American Journal of Epidemiology Advance Access originally published online on November 28, 2006
American Journal of Epidemiology 2007 165(4):474; doi:10.1093/aje/kwk098
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LETTERS TO THE EDITOR |
TWO OF THE AUTHORS REPLY
Northern California Cancer Center, Fremont, CA 94328
(e-mail: pam{at}nccc.org)
We thank Sioka et al. for their letter (1) and their interest in our paper (2). We are in agreement with these and other authors (3, 4) that while the risk of some specific malignancies may be at least partly due to the iatrogenic effects of thyroid cancer treatment, the occurrence of second primary malignancies at many sites is probably due primarily to shared environmental or genetic risk factors. We would like, however, to raise several cautions regarding interpretation of the data presented by Sioka et al. (1).
First, referral bias should be taken into account when interpreting results from a case series of patients. Second, caution should be exercised in interpreting results related to an exposure, such as iodine-131 treatment, when there is no unexposed group included in the analysis. Third, sufficient statistical power is a nonnegligible problem when assessing second malignancies following a relatively rare tumor. While we saw no evidence of radiation-induced secondary primary malignancies, our power to detect such effects was limited, even with approximately 2,500 irradiated patients followed for more than 5 years; fewer than five second primary malignancies were observed for many sites within this subgroup (2). Fourth, when examining preexisting cancers, survival bias and (in case series) selection bias are of particular concern.
A stronger method for examining the effects of thyroid cancer risk following the diagnosis of another cancer, such as breast or colon cancer, would be to identify a population diagnosed with the "preexisting" cancer of interest and then follow these persons up to assess the risk of a second malignancy occurring in the thyroid gland, as was done by Ronckers et al. (3). Nonetheless, we appreciate the additional information provided by Sioka et al. (1).
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ACKNOWLEDGMENTS |
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Conflict of interest: none declared.
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References |
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 TOP References |
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- Sioka C, Zouboulidis A, Fotopoulos A, et al. (2007) Re: "Risk of second primary malignancies in women with papillary thyroid cancer." (Letter). Am J Epidemiol 165:473.
[Free Full Text] - Canchola AJ, Horn-Ross PL, Purdie DM. (2006) Risk of second primary malignancies in women with papillary thyroid cancer. Am J Epidemiol 163:521–7.
[Abstract/Free Full Text] - Ronckers CM, McCarron P, Ron E. (2005) Thyroid cancer and multiple primary tumors in the SEER cancer registries. Int J Cancer 117:218–88.[CrossRef]
- Rubino C, de Vathaire F, Dottorini ME, et al. (2003) Second primary malignancies in thyroid cancer patients. Br J Cancer 89:1638–44.[CrossRef][Web of Science][Medline]
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  L. Myer, L. Kuhn, L. Denny, and T. C. Wright Jr. THE AUTHORS REPLY Am. J. Epidemiol., February 15, 2007; 165(4): 475 - 476. [Full Text] [PDF]
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