© 2005 Oxford University Press
CORRESPONDENCE |
RESPONSE: Re: Familial Clustering of Hodgkin Lymphoma and Multiple Sclerosis
Affiliations of authors: Department of Epidemiological Research, Statens Serum Institut, Copenhagen, Denmark (HH, KR, NMN, LM, MM); National Institute of Public Health, Copenhagen, Denmark (NK-H); Department of Neurology, Aarhus University Hospital in Aalborg, Aalborg, Denmark (NK-H); Department of Cancer Prevention and Documentation, Danish Cancer Society, Copenhagen, Denmark (HHS)
Correspondence to: Henrik Hjalgrim, MD, Department of Epidemiological Research, Statens Serum Institut, Copenhagen, Denmark (e-mail: hhj{at}ssi.dk).
In a series of analyses, we recently observed an increased risk of multiple sclerosis (relative risk [RR] = 2.25, 95% confidence interval [CI] = 1.24 to 4.06) in first-degree relatives of patients with Hodgkin lymphoma and, independently hereof, an analogously increased occurrence of young-adult Hodgkin lymphoma (RR = 1.93, 95% CI = 1.01 to 3.71) in first-degree relatives of patients diagnosed with multiple sclerosis in a Danish register–based investigation (1).
The observed clustering of these two conditions is interesting because it may reflect that they share common risk factors and because the clustering may form the basis for new etiological hypotheses. In recognition of the absence of similar reports in the literature, however, we encouraged other researchers to verify our observations.
Landgren et al. have taken up this challenge and (re)analyzed Danish and Swedish cancer and hospital discharge register data. Using more elaborate statistical techniques combining the time periods before and after the exposure-defining lymphoma diagnosis, they observe a 1.98-fold increased risk for multiple sclerosis in first-degree relatives of Danish Hodgkin lymphoma patients and a, more modest, 1.29-fold increased risk for multiple sclerosis in first-degree relatives of Swedish patients with Hodgkin lymphoma. Thus, these new Danish-Swedish analyses suggest a relative risk of 1.51 (95% CI = 1.07 to 2.15) for multiple sclerosis in the combined group of all first-degree relatives of patients with Hodgkin lymphoma.
Although different statistical approaches make direct comparison of risk estimates difficult, we agree with the authors that there is good agreement between our previous results using the Danish Multiple Sclerosis Register and the current series of analyses using Danish hospital discharge data. In our view, however, this consistency does not necessarily imply that the results of analyses of Swedish data should be interpreted uncritically. The possible implications of the various gaps in the Swedish Family Register and parental information missing for 50% of offspring dead before 1991 (2) are unclear to us, and we also disagree that diagnostic misclassification in the hospital discharge data, the extent of which may well differ between Denmark and Sweden, can automatically be presumed unimportant.
With these precautions, therefore, it would seem that the most important finding of the current investigation is the confirmation of the previously observed familial clustering of Hodgkin lymphoma and multiple sclerosis (1).
REFERENCES
(1) Hjalgrim H, Rasmussen S, Rostgaard K, Nielsen NM, Koch-Henriksen N, Munksgaard L, et al. Familial clustering of Hodgkin lymphoma and multiple sclerosis. J Natl Cancer Inst 2004;96:780–4.
(2) Goldin LR, Pfeiffer RM, Gridley G, et al. Familial aggregation of Hodgkin lymphoma and related tumors. Cancer 2004;100:1902–8.[CrossRef][Web of Science][Medline]
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J Natl Cancer Inst 2005 97: 543-544.
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