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The purpose of this prospective study was to determine the diagnostic impact and influence on patient treatment of posttherapeutic 131I SPECT-CT when the findings on planar posttherapeutic whole-body scintigraphy (ptWBS) were inconclusive. A total of 53 SPECT-CT scans were performed in 41 patients with thyroid cancer after high-dose 131I therapy (2.944 to 7.526 GBq 131I) because of diagnostic uncertainty on ptWBS. Physiological uptake in the salivary glands, gastric mucosa, gut, nasal mucosa, urinary tract and liver were considered to be normal. Any other foci of increased 131I uptake, except iodine uptake clearly located in the thyroid bed, were considered to be abnormal. The data were evaluated on a lesion and a patient basis. Regarding neck lesions, SPECT-CT provided a diagnostic impact in 26/90 lesions (28.9%) and confirmed the diagnosis in 64/90 lesions (71.1%). On a patient basis, SPECT-CT changed N status in 12/33 patients (36.4%), provided a diagnostic impact in 21/33 patients (63.6%) and led to a treatment change in 8/33 patients (24.2%). Regarding lesions distant from the neck, SPECT-CT confirmed the diagnosis in 62/71 lesions (87.3%) and had a diagnostic impact in 9/71 lesions (12.7%). On a patient basis, SPECT-CT changed M status in 4/19 patients (21.1%), had a diagnostic impact in 14/19 patients (73.7%) and led to a treatment change in 2/19 patients (10.5%). Considering all patients, SPECT-CT led to a treatment change in 10/41 patients (24.4%). Integrated SPECT-CT is a useful tool, especially in cases of diagnostic uncertainty and helps to individualize patient management. [ABSTRACT FROM AUTHOR]