Poorly differentiated thyroid cancer lesions often lose the ability to concentrate radioactive [131I]iodine (RAI) and exhibit increased metabolic activity, as evidenced by enhanced glucose uptake. We incorporated [18F]fluorodeoxyglucose (FDG) positron emission tomography (PET) scanning into the routine follow-up of a cohort of thyroid cancer patients undergoing annual evaluations. One hundred and twenty-five patients who had previous thyroidectomies were included. They had diagnostic RAI whole body scans, serum thyroglobulin measurements, and additional imaging studies as clinically indicated. During 41 months of follow-up, 14 patients died. Univariate analysis demonstrated that survival was reduced in those with age over 45 yr, distant metastases, PET positivity, high rates of FDG uptake, and high volume of the FDG-avid disease (>125 mL). Survival did not correlate with gender, RAI uptake, initial histology, or grade. Multivariate analysis demonstrated that the single strongest predictor of survival was the volume of FDG-avid disease. The 3-yr survival probability of patients with FDG volumes of 125 mL or less was 0.96 (95% confidence interval, 0.91, 1.0) compared with 0.18 (95% confidence interval, 0.04, 0.85) in patients with FDG volume greater than 125 mL. Only 1 death (of leukemia) occurred in the PET-negative group (n = 66). Of the 10 patients with distant metastases and negative PET scans, all were alive and well. Patients over 45 yr with distant metastases that concentrate FDG are at the highest risk. Once distant metastases are discovered in patients with differentiated thyroid carcinoma, FDG-PET can identify high and low risk subsets. Subjects with a FDG volume greater than 125 mL have significantly reduced short term survival.

Radioactive iodine (131I) is an important therapeutic option for the treatment of metastatic thyroid carcinoma. Survival in patients with metastases that concentrate radioiodine is better than those whose metastatic lesions do not take up radioiodine. Survival is markedly reduced in patients who have metastatic lesions that concentrate 18F-fluorodeoxyglucose (FDG) on positron emission tomography (PET). In this retrospective study, we evaluated the ability of 131I to destroy FDG-avid metastatic lesions in thyroid cancer patients. Twenty-five patients with positive FDG-PET scans received at least one dose of 131I treatment before a second FDG-PET was performed. The average interval between the two PET scans was 12.9 months. The average interval between the 131I treatment and the follow-up FDG-PET was 10.1 months. We measured posttherapy changes in lesional volume, in standard uptake values (SUV) of FDG, and in serum thyroglobulin (Tg) levels. The total volume of FDG-avid metastases rose significantly (p = 0.036) from a mean of 159 mL to 235 mL after 131I therapy, the maximum SUV rose from 9.3 to 11.9, the median Tg at the time of the second PET scan was 132% of that at baseline. Statistical analyses demonstrated no significant changes in maximum SUV, or serum Tg levels after 131I in the FDG-PET-positive group. In a control group of FDG-PET-negative patients, the serum Tg decreased to 38% of baseline after 131I therapy (p < 0.001). We conclude that high-dose 131I therapy appears to have little or no effect on the viability of metastatic FDG-avid thyroid cancer lesions.

In internal emitter therapy, an accurate description of the absorbed dose distribution is necessary to establish an administered dose-response relationship, as well as to avoid critical organ toxicity. This work describes the implementation of a dosimetry method that accounts for the radionuclide decay spectrum, and patient-specific activity and density distributions. The dosimetry algorithm is based on a Monte Carlo procedure that simulates photon and electron transport and scores energy depositions within the patient. The necessary input information may be obtained from a registered set of CT and SPECT or PET images. The algorithm provides the absorbed dose rate for the radioactivity distribution provided by the SPECT or PET image. The algorithm was benchmarked by reproducing dosimetric quantities using the Medical Internal Radionuclide Dose (MIRD) Committee's Standard Man phantom and was used to calculate absorbed dose distributions for representative case studies.

Poorly differentiated thyroid cancer lesions often lose the ability to concentrate radioactive [131I]iodine (RAI) and exhibit increased metabolic activity, as evidenced by enhanced glucose uptake. We incorporated [18F]fluorodeoxyglucose (FDG) positron emission tomography (PET) scanning into the routine follow-up of a cohort of thyroid cancer patients undergoing annual evaluations. One hundred and twenty-five patients who had previous thyroidectomies were included. They had diagnostic RAI whole body scans, serum thyroglobulin measurements, and additional imaging studies as clinically indicated. During 41 months of follow-up, 14 patients died. Univariate analysis demonstrated that survival was reduced in those with age over 45 yr, distant metastases, PET positivity, high rates of FDG uptake, and high volume of the FDG-avid disease (>125 mL). Survival did not correlate with gender, RAI uptake, initial histology, or grade. Multivariate analysis demonstrated that the single strongest predictor of survival was the volume of FDG-avid disease. The 3-yr survival probability of patients with FDG volumes of 125 mL or less was 0.96 (95% confidence interval, 0.91, 1.0) compared with 0.18 (95% confidence interval, 0.04, 0.85) in patients with FDG volume greater than 125 mL. Only 1 death (of leukemia) occurred in the PET-negative group (n = 66). Of the 10 patients with distant metastases and negative PET scans, all were alive and well. Patients over 45 yr with distant metastases that concentrate FDG are at the highest risk. Once distant metastases are discovered in patients with differentiated thyroid carcinoma, FDG-PET can identify high and low risk subsets. Subjects with a FDG volume greater than 125 mL have significantly reduced short term survival.

Dedicated small-animal imaging devices, e.g. positron emission tomography (PET), computed tomography (CT) and magnetic resonance imaging (MRI) scanners, are being increasingly used for translational molecular imaging studies. The objective of this work was to determine the positional accuracy and precision with which tumors in situ can be reliably and reproducibly imaged on dedicated small-animal imaging equipment. We designed, fabricated and tested a custom rodent cradle with a stereotactic template to facilitate registration among image sets. To quantify tumor motion during our small-animal imaging protocols, ‘gold standard’ multi-modality point markers were inserted into tumor masses on the hind limbs of rats. Three types of imaging examination were then performed with the animals continuously anesthetized and immobilized: (i) consecutive microPET and MR images of tumor xenografts in which the animals remained in the same scanner for 2 h duration, (ii) multi-modality imaging studies in which the animals were transported between distant imaging devices and (iii) serial microPET scans in which the animals were repositioned in the same scanner for subsequent images. Our results showed that the animal tumor moved by less than 0.2−0.3 mm over a continuous 2 h microPET or MR imaging session. The process of transporting the animal between instruments introduced additional errors of ∼0.2 mm. In serial animal imaging studies, the positioning reproducibility within ∼0.8 mm could be obtained.