacement of the skull base foramina with soft tissue.

However, granulation tissue and posttreatment scarring may also result in obliteration of fat planes and apparent soft-tissue infiltration into the skull base foramina.

Close correlation with prior images and clinical symptoms can help distinguish perineural tumor spread from the effects of treatment.

Follow-up imaging may be required in equivocal cases.

A case of adenoid cystic carcinoma of the left sublingual gland. Five years after surgery and RT, she presented with left cheek pain. Coronal contrast-enhanced fat-saturated T1-weighted MR image shows a tubular mass located along the course of the left cranial nerve V3 and involving the cavernous sinus. Note the enlargement of the left foramen ovale and the loss of the normal fat pad.

Complications after Surgery

Most complications related to surgery occur in the early period after treatment.

1. A fluid collection is sometimes seen after surgery, and serous retention often resolves spontaneously, requiring no further treatment.

2. Chylous fistula

Occurs in 1%–2% of patients after neck dissection, especially when level IV nodes are dissected.

Chylous fistula is often located in the lower left portion of the neck, so this characteristic location helps raise the suspicion of this complication.

Early surgical complications such as serous retention, abscess, hematoma and chylous fistula often show imaging findings similar to those of a fluid collection with peripheral enhancement at CT and MR imaging.

Fistula after tumor resection, left-sided radical neck dissection, and tracheostomy in a 69-year-old man with SCC of the left side of the base of the tongue.

At 10 days after surgery, he presented with a fever. Coronal contrast-enhanced CT image shows the formation of a pharyngocutaneous fistula in the left side of the neck.

3. Flap necrosis is a rare complication.Flap failure is associated with vascular thrombosis, the majority of which is venous in origin, occurring within 3 days after surgery. Therefore, the detection of thrombus in the artery and vein is important in the evaluation of the viability of the flap.

SCC of the hypopharynx. A partial hypopharyngectomy, right-sided modified neck dissection, and pharyngeal reconstruction with a free jejunal flap. (a) Non enhanced CT image obtained 5 days after surgery shows slightly increased attenuation of the jejunal free flap. (b) Contrast-enhanced CT image shows no enhancement in the flap. Note the occlusion of the right internal jugular vein.

Late complications of Radiation Therapy

Mucosal Necrosis Uncommon but important late toxic effect of RT, greatest during the first 6–12 months after RT.

May cause pain and interferes with the patient’s ability to chew and swallow.

In more than 95% of cases, soft-tissue necrosis heals spontaneously, but healing may take 6 months or more.

At CT and MR imaging, mucosal necrosis shows a lack of mucosal enhancement with or without ulceration. Pockets of gas adjacent to the lesion should raise suspicion for tissue necrosis. Gas is better identified by CT, compared with MRI.

Clinical correlation is essential for mucosal complications because it is easier to be diagnosed clinically than by imaging.

SCC of the larynx. 5 months after RT, complained of severe odynophagia. Contrast-enhanced CT shows the non enhancing area in the hypopharyngeal mucosa and a small gas pocket adjacent to the lesion, findings that indicate tissue necrosis.

Osseous Complications Osteoradionecrosis

in general, it is a condition in which irradiated bone becomes devitalized and exposed through the overlying skin or mucosa, persisting without healing for at least 3 months.

Occurs 1–3 years after radiation therapy.

Sites: skull base, temporal bone, mandible, maxilla and hyoid bone. Of these, the mandible is the most commonly affected because of its superficial location and relatively poor blood supply.

CT demonstrates a focal lytic area with cortical destruction, sequestra formation and loss of the trabeculation pattern.

A 64-year-old man with SCC of the right tonsil. Coronal CT images show a focal lytic area with cortical destruction and a pathologic fracture in the right mandibular ramus.

Osteoradionecrosis MR images show abnormal signal intensity in the bone marrow, with cortical destruction. Pathologic fracture, soft-tissue thickening, and fistula formation are sometimes seen.

A 70-year-old man with SCC of the right tonsil. Surgery and RT. Axial T1-weighted (a) and fat-saturated T2-weighted (b) MR images show a T1-hypointense and heterogeneous slightly T2-hyperintense area in the right mandibular body.

Osteoradionecrosis

Although these imaging findings mimic those of tumor recurrence, the presence of an associated soft-tissue mass favors a diagnosis of tumor recurrence.

The identification of cortical defects remote from the primary tumor site favors the diagnosis of osteoradionecrosis.

SCC of the left lower gingiva. Axial CT image shows a focal lytic area with cortical destruction in the mandible. Note the associated soft-tissue mass. The histopathologic findings from biopsy helped confirm the tumor recurrence.

Vascular Complications

Accelerated atherosclerosis and thrombosis of the IJV or carotid artery are well-known complications after RT.

Formation of a pseudoaneurysm of the internal carotid artery is another rare complication.

It occurs most often in patients who have undergone high-dose radiation therapy, with a latency period between 4 months and 20 years.

The imaging findings mimic those of other atherosclerotic disease and cannot be differentiated from them.

Radiation-induced vasculopathy is often bilateral and related to the irradiated field.

SCC of the larynx treated with RT. Axial contrast-enhanced CT images obtained at 2 years (a) and 5 years (b) after radiation therapy show the progression of bilateral thickening of the walls of the common carotid arteries.

Radiation-induced Lung DiseaseRT for patients with head and neck cancer often includes the apical aspect of the thorax, to encompass the supraclavicular and level IV nodal areas and results in bilateral apical radiation-induced lung changes.

Clinical manifestations:1- Acute radiation pneumonitis; occurs within 1–3 months after RT.2-Late radiation lung fibrosis; occurs within 6–12 months after RT.

Imaging findings of radiation pneumonitis are focal ground-glass attenuation or consolidation or both.

Radiation pneumonitis gradually resolves but progresses to radiation lung fibrosis if the damage is severe.

Radiation lung fibrosis is shown to be a well-defined area of volume loss, linear scarring and traction bronchiectasis

A 60-year-old woman with SCC of the tongue after surgery and RT. (a)Radiation pneumonitis: CT obtained 3 months after RT shows multiple foci of ground-glass attenuation and consolidation in the bilateral lung apices. (b)Radiation fibrosis: CT obtained 6 months after RT shows an area of well-defined ground-glass attenuation with volume loss and linear scarring.

Radiation-induced Brain Necrosis

Irradiation of the skull base for the treatment of patients with skull base tumors or skull base invasion may cause damage to adjacent brain tissue.

Radiation-induced focal brain necrosis often occurs within 2 years after radiation therapy.

Focal brain necrosis is depicted as a ring-enhancing mass with variable edema on CT and MR images.

A 54-year-old woman after radiation therapy for a chondrosarcoma of the left cavernous sinus and skull base. Two years after RT, she complained of nausea and memory loss. (a)Axial T2-weighted MR image shows the mass with surrounding edema in the left temporal lobe. (b)Coronal contrast-enhanced fat-saturated T1-weighted MR image shows irregular ring enhancement of the mass.

Differentiation from metastasis, high-grade glioma or abscess is difficult.

Diffusion-weighted MR imaging, MR spectroscopy, and PET/CT may be used for differentiation.

With DW MR imaging, there is elevated values of the ADC in areas of brain necrosis, compared with the values in intact normal brain parenchyma and tumor. MR spectroscopy demonstrates the metabolite changes, such as reduced N-acetylaspartate and elevated lactate levels, in the necrotic tissues.

At PET/CT, decreased metabolic activity in brain necrosis may be seen.

Radiation-induced Neoplasm

Radiation-induced neoplasm is rare.

Various types including meningioma, sarcoma (osteosarcoma, malignant fibrous histiocytoma), osteochondroma, schwannoma, osteoblastoma, squamous cell carcinoma and lymphoma. The diagnostic criteria of postirradiation osteosarcoma include a lesion centered in irradiated bone without a primary malignant osteoblastic lesion, arising after a latency period of at least 3 years after the completion of RT.

Imaging findings mimic those of primary osteosarcoma.

A 45-year-old woman after RT for a SCC of the nasopharynx. 9 years after RT, she presented with left mandibular pain and swelling. Contrast-enhanced fat-saturated T1- weighted MR shows a heterogeneously enhancing mass at the location of the left lateral pterygoid muscle, with destruction of the left mandibular ramus.

Conclusions

In patients who have been treated for head and neck cancer, the typical posttreatment changes are based on the various types of therapy, including surgery with and without reconstruction, neck dissection and radiation therapy.

Knowledge of the various treatment methods and their expected and unexpected posttreatment imaging findings helps to make an accurate diagnosis and avoid unnecessary further diagnostic work-up.

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