Sialadenitis After Radioactive Iodine for Thyroid Cancer
Radioactive iodine therapy (RAI) is a well-established treatment modality in the management of differentiated papillary and follicular thyroid cancers for the ablation of residual thyroid tissue after total thyroidectomy. In addition to uptake by thyroid tissue, RAI will also concentrate in the salivary gland ducts and be secreted in saliva. The first symptom after RAI is usually obstructive in nature affecting the ducts. Patients will have pain and swelling over the salivary glands with decreased salivary flow.
The prevalence of sialadenitis varies. Bilateral sialadenitis is more frequent than unilateral sialadenitis, with the parotid glands more commonly involved than submandibular glands. The increased sensitivity of the parotid gland is partially due to the larger content of serous cells that are more susceptible to the deleterious effects of ionizing radiation. Mucin secreted by mucin cells in submandibular and sublingual glands acts as a protective oral mucosal barrier. A higher clearance rate of saliva secretion in submandibular glands may also be a component.
The sodium/iodine symporter, highly expressed in the striated ducts and not in acinar cells, concentrates RAI. Initial obstructive symptoms impacting ducts has two mechanisms. The RAI is taken up by salivary striated ducts and causes inflammation and increased periductal pressure and ductal constriction. This will cause reduction in salivary flow, retention of saliva, as well as pain and swelling of the salivary glands. Second, the inflammatory infiltrate will cause ductal lumen narrowing due to the formation of mucus plugs in the salivary gland duct. The mucus plugs are formed when there is a nidus of radiation induced inflammatory cells. The narrowed ductal lumen creates an obstruction with stagnation and mucus precipitation. Salivary retention will cause pain and swelling, most severe during mealtimes due to increase saliva production. Patients will feel a salty taste because the intralobular ducts have not adequately resorbed sodium and chloride ions from saliva.
RAI also has an impact on the parenchyma of the salivary glands. Initially, salivary gland parenchymal uptake of RAI may be normal, but because of early damage to the ductal wall, clearance is delayed and results in retention of RAI. Acinar cells in proximity to ductal lumina may be subjected to I131 damage. Over time, vascular fibrosis becomes manifest.
Symptoms may develop immediately after RAI and may be transient or long term. The mucus plug in the ductal lumen is soft and when there is spontaneous extrusion of the mucus plug, the symptoms subside. Long-term chronic sialadenitis symptoms (infection, xerostomia, taste alteration, dental caries, Candida infection) occur with acinar cell atrophy after prolonged ductal obstruction. There is also risk of retrograde ascending secondary ductal infection from oral bacteria causing infection with more pain and swelling.
Diagnosis of RAI induced chronic sialadenitis is by patient history and symptoms. Ultrasound and computed tomography (CT) scan are not helpful. Scintigraphy with intravenous radioisotope Technetium-99m pertechnetate can help evaluate salivary gland function.
Treatment is mainly symptomatic with massage, hydration, sialogogues, and avoidance of anticholinergic medications. Cholinergic medication such as pilocarpine or cevimeline can potentially increase salivary outflow. Antibiotics can be used for bacterial infection. Many cases subside without treatment.
Sialendoscopy can be elected in patients unresponsive to medical treatment for diagnosis and symptomatic relief ( Figs. 33.1 , 33.2 ; ). The duct mucosa is pale with congested capillaries ( Fig. 33.3 ; ). Mucus plugs can be irrigated with saline solution. Cases of partial and near total stenosis can be dilated with a sialendoscope tip, hydrostatic pressure, guidewire, malleable dilators, basket, or balloon with resolution of symptoms. Instillation of steroid and stenting are recommended for severe or recurrent cases. Over 90% of patients receive some improvement in symptoms and over 50% of patients experience complete resolution of symptoms sustained over 18 months. Resection of salivary gland is only reserved for those patients not helped by other means of treatment.
