More information about text formats
Letter to the Editor
The article in question：
Crewe JM, Threlfall T, Clark A, Sanfilippo PG, Mackey DA. Pterygia are indicators of an increased risk of developing cutaneous melanomas. Br J Ophthalmol 2017.
Address：#180 Fenglin Rd., Department of Ophthalmology, Zhongshan Hospital Affiliated to Fudan University, Shanghai 200032, P.R. China
Phone: +86-186 1688 1220 or +86-21-64041990 ext. 2684
We read with great interest the paper by Crewe et al.1 The authors showed that patients with pterygium had higher risk of cutaneous melanomas (CM) in a large retrospective matched-cohort study in Western Australia (WA), and suggested pterygium as an indicator for CM. The finding was interesting. However, we doubt the conclusion and its clinical relevance and public health significance.
Compared to control group, patients with pterygium had a 20% or 24% increased risk of developing CM in terms of odds ratio(OR) or incidence rate ratio (IRR), respectively. The incidence rate difference（IRD）, however, was only 27.7/100 000 person-years (PY) (Table 5., by subtracting the IR of the control group from that of the pterygium group, i.e. (186.5-158.8)/100 000 PY). The rate difference corres...
Compared to control group, patients with pterygium had a 20% or 24% increased risk of developing CM in terms of odds ratio(OR) or incidence rate ratio (IRR), respectively. The incidence rate difference（IRD）, however, was only 27.7/100 000 person-years (PY) (Table 5., by subtracting the IR of the control group from that of the pterygium group, i.e. (186.5-158.8)/100 000 PY). The rate difference corresponds to a number needed to harm (NNH) of 3610 (the reciprocal of the rate difference), which means that if 3610 individuals with pterygium that needed to be excised are followed for one year, approximately one additional CM could be detected. However, the annual cases of pterygium treatment in WA were less than 1/3 of this NNH value (figure 2; around 1000 pterygium cases were treated in WA hospitals in 2013) 1. In other words, only one person could gain benefit if all cases of pterygium treatment in WA be carefully followed up for more than 3 years. It could potentially result in unnecessary biopsies, lead to overdiagnosis and overtreatment, and cause enormous waste of resources,2 not to mention the considerable anxiety exposed to the patients and their relatives year by year.
Giving the strategy of the exposure group (the patients with pterygium) sampling, there was obvious selection bias in Crewe’s study. The reported population prevalence of pterygium varies from 2.8% to 7.8% in Australia.3-5 But only the cases with hospital treated pterygium were included in the study, which overrepresented the serious pterygium cases with higher cumulative risk of ultraviolet radiation exposure, thus may be predisposed to CM. On the other hand, patients may have their pterygia removed for cosmetic reasons. An increased detection rate for CM could also be observed in such patients as they might be alert to any changes of their appearance. Thus, the selection of the exposure group in this cohort study could falsely increase the association between pterygium and CM. As acknowledged in the paper, only a small proportion of pterygium cases were included in the study. If, however, the potentially biased conclusion be extrapolated to general pterygium cases, it could do even more harm than good.
A real-world study with big data is appealing because of its representation of the wider population. However, even a small effect, if any, can be statistically significant with a large sample size. Caution should especially be taken in interpreting the findings and their clinical relevance and significance.
Moreover, a typo in Table 5 needs to be corrected. The age groups should be labeled as “>50 years” and “<49 years.”
Minqian Shen. 1,2
1. Zhongshan Hospital Affiliated to Fudan University
2. Center for Evidence-based Medicine, Fudan University
Financial Disclosures: The authors have no financial disclosures.
1. Crewe JM, Threlfall T, Clark A, Sanfilippo PG, Mackey DA. Pterygia are indicators of an increased risk of developing cutaneous melanomas. Br J Ophthalmol 2017.
2. Force USPST, Bibbins-Domingo K, Grossman DC, et al. Screening for Skin Cancer: US Preventive Services Task Force Recommendation Statement. JAMA 2016;316(4):429-35.
3. Pham TQ, Wang JJ, Rochtchina E, Mitchell P. Pterygium, pinguecula, and 5-year incidence of cataract. Am J Ophthalmol 2005;139(6):1126-8.
4. Landers J, Henderson T, Craig J. Prevalence of pterygium in indigenous Australians within central Australia: the Central Australian Ocular Health Study. Clin Exp Ophthalmol 2011;39(7):604-6.
5. McCarty CA, Fu CL, Taylor HR. Epidemiology of pterygium in Victoria, Australia. Br J Ophthalmol 2000;84(3):289-92.