A patient with adenoid cystic carcinoma of the skull base was treated with radiotherapy because of perineural tumor invasion (PTI). Treatment evaluation using magnetic resonance (MR) imaging showed disease progression of PTI outside the irradiated area, whereas it was difficult to evaluate the therapeutic effect of radiotherapy because the irradiated lesions still had contrast enhancement. Thallium-201 SPECT/CT image fusion was performed to evaluate tumor extent and viability after therapy. During the follow-up period, disease progression was noted in the areas of abnormal uptake on the thallium-201 SPECT/CT fusion images, but the lesions with no uptake remained unchanged on follow-up MR imaging. Thallium-201 SPECT/CT image fusion can be used for precise localization of viable PTI in head and neck cancer patients who had been previously treated.
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