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Head and Neck Imaging

Radiation therapy, tissue changes

after irradiation of a neck cancer, a number of tissue changes become visible on CT and MR images of the neck. These expected postradiation alterations should not be misinterpreted as evidence of persistent or recurrent tumour.

Within the first two weeks after radiotherapy, there is an acute inflammatory reaction within the deep tissues. Increased permeability, due to detachment of the lining endothelial cells within small blood and lymphatic vessels, results in interstitial oedema. After this initial period of a few weeks, there is progressive thickening of the connective tissue. Endothelial proliferation is also seen, eventually resulting in complete obstruction of the vessels. The reduction in venous and lymphatic drainage results in further accumulation of interstitial fluid. The fibrosis then becomes progessively more advanced, but the interstitial oedema may be reduced by formation of collateral capillary and lymphatic channels.

The changes visible on post-treatment CT and MR images depend on the radiation dose and rate, the irradiated tissue volume, and the time elapsed since the end of radiation therapy. Changes which may be seen include (Fig.1):

  • symmetrical thickening of the skin and platysma muscle;

  • reticulation of the subcutaneous fat and the deep tissue fat layers;

  • oedema in the retropharyngeal space;

  • increased enhancement of the major salivary glands, followed by size reduction of these glands: postirradation sialadenitis;

  • atrophy of lymphatic tissue, in both the lymph nodes and Waldeyers ring;

  • symmetrical thickening and increased enhancement of the pharyngeal walls; and

  • symmetrical thickening of the laryngeal structures, with increased density of the fat in the pre epiglottic space and paraglottic space

    These tissue changes are most pronounced during the first few months after the end of radiation therapy, and diminish or even resolve with time. It is important to note is that the expected tissue changes after radiation therapy appear symmetrical. Comparison with a baseline study is often helpful: progressive disease is very suggestive of recurrent tumour or a treatment complication.

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    Fig.1

    Axial contrast-enhanced CT images of neck, 6 months after radiation treatment for a tonsillar cancer. a. Level of hyoid bone. Thickening of platysma muscles (arrowheads) and subcutaneous reticulation. Pronounced enhancement of both submandibular glands (arrow), due to radiation sialadenitis. Increased density in pre-epiglottic space (p). Thickened ary-epiglottic folds (a). Retropharyngeal edema (r). b. Level of parotid glands, both showing pronounced enhancement: radiation sialadenitis.
    Radiation therapy, tissue changes, Fig.1 (a)
    Radiation therapy, tissue changes, Fig.1 (b)