Effective dose and thyroid cancer risk in panoramic imaging

On panoramic radiographs, sensitive organs, including the thyroid, are exposed to radiation. Thyroid cancer is one of the most common cancers in Iran. That is why, in this study the effective dose and risk of thyroid cancer were estimated on panoramic radiography.

Seventy GR200 thermoluminescence (TLD) dosimeters were used to estimate the absorbed dose of thyroid. The dosimeters were calibrated and placed on the patients’ necks during panoramic radiography. After dosimeters were read, the mean absorbed dose and effective thyroid dose were calculated in three groups with different radiation conditions. Lifetime Attributable Risk (LAR) of thyroid cancer was estimated using the model presented in the BEIR VII report. GraphPad Prism statistical software was used to analyze the data. 

The mean absorbed dose of thyroid lobes in groups M, L, XL (According to mandibular size) was estimated to be 0.116±0.01, 0.123±0.04 and 0.03±0.134 mg, respectively. The right thyroid lobe in group XL with absorption dose of 0.143±0.05 mg and the left lobe in group M with absorption dose of 0.106±0.03 mg had the highest and the lowest absorption doses, respectively. The difference between the absorbed doses of the right and left thyroid lobes in any of the three groups was not statistically significant. Thyroid absorption doses in these three groups were not statistically significant. The highest risk of thyroid cancer in the age range of 15-60 years was related to the age of 15, which was estimated to be 0.238 in women and 0.042 in men per 100,000 people.

In lower ages and among women, the risk of thyroid cancer is higher than that of men. Also, due to the impossibility of limiting thyroid radiation in panoramic radiography using lead thyroid collar that causes metal artifacts, we should reduce the number of panoramic radiographs as much as possible, especially at lower ages.