So it seems a group of people at Ottawa Hospital in Ontario has just had a paper published in JNMT that covers pretty much the same material I worked on and just submitted to the same journal. My contact at GSK told me it was something they also sponsored (like my project), but wasn't aware they had submitted it for publication.
Trying to get my hands on a copy of the article so I can read it and see what they looked at. Fortunately they come up with the same conclusions and results I did. From the abstract it seems like some of the things they investigated were different from what I looked at, so hopefully it's different enough from mine that the reviewers decide it's still worthy of publication.
Objective: Residence time measurements obtained by serial whole body conjugate view imaging are commonly used in patient specific dosimetry for radioimmunotherapy (RIT) applications. In order to determine the effect of collimator selection on residence time measurements for 131I, the accuracy of 131I half-life measurements using multiple gamma camera and collimator combinations was investigated. Method: Serial anterior and posterior whole body images were acquired over a period of 15 days using 4 different gamma cameras with medium and/or high energy collimators. Background corrected geometric mean counts from the images were fitted to a mono-exponential curve to determine the half-life of 131I for the different gamma camera/collimator combinations. Results: An average half-life of 8.15 days with a standard deviation (SD) of 0.07 days was obtained from all camera/collimator combinations. A half life of 8.12 days (SD 0.11 d) was obtained for the high energy collimators, and 8.18 d (SD 0.04 d) for the medium energy collimators. These values are all very close to the 8.02 day 131I physical half-life and were not found to be statistically different (p=0.44). Similar results were also obtained for the measured half-life for single head gamma camera configurations (mean half life 8.15 d, SD 0.12 d). The therapeutic 131I-tositumomab dose resulting from the differences in measured half-life ranged between 2.58--2.6 GBq (69.8--70.4 mCi). Conclusion: There is no significant difference in 131I half-life and residence time measurements made using medium or high energy collimators in dual head or single head imaging configurations.
131I-Tositumomab has been used in treating patients with non-Hodgkin's lymphoma. It is generally recommended that high-energy collimators be used to image patients before they receive 131I-tositumomab therapy, to determine the effective half-life for therapeutic dose and gross biodistribution. Because many nuclear medicine departments do not possess high-energy collimators, this study was designed to assess the suitability of using medium-energy collimators. The effect of scanning speed was also investigated, in an attempt to optimize the acquisition time. Methods: Measurements were taken using an elliptic anthropomorphic torso phantom and an organ-scanning phantom fitted with fillable spheres (1-5 cm in diameter) and organ inserts. Three phantom studies were performed with differing initial 131I concentrations in the organs, the spheres, and the thoracic and abdominal chambers. Images were acquired with both high-energy and medium-energy collimators and at acquisition speeds of 20 and 100 cm/min. The half-life for each combination (study/collimator/speed) was calculated from a linear fit of the data. The contrast of the tumor sphere was assessed using 2 identical regions, placed on and beside the sphere, and averaged over several time points. Biodistribution and image quality were visually assessed by 2 independent observers. Results: Measured half-life values and visual assessment of biodistribution showed no significant difference between the 2 collimators (P = 0.32) or acquisition speeds (P = 0.85). A significant difference in the contrast of the tumor spheres was observed between the 2 collimators (P < 0.01) but not between acquisition speeds. Visual assessment of the images showed increased noise on the image acquired at 100 cm/min, although this noise did not affect lesion detectability. Conclusion: Measured half-life is not significantly different between the 2 collimators; hence, calculation of the residence time would be nearly the same. Medium-energy collimators can be used to accurately calculate the 131I-tositumomab therapeutic dose and detect alterations in biodistribution.