VARIOUS complications have been reported following the diagnostic use of colloidal thorium dioxide (Thorotrast) since its introduction by Radt (9) in 1929. These have included aplastic anemia, thrombosis, and the production of sarcomata (3, 4, 5, 7, 8, 14). By far the most frequent complication, however, has been the formation of fibrous nodules and masses following inadvertent perivascular injection of the radioactive medium (1, 2, 6, 10, 11, 15, 16, 17). This communication is concerned with a case in which sequelae of major import developed eleven years after the injection of Thorotrast into the soft tissues of the neck during attempted carotid arteriography. Of special interest is an estimate of the total tissue radiation dosage, made according to the method suggested by Rundo (13). Case Report A 35-year-old white woman was first admitted to the hospital in June 1945, complaining of sudden onset of severe frontal headache during exertion. Shortly thereafter she experienced aphasia, incontinence, and a right hemiplegia. Three days after admission an attempt at cerebral arteriography with Thorotrast was unsuccessful. Six milliliters of the medium were extravasated into the soft tissues of the left side of the neck (Fig. 1). A subsequent arteriogram showed poor filling of the left anterior and middle cerebral arteries. This suggested an aneurysm near the junction of the anterior cerebral and carotid arteries. The patient improved rapidly and was discharged with the diagnosis of probable rupture of a congenital cerebral vascular aneurysm. With the exception of a“lump in the left side of the neck,” the patient was well until April 1955, when hoarseness developed within a few hours. This continued and became progressively more marked. In February 1956, difficulty in the use of the tongue was noticed. In October, the patient was readmitted for evaluation of her symptoms and of the left cervical mass. Physical examination on admission revealed a Horner's syndrome on the left and atrophy of the left half of the tongue. A hard mass was palpable in the left side of the neck, extending from the posterior edge of the clavicle to the angle of the mandible. There were minimal right facial weakness and a right foot drop, but no sensory changes were noted. Laryngoscopy disclosed a left vocal cord paralysis. Routine laboratory determinations were all within normal limits. Roentgenograms showed residual radiopaque material in the left side of the neck, extending into the superior mediastinum (Fig. 2). The spleen and liver were also shown to contain radiopaque material (Fig. 3). Because of the possibility of continued expansion of the cervical lesion with ultimate involvement of vital structures, and in view of possible neoplastic transformation, surgical excision was performed in January 1957. A hard, calcified mass involving the carotid region was encountered, which extended superiorly to the foramen lacerum and inferiorly into the superior mediastinum. With sacrifice of segments of the carotid arteries, the jugular vein, and the vagus nerve, the mass was dissected free of the esophagus and trachea, to which it was firmly adherent. Its mediastinal portions were removed except for a portion about the subclavian artery. The hypoglossal nerve was sacrificed, but the phrenic nerve, which was uninvolved, was left intact. The patient recovered rapidly following the surgical procedure, with no increase in neurological deficit. To date she has shown no evidence of recurrence of the mass or of progression of the findings (Fig. 4).