Rani Al-Senan MS, Presenter: Nothing to Disclose

Mustapha Hatab PhD, Abstract Co-Author: Nothing to Disclose

There have been studies which used neck surface dose to estimate thyroid dose. Our results caution against this assumption unless neck diameter and gland’s depth are known as they have a strong influence on thyroid dose. One major drawback of commonly used Monte Carlo-based CT dosimetry software packages is the use of a standard phantom size which may result in over or under estimated thyroid dose.

To investigate the relationship between neck surface dose and thyroid dose for pediatric CT and to evaluate the effect of child’s neck circumference and depth of thyroid gland on thyroid dose. Also, to compare the estimated thyroid dose with that calculated form Monte Carlo-based software.

Neck surface dose was obtained by placing one OSL dosimeter (OSLD) over the child’s thyroid prior to a trauma CT study. A total of 25 pediatric subjects ranging in ages from 1–3 yrs old were selected. Average neck circumference and anterior depth of thyroid were obtained from a DICOM viewer. To evaluate the relationship between OSLD dose and thyroid dose, a study using acrylic phantoms and sleeves with various diameters was carried out. Two holes were drilled at different distances from the phantom surface. For each phantom size OSLDs were placed inside the holes and on surface. Phantoms were scanned with a standard axial protocol (16x2.5mm, 120 kVp/400 mAs). Ratios of surface dose to “in-hole” dose were used to convert patients’ OSLD dose to thyroid dose. Thyroid dose was also obtained from Monte Carlo-based ImPACT software. Age correction was applied. A paired t-test was performed to compare the thyroid dose obtained from the two methods.

Average neck diameter and thyroid depth for all subjects were 11.4cm (±2.75) and 0.67cm (±0.27), respectively. Ratio of surface dose to “in-hole” dose from phantoms study varied from 0.76 (for 12.7cm diameter/1.3cm hole depth) to 1.17 (11.4cm diameter/0.6cm hole depth), and was close to unity with 2.4cm hole depth and 11.4cm phantom. Comparison with ImPACT showed overestimation by ImPACT in all subjects. Overestimation varied from 11% up to 135%. Paired t-test for the two approaches was statistically significant (p <0.001).

Al-Senan, R, Hatab, M, Evaluation of Thyroid Dosimetry Methods for Pediatric CT Exams.  Radiological Society of North America 2010 Scientific Assembly and Annual Meeting, November 28 - December 3, 2010 ,Chicago IL. http://archive.rsna.org/2010/9016007.html