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Objective:
Treatment of differentiated thyroid carcinoma (DTC) typically involves thyroidectomy followed by radioablation with radioiodine I-131 (RAI). Based on the 2015 American Thyroid Association Management (ATA) Guidelines, patient stratification is based on age, staging at surgery, including lymphovascular invasion or distant metastases, and biopsy-based cytology. This often translates into empirical dosing with radioiodine1. Following the treatment of DTC with RAI, we utilized posttreatment SPECT/CT imaging to better delineate and stage of iodine-avid thyroid cancer remnants and metastases. We used commercially available voxel-based dosimetry software to determine postRAI dosimetry in residual tumor and/or distant metastases and compared this with the nontarget tissue delivered dose.

Materials and Methods:
The study protocol was approved by our institutional IRB. Five patients between the ages of 28 and 74 years old were enrolled. Inclusion criteria included a history of differentiated thyroid carcinoma (DTC), thyroidectomy and suspicion of metastatic disease either locally in the cervical soft tissues or in the mediastinum and chest. Following dosing with I-131, we obtained posttreatment SPECT/CT imaging both by 24 hours and by 168 hours posttreatment. Images were acquired from the skull base to the liver dome on a Siemens Symbia Evo 1 SPECT/CT with a high-energy collimator. Images were sent to a Windows based workstation running dosimetry software (Velocity from Varian Medical Systems). Follow-up with laboratory serum thyroglobulin and antithyroglobulin antibody levels as well by imaging when necessary was done as per treatment protocol in place at our institution. Correlation was done with clinical information regarding the tumor size and focality at surgery as well as histologic and molecular pathology.

Results:
The delivered dose to iodine-avid remnant tissue varied considerably with the prescribed RAI dose. Features such as size of tumor and follicular cell type were associated with a larger delivered dose, similar to previous work. The mean dose absorbed by nontarget tissue (the salivary glands) was 92.6 ±31.5 Gy, about a factor of three below target tissue dose, on average. Additionally, the mean dose delivered to the thyroid bed and cervical lymph nodes varied considerably (= 292 ± 103.6 Gy and = 908.8 ± 333.7 Gy, respectively). One patient had an iodine-avid osseous metastasis that received a dose of 216.1 Gy. The radioiodine-avid lesions received over 100 Gy, the threshold for a lethal tumor dose.

Conclusion:
Compared with conventional planar imaging, postRAI SPECT/CT detects more lesions. Additionally, we demonstrate a range of target tissue doses and a significant difference in the dose delivered depending on the clinical and histological tumor features. Treatment adequacy is predicted based on measurements using voxel-based dosimetry software. Together with clinical features, this adds prognostic information useful for patient management.