Eur J Epidemiol DOI 10.1007/s10654-015-9992-x

LETTER TO THE EDITOR

Self-reported skin cancer is unreliable Ann-Sofie Sonne Holm • Hans Christian Wulf

Received: 30 October 2014 / Accepted: 9 January 2015 Ó Springer Science+Business Media Dordrecht 2015

Self-reported information is a tool commonly used in both clinical and epidemiological studies and is in many cases the only available source of information. And although some studies use medical records or registers, the majority of studies use self-reported information [1]. However, if the validity of this information is erroneous, the scientific results generated will in consequence be correspondingly erroneous. The validity of self-reported information has been widely discussed for many years, but there are very few studies examining the validity of self-reported skin cancer [2–5]. The incidence of skin cancer may be difficult to assess, as in most countries non-melanoma skin cancer (NMSC) incidence is not routinely registered. Epidemiological data on NMSC incidence are consequently sparse [6]. However, Denmark has extensive registration of NMSC in the nationwide pathology database (Patobank). Against this background, the objective of the present study was to evaluate the validity of self-reports regarding having had cutaneous malignant melanoma (CMM), NMSC, and precursors to skin cancer (PSC) and subsequently to explore possible explanations for potential discrepancies between self-reports and Patobank. In 2011 a nationwide individualised skin cancer prevention campaign was carried out. A large walk-in bus, which visited 30 towns, five beaches, eight camping grounds and two marinas all over Denmark, provided the setting. These locations were selected based on the fact that there were a lot of people gathered these places. Prior to the visit, the arrival of the bus was announced in local media.

A. S. Holm (&)  H. C. Wulf Department of Dermatology D92, Copenhagen University Hospital, Bispebjerg, Bispebjerg Bakke 23, 2400 Copenhagen, Denmark e-mail: [email protected]

Thus, participation in the study was based on voluntariness. Participants completed a questionnaire on whether a doctor had ever diagnosed them with CMM, NMSC, or PSC. To verify the self-reported information, all the participants’ pathology test results were obtained through Patobank. Patobank contains information on all pathology tests from all the Danish pathology departments and clinics in the country. All diagnoses are connected to a personal 10-digit number for personal identification (CPR) [6–8]. Skin metastases and cases of skin cancer on the genitalia and anal region were excluded from the study. Individuals who had died or emigrated since the completion of the questionnaire as well as individuals who had not supplied their CPR number were excluded. Individuals were registered as having CMM (n = 37) if the diagnosis CMM occurred in Patobank, and those diagnosed with basal cell carcinoma (BCC) (n = 106), squamous cell carcinoma (SCC) (n = 13), or sebaceocellulært carcinoma (n = 1) were registered as having NMSC. If the diagnosis of keratosis actinica (n = 86), Bowen’s disease (n = 3), keratoacanthoma (n = 11) or dysplasia (n = 24) occurred in Patobank, the individuals were registered as having PSC. Positive prediction values (PPV), and a negative prediction values (NPV) were calculated (Table 1) and presented with 95 % confidence interval (CI). Patobank were used as the golden standard, which is discussed later in this letter. By using logistic regression predictors for false reporting were analysed. Twenty-eight percent of the participants were excluded due to the exclusion criteria described above, after which the dataset consisted of 2,898 individuals. The vast majority (94 %) were excluded due to missing CPR number, while 1.5 % were dead and 4.5 % had emigrated. The PPVs (CI) for CMM, NMSC and PSC were 71.7 % (58.7; 84.7), 76.0 % (67.8; 84.2), and 37.9 % (28.5; 47.3) respectively. The NPVs

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A.-S. S. Holm, H. C. Wulf Table 1 Formulas for calculation of positive and negative prediction values and the data necessary to calculate these for CMM, NMSC and PSC respectively

Skin cancer according to Patobank

Self-reported skin cancer ?

-

?

True-positive (TP) (33,79,39)

False-negative (FN) (4,37,57)

-

False-positive (FP) (13,25,64)

True-negative (TN) (2,847, 2,757, 2,738)

TP TN ; NPV ¼ TNþFN PPV ¼ TPþFP

(CI) were 99.9 % (99.1; 100), 98.7 % (96.0; 100), and 98.0 % (92.9; 100) for CMM, NMSC, and PSC, respectively. For CMM and NMSC collectively there was a PPV of 78.1 %. Individuals whose diagnosis dated more than five years back in time were approximately three times more likely to give a false-negative report than individuals diagnosed within the last five years. Further, having four or more dermatological diagnoses were significantly associated with false-negative reporting. There was no association between false-negative reporting and either sex or age. Compared to individuals younger than 55 years of age, those aged 55–64, 65–74, and C75 years of age were approximately 4.5, 8.5, and 18 times more likely to give a false-positive report, respectively. Moreover, analysis showed that the likelihood of giving a false-positive report was associated with increasing number of dermatological diagnoses. Individuals with one or more dermatological diagnoses are approximately three times as likely to give a false-positive answer as those with no dermatological diagnosis. All associations remained after adjusting for age. Analysis revealed no correlation between false-positive reporting and sex (Table 2). Four previous studies have been identified examining the validity of self-reported skin cancer [2–5]. However one study [3] included only five individuals and will not be commented on here. Another study [5] did not provide the PPV and NPV, why these were subsequently calculated. For CMM, studies have found PPVs ranging from 19 to 57 % [2, 4, 5]. The divergent results, compared to the present study could possibly be explained by the limited reach in the other studies, e.g. one did not include outpatient settings [2]. Two former studies claim to examine NMSC, however one of the studies [5] validate the selfreported answers by comparing them to a register which do not register NMSC. The other study, have found a PPV of 94.4 % [4] however the study recruited participants from a dermatological clinic. Thereby a higher value is to be expected, since the values of PPV and NPV may vary with the prevalence of the disease in the population. In general, increasing prevalence will result in increased PPV and decreased NPV. The NPV for CMM and NMSC is high in all the other studies as well as in the present study [2, 4, 5].

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Only one study [4] has been identified concerning CMM or NMSC and false-reporting. Therefore, studies on other cancer types are included in the comments below. In the present study, sex was not significantly associated with false reporting, which is in accordance with the main conclusion in other studies [4, 9, 10]. With respect to age, two studies [4, 10] found slightly differing results, but did not distinguish between false-negative and false-positive, for which reason it is difficult to compare the results with those presented here. The present study found an association between false-positive reporting and age, which is in accordance with other studies examining overall cancer [10] and one examining breast cancer [5]. In the present study, false-negative reporting was not associated with age, which largely accords with other studies [3, 5, 9]. In the present study years since diagnosis were significantly associated with giving a false-negative report. This is in accordance with another study finding increased time since a cancer diagnosis to be associated with incorrect reporting [3]. Another study [5] did not find this association which may be because the years stated were self-reported, while in the present study the year of diagnosis was extracted from Patobank. False-negative reporting was inversely associated with having four or more dermatological diagnoses, and this association is comparable with the association found in another study [3]. A false-positive report was associated with an increasing number of dermatological diagnoses. In assessing the significance regarding predictors of false-positive reports, it should be noted that the individuals who reported false-positives represents only 2 % of the answers for people without skin cancer. The absolute value of these findings may thereby be greater in a theoretical than a practicable perspective. Furthermore, disease-related characteristics in regard to various cancer types have been found to be associated with incorrect self-reporting. Individuals with a more serious cancer form are most likely to give a correct report [3, 5, 10]. Based on this one would expect PPV to be highest for CMM. Yet, this was not the case in the present study. However, the number of cases from which the PPV for CMM is calculated is somewhat smaller than for NMCS, and consequently greater uncertainty is attaches to these results.

Self-reported skin cancer is unreliable Table 2 Predictors of false-negative and false-positive reporting of skin cancer People with skin cancer Variables

Truepositive

Falsenegative

Analyses

N

%

N

%

P value

Men

65

83.3

13

16.7

1

Women

95

79.8

24

20.2

1.26

Sex

OR

95 % CI

Truenegative

Falsepositive

Analyses

N

%

N

P value

776

98.0

16

2.0

1,528

97.4

41

2.6

%

0.539

Age \ 55 years

30

90.9

55-64 years

48

65-74 years

68

C 75 years

26

95 % CI

[0.60; 2.66]

1 1.30

[0.73; 2.33]

\0.001

3

9.1

87.3

7

12.7

0.604

1.46

79.1

18

20.9

0.141

2.65

72.2

10

27.8

0.058

3.85

1

1,699

99.2

13

0.8

1

[0.35; 6.08]

466

96.7

16

3.3 \0.001

4.49

[2.14; 9.40]

[0.73; 9.67]

380

93.8

25

6.2 \0.001

8.60

[4.36; 16.96]

[0.96; 15.49]

66

88.0

9

12.0 \0.001

17.82

[7.36; 43.17]

0.004

0.002

0 1

– 35

– 71.4

– 14

– 28.6



2

33

82.5

7

17.5

0.655

0.53

3

20

66.7

10

33.3

0.655

C4

84

92.3

7

7.7

0.002

Years since diagnosis

OR

0.377

0.149

No. of dermatological diagnoses*

*

People without skin cancer

– 1



1,711 479

98.6 96.4

25 18

1.4 3.6

0.003

1 2.57

[1.39; 4.75]

[0.19; 1.48]

216

96.0

9

4.0

0.008

2.85

[1.31; 6.19]

1.25

[0.47; 3.33]

105

95.5

5

4.5

0.018

3.26

[1.22; 8.69]

0.21

[0.08; 0.56]

114

95.0

6

5.0

0.006

3.06

[1.45; 8.96]













0.003

B5 years

129

87.2

19

12.8

1

[5 years

43

69.4

19

30.6

3.00

– [1.46; 6.19]

Dermatological diagnosis includes skin cancer

The discrepancy in the present study between selfreports and Patobank could be due to poor communication between doctor and patient. It is possible that some patients have never been made aware of the results of their biopsy, especially in cases where they have been treated at the same visit as the biopsy was performed. Moreover, it is likely that pathology diagnoses which have been classified as malignancies in the present study may have been discussed with the patient in such a way that the patient did not consider them to be cancers. Consequently it was examined whether the non-inclusion of dysplasia as a PSC modified the results, which proved not to be the case. The fact that Patobank is use as a golden standard can be debated, as basal cell carcinoma is sometimes treated without histological verification. However, it is much less likely, if not improbable, that CMM is treated without histological verification and the PPV for NMCS and CMM are relatively similar (71.1 and 76.0 %, respectively). Thus, the estimated obtained is not considered erroneous. However, the difference between the amount treated on a clinical suspicion alone and the amount treated based on a histological verification potentially is even greater for PSC. Thus, it is likely that the ‘‘true’’ PPV for PSC is more similar with the PPV for CMM and NMSC than the PPV found in the present study.

In many countries the registration of BCC is often nonexistent. However, the present study utilised the unique opportunity provided by Patobank to examine the validity of both CMM and a number of NMSCs including BCC. Patobank has recorded BCC, SCC, and other rare histological types of cancers of the skin separately since 1978 [6], and the coverage of skin cancer is almost 100 % [7]. However, it is possible that there are a few cases that are not included in Patobank occurred before the registration was 100 %. Further, therapeutical destruction of the biological material without prior biopsy can be the cause of an unknown number of cases going unregistered [6–8]. In addition, it is possible that the fact that participation in the study was based on voluntariness means that it attracted a selected group with an interest in solar skin damage and skin in general. There was no significant difference between the excluded and included material in regard to age (P = 0.858), sex (P = 0.858), or self-reported skin cancer (P = 0.944) and thereby no selection bias was detected. Nevertheless it is possible that these two groups differ from each other in other ways not examined in the present study. The present study provides important novel knowledge and is particularly relevant for epidemiological studies, as it to the best of our knowledge is the first study to examine

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A.-S. S. Holm, H. C. Wulf

the validity of both self-reported CMM and NMSC at the population level. The present study has revealed that the PPV for skin cancer is relatively poor and that people are more likely to know whether they have had skin cancer, but unable to distinguish between the different kinds. In conclusion, these results suggest that epidemiological studies that rely on self-reported cancer information most probably are biased, which should be taken into consideration in future studies. More specifically, studies that examine the incidence of skin cancer will most likely overestimate the incidence of skin cancers, if the study is conducted through self-reports. Studies using self-reports to examine the causal link between skin cancer and other factors may consequently blur any correlations that would otherwise be present and an even greater data materiel will be necessary to display the associations. Acknowledgments The LEO Foundation and The Association of Danish Pharmacies hosted the skin cancer campaign. Conflict of Interest Dr. Wulfs/Miss Holms institution received grants from The LEO Foundation, during the conduct of the study. Outside the submitted work Dr. Wulf has received speaking honorarium from LEO Pharma and Galderma.

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2. Bergmann MM, Calle EE, Mervis CA, Miracle-McMahill HL, Thun MJ, Health CW. Validity of self-reported cancers in a Propsective Cohort Study in comparison with data from state cancer registries. Am J Epidemiol. 1998;147(6):556–62. 3. Desai MM, Bruce ML, Desai RA, Druss BG. Validity of selfreported cancer history: a comparison of health interview data and cancer registry records. Am J Epidemiol. 2001;153(3): 299–306. 4. Ming ME, Levy RM, Hoffstad OJ, Filip J, Gimotty PA, Ming ME. Validity of patient self-reported history of skin cancer. Arch Dermatol. 2004;140(6):730–5. 5. Parikh-Patel A, Allen M. Wright WE, the California Teachers Study Steering Committee. Validation of self-reported cancers in the California Teachers Study. Am J Epidemiol. 2003;157(6): 539–45. 6. Birch-Johansen F, Jensen A, Mortensen L, Olesen AB, Kjær SK. Trends in the incidence of nonmelanoma skin cancer in Denmark 1978–2007: rapid incidence increase among young Danish women. Int J Cancer. 2010;127(9):2190–8. 7. Bjerregaard B, Larsen OB. The Danish pathology register. Scand J Public Health. 2011;39(7 suppl):72–4. 8. The Danish registry of pathology. 2014. http://www.patobank.dk (In Danish). 9. Manjer J, Merlo J, Berglund G. Validity of self-reported information on cancer: determinants of under- and over-reporting. Eur J Epidemiol. 2004;19(3):239–47. 10. Navarro C, Chirlaque MD, Tormo MJ, Pe´rez-Flores D, Rodrı´quez-Barranco M, Sa´nchez-Villegas A, et al. Validity of self reported diagnoses of cancer in a major Spanish prospective cohort study. J Epidmiol Commun H. 2006;60(7):593–9.

Self-reported skin cancer is unreliable.

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