Estimation of Chest Wall Attenuation of 123I Emissions in Substernal Goiter: A Phantom Study.

In patients with substernal goiter, the generally accepted theory is that thyroid uptake measurements with iodine isotopes are underestimated because of attenuation by the chest wall. The extent of this underestimation is not well known. In this study, we calculated the attenuation of 123I emissions using a cadaver chest wall with a thyroid probe to better understand the potential severity of this underestimation. Methods: A 11.1-MBq capsule of 123I was measured using a thyroid probe directly in a standard neck phantom and behind a cadaver chest wall that included the soft tissues and bony structures (sternum). Results: The calculated attenuation of the iodine capsule was 18% for the neck phantom and 35% for the cadaver chest wall. Conclusion: Thyroid uptake in cases of substernal goiter may be underestimated by standard techniques using a neck phantom. The composition of the chest wall can vary greatly, and the substernal extent of the goiter would be difficult to calculate with high accuracy on a routine basis. Comparison between the cadaveric specimen and the phantom does give us a rough estimation of the differences in attenuation. Our findings suggest that attenuation by the chest wall can be substantial. Knowledge of the extent of the substernal component of the thyroid gland may be useful if the uptake measurement is used to calculate doses for treating hyperthyroidism in patients with substernal goiter.

Expanding the diversity of mycobacteriophages: insights into genome architecture and evolution.

Mycobacteriophages are viruses that infect mycobacterial hosts such as Mycobacterium smegmatis and Mycobacterium tuberculosis. All mycobacteriophages characterized to date are dsDNA tailed phages, and have either siphoviral or myoviral morphotypes. However, their genetic diversity is considerable, and although sixty-two genomes have been sequenced and comparatively analyzed, these likely represent only a small portion of the diversity of the mycobacteriophage population at large. Here we report the isolation, sequencing and comparative genomic analysis of 18 new mycobacteriophages isolated from geographically distinct locations within the United States. Although no clear correlation between location and genome type can be discerned, these genomes expand our knowledge of mycobacteriophage diversity and enhance our understanding of the roles of mobile elements in viral evolution. Expansion of the number of mycobacteriophages grouped within Cluster A provides insights into the basis of immune specificity in these temperate phages, and we also describe a novel example of apparent immunity theft. The isolation and genomic analysis of bacteriophages by freshman college students provides an example of an authentic research experience for novice scientists.