Background
Squamous cell carcinoma (SCC) is the second most common skin cancer after basal cell carcinoma (BCC). High cumulative ultraviolet (UV) radiation exposure is the major environmental risk factor for SCC along with other host factors that are markers of UV susceptibility such as fair skin and fair/red hair [
1‐
3].
Physical activity influences the risk of several types of cancer. Existing evidence strongly supports the role of regular physical activity in reducing risk of cancers of the colon, breast, and endometrium, and to a lesser extent cancers of the lung and pancreas [
4]. It is also suggested that a similar association may exist for other cancers [
5].
Proposed biological mechanisms through which physical activity may affect cancer risk or progression include changes in sex hormones, metabolic hormones and growth factors, inflammatory markers, and immune function [
5,
6]. Beneficial effects of physical activity are also conceivable for skin cancer, specifically those that improve immune function and those that increase detoxification of reactive oxygen species or increase DNA repair, thus reducing sun exposure-induced DNA damage [
5,
7,
8].
Data on the association between physical activity and skin cancer are so scarce however [
9], that conclusions on physical activity and body fatness as possible determinants of skin cancer could not be reached for lack of evidence [
4]. A potential positive association between amount of recreational physical activity and skin cancer has been suggested in athletes, known to have very high UV radiation exposure levels while performing outdoor sports [
10‐
12]. In addition to sun exposure, exercise-induced immunosuppression has also been suggested to increase risk of keratinocyte carcinomas and cutaneous melanoma in athletes [
12,
13].
The very few population-based studies on recreational or occupational physical activity and skin cancer incidence are confined to melanoma [
9] or combined keratinocyte cancers (SCC and BCC) [
14‐
16]. As for athletes, in the general population confounding of the association by sun exposure remains a potential explanation for any positive associations [
16]. Evidence on occupational sun exposure as a factor associated with keratinocyte cancers remains unclear based on the few investigations exploring outdoor work or occupations in relation to these cancers [
17‐
19].
Our aim was to examine the associations of recreational and occupational physical activity and incidence of SCC in a long-term community-based cohort study of skin cancer in Australia taking into account potential confounding by sun exposure and other established risk factors for SCC.
Results
For the present study, participants with missing data on physical activity items (n = 282) and body mass index (BMI, kg/m2) (n = 168) were excluded, leaving an analytical cohort of 1,171 men and women. Participants who were excluded from the analysis tended to be younger than their included counterparts, but the difference in mean age at baseline was only statistically significant in men (47.2 vs 50.5 yrs). Further, excluded women were more likely to have a lower education, while excluded men were more likely to have a lower grading of clinical elastosis and were less likely to have a keratinocyte cancer prior to 1992 compared to their respective counterparts included in the study.
Study participants aged 25 to 75 years at baseline (1992) were followed for an average period of 14.4 (± 3.8) years, yielding a total of 16 887 person-years. Characteristics of the study population stratified by case status and sex are presented in Table
1,
2. Compared to those unaffected, persons with SCC were on average older, more likely to burn than tan on strong sun exposure, have a higher grade of freckling on the back, and severe clinical elastosis of the neck (Table
1). Study participants with or without SCC did not differ in their leisure or occupational sun exposure in 1992 or their cumulative sun exposure over time (data not shown). Twenty-eight percent had a history of keratinocyte cancer (SCC and/or BCC) before the baseline examination in 1992.
Table 1
Characteristics by skin cancer status, the Nambour Skin Cancer Study (N = 1,171)
|
Mean (SD)
|
Age (years) | 57.3 (9.9) | 48.1 (12.1) | < 0.001 | 59.7 (11.7) | 48.3 (13.0) | < 0.001 |
BMI, kg/m2
| 26.2 (5.0) | 25.8 (4.7) | 0.515 | 26.6 (3.8) | 26.5 (3.6) | 0.868 |
|
N (%)
|
History of skin cancer | 48 (53.3) | 123 (21.4) | < 0.001 | 64 (65.3) | 88 (21.6) | < 0.001 |
Education | | | | | | |
High school only | 63 (70.0) | 366 (63.7) | 0.446 | 37 (37.8) | 167 (40.9) | 0.284 |
Certificate/diploma | 3 (3.3) | 34 (5.9) | | 31 (31.6) | 147 (36.0) | |
University | 22 (24.4) | 147 (25.6) | | 22 (22.4) | 59 (14.5) | |
Trade/other | 2 (2.2) | 28 (4.9) | | 8 (8.2) | 35 (8.6) | |
Pack-years of smoking | | | | | | |
None | 56 (62.2) | 383 (66.6) | 0.599 | 35 (35.7) | 181 (44.4) | 0.115 |
1-7 | 13 (14.4) | 92 (16.0) | | 16 (16.3) | 65 (15.9) | |
> 7-20 | 10 (11.1) | 49 (8.5) | | 13 (13.3) | 67 (16.4) | |
> 20 | 11 (12.2) | 51 (8.9) | | 34 (34.7) | 95 (23.3) | |
Tanning ability of skin | | | | | | |
Always burn | 33 (36.7) | 130 (22.6) | 0.003 | 29 (29.6) | 55 (13.5) | < 0.001 |
Burn then tan | 54 (60.0) | 377 (65.6) | | 62 (63.3) | 310 (76.2) | |
Only tan | 3 (3.3) | 68 (11.8) | | 7 (7.1) | 42 (10.3) | |
Painful sunburns throughout life (1992) |
Never | 10 (11.1) | 62 (10.8) | 0.084 | 15 (15.3) | 48 (11.8) | 0.798 |
Once | 21 (23.3) | 114 (19.8) | | 14 (14.3) | 55 (13.5) | |
2-5 times | 29 (32.2) | 263 (45.7) | | 40 (40.8) | 176 (43.2) | |
More than 5 times | 30 (33.3) | 136 (23.7) | | 29 (29.6) | 128 (31.4) | |
Freckling of the back (1992)a
|
Nil | 18 (20.5) | 197 (35.1) | 0.001 | 19 (19.6) | 125 (32.0) | < 0.001 |
Mild | 29 (33.0) | 215 (38.3) | | 25 (25.8) | 137 (35.0) | |
Moderate | 30 (34.1) | 103 (18.3) | | 20 (20.6) | 76 (19.4) | |
Severe | 11 (12.5) | 47 (8.4) | | 33 (34.0) | 53 (13.6) | |
Clinical elastosis of the neck (1992)a
|
Nil | 5 (5.6) | 160 (27.8) | < 0.001 | 2 (2.1) | 72 (17.7) | < 0.001 |
Mild | 38 (42.2) | 282 (49.0) | | 35 (36.1) | 205 (50.4) | |
Severe | 47 (52.2) | 133 (23.1) | | 60 (61.9) | 130 (31.9) | |
Table 2
Type of physical activity by skin cancer status, the Nambour Skin Cancer Study (N = 1,171)
Recreational activity, total (hrs/wk) |
None | 32 (35.6) | 190 (33.0) | 0.823 | 33 (33.7) | 169(41.4) | 0.093 |
F ≤ 1 | M ≤ 1.5 | 18 (20.0) | 135 (23.5) | | 17 (17.3) | 88 (21.6) | |
F ≤ 3 | M ≤ 4 | 19 (21.1) | 131 (22.8) | | 21 (21.4) | 81 (19.9) | |
F > 3 | M > 4 | 21 (23.3) | 119 (20.7) | | 27 (27.6) | 70 (17.2) | |
Walking (hrs/wk) | | | | | | |
None | 40 (44.4) | 278 (48.3) | 0.479 | 43 (43.9) | 261 (64.0) | < 0.001 |
F < 1 | M < 1.2 | 17 (18.9) | 88 (15.3) | | 11 (11.2) | 57 (14.0) | |
F < 2.5 | M < 2.5 | 13 (14.4) | 107 (18.6) | | 20 (20.4) | 46 (11.3) | |
F ≥ 2.5 | M ≥ 2.5 | 20 (22.2) | 102 (17.7) | | 24 (24.5) | 44 (10.8) | |
Moderate activity (hrs/wk) |
None | 74 (82.2) | 434 (75.5) | 0.374 | 72 (73.5) | 290 (71.1) | 0.895 |
F < 1.5 | M < 1.75 | 8 (8.9) | 72 (12.5) | | 13 (13.3) | 59 (14.5) | |
F ≥ 1.5 | M ≥ 1.75 | 8 (8.9) | 69 (12.0) | | 13 (13.3) | 59 (14.5) | |
Vigorous activity (hrs/wk) |
None | 85 (94.4) | 527 (91.7) | 0.363 | 92 (93.9) | 350 (85.8) | 0.030 |
F ≤ 8 | M ≤ 12 | 5 (5.6) | 48 (8.3) | | 6 (6.1) | 58 (14.2) | |
Recreational activity, total (MET-hr/wk) |
None | 32 (35.6) | 190 (33.0) | 0.911 | 33 (33.7) | 169 (41.4) | 0.417 |
F ≤ 3.7 | M ≤ 6 | 19 (21.1) | 132 (23.0) | | 19 (19.4) | 81 (19.9) | |
F ≤ 10.7 | M ≤ 18 | 21 (23.3) | 125 (21.7) | | 23 (23.5) | 82 (20.1) | |
F > 10.7 | M > 18 | 18 (20.0) | 128 (22.3) | | 23 (23.5) | 76 (18.6) | |
Occupational activity | | | | | | |
Sedentary | 25 (29.8) | 192 (37.4) | 0.073 | 23 (24.2) | 104 (29.1) | 0.200 |
Standing | 9 (10.7) | 84 (16.3) | | 37 (38.9) | 146 (40.8) | |
Manual | 50 (59.5) | 238 (46.3) | | 35 (36.8) | 108 (30.2) | |
At baseline, 33% of all women and 40% of all men were classified as physically inactive, i.e. they did not report any time spent walking or performing more moderate/vigorous recreational activities. Among physically active study participants, men reported more hours of total recreational activity than women. On average, active men (n = 304) spent a mean (SD) of 4.2 (4.5) hrs/wk and active women (n = 443) 2.9 (3.3) hrs/wk on any recreational activity. Walking was the predominant activity (men: 2.3 (3.7); women 1.9 (2.8) hrs/wk).
Women did not differ by case status in any of the recreational physical activity measures, whereas males with SCC reported more hours of walking, but less hours of vigorous exercise, compared to their counterparts with no SCC (Table
2). Regarding occupational activity, affected females tended to have a higher proportion of manual activity and lower proportion of sedentary occupations than nonaffected, but this difference did not reach statistical significance. Overall, women in manual activity tended to do their work mainly indoors (45%) or both indoors and outdoors (47%), whereas men in manual activity performed their work predominantly outdoors (66%).
In age-adjusted and multivariable-adjusted models, neither total recreational physical activity nor any individual type of activity or MET-hrs were significantly associated with risk of SCC when analysed by persons (Table
3) or tumor counts (Tables
4) in both sexes.
Table 3
Relative risks (RRs) and 95% CI of SCC (1992-2007) according to type of physical activity, stratified by sex
Recreational activity, total (hrs/wk) | | (n = 665) | (n = 650) | | (n = 506) | (n = 486) |
None | 32/222 | Reference | Reference | 33/202 | Reference | Reference |
F ≤ 1 | M ≤ 1.5 | 18/153 | 0.88 (0.53, 1.48) | 0.92 (0.56, 1.53) | 17/105 | 0.82 (0.50, 1.36) | 0.89 (0.54, 1.46) |
F ≤ 3 | M ≤ 4 | 19/150 | 0.96 (0.57, 1.60) | 1.04 (0.62, 1.74) | 21/102 | 1.23 (0.78, 1.94) | 1.29 (0.82, 2.04) |
F > 3 | M > 4 | 21/140 | 0.87 (0.53, 1.41) | 0.85 (0.52, 1.38) | 27/97 | 1.45 (0.93, 2.25) | 1.33 (0.86, 2.05) |
p-trend | | 0.647 | 0.557 | | 0.053 | 0.135 |
Walking (hrs/wk) | | | | | | |
None | 40/318 | Reference | Reference | 43/304 | Reference | Reference |
F < 1 | M < 1.2 | 17/105 | 1.30 (0.81, 2.10) | 1.36 (0.85, 2.17) | 11/68 | 0.99 (0.56, 1.74) | 1.04 (0.59, 1.82) |
F < 2.5 | M < 3.5 | 13/120 | 0.81 (0.44, 1.46) | 0.99 (0.55, 1.78) | 20/66 | 1.61 (1.06, 2.47) | 1.64 (1.07, 2.51) |
F ≥ 2.5 | M ≥ 3.5 | 20/122 | 0.98 (0.60, 1.62) | 1.06 (0.65, 1.74) | 24/68 | 1.63 (1.06, 2.50) | 1.37 (0.90, 2.08) |
p-trend | | 0.734 | 0.997 | | 0.026 | 0.142 |
Moderate activity (hrs/wk) |
None | 74/508 | Reference | Reference | 72/362 | Reference | Reference |
F < 1.5 | M < 1.7 | 8/80 | 0.72 (0.37, 1.43) | 0.67 (0.35, 1.29) | 13/72 | 0.88 (0.52, 1.48) | 0.95 (0.58, 1.57) |
F ≥ 1.5 | M ≥ 1.7 | 8/77 | 0.79 (0.41, 1.51) | 0.66 (0.35, 1.27) | 13/72 | 0.96 (0.58, 1.60) | 1.05 (0.64, 1.70) |
p-trend | | 0.389 | 0.141 | | 0.859 | 0.868 |
Vigorous activity (hrs/wk) |
None | 85/612 | Reference | Reference | 92/442 | Reference | Reference |
F ≤ 8 | M ≤ 12 | 5/53 | 1.49 (0.69, 3.21) | 1.30 (0.63, 2.65) | 6/64 | 1.02 (0.47, 2.21) | 1.08 (0.54, 2.18) |
p-value | | 0.410 | 0.533 | | 0.953 | 0.831 |
Recreational activity, total (MET-hr/wk) |
Sedentary | 32/222 | Reference | Reference | 33/202 | Reference | Reference |
F ≤ 3.7 | M ≤ 6 | 19/151 | 0.98 (0.55, 1.52) | 0.96 (0.58, 1.57) | 19/100 | 0.93 (0.58, 1.50) | 1.01 (0.63, 1.61) |
F ≤10.7 | M ≤ 18 | 21/146 | 0.96 (0.58, 1.58) | 1.07 (0.64, 1.81) | 23/105 | 1.16 (0.74, 1.82) | 1.16 (0.74, 1.83) |
F > 10.7 | M > 18 | 18/146 | 0.82 (0.50, 1.37) | 0.78 (0.47, 1.30) | 23/99 | 1.41 (0.88, 2.23) | 1.31 (0.84, 2.06) |
p-trend | | 0.481 | 0.318 | | 0.114 | 0.208 |
Occupational activity | | | | | | |
Sedentary | 25/217 | Reference | Reference | 23/127 | Reference | Reference |
Standing | 9/93 | 0.85 (0.55, 1.32) | 0.69 (0.46, 1.05) | 37/183 | 0.84 (0.54, 1.30) | 0.89 (0.58, 1.36) |
Manual | 50/288 | 0.70 (0.37, 1.34) | 0.64 (0.33, 1.24) | 35/143 | 1.00 (0.68, 1.48) | 1.13 (0.76, 1.69) |
p-value | | 0.456 | 0.131 | | 0.663 | 0.532 |
Table 4
Relative risks (RRs) and 95% CI of SCC counts (1992-2007) according to type of physical activity, stratified by sex
Recreational activity, total (hrs/wk) | | (n = 665) | (n = 650) | | (n = 506) | (n = 486) |
None | 62 | Reference | Reference | 59 | Reference | Reference |
F ≤ 1 | M ≤ 1.5 | 27 | 0.79 (0.42, 1.49) | 0.89 (0.48, 1.65) | 57 | 1.09 (0.58, 2.07) | 1.03 (0.55, 1.94) |
F ≤ 3 | M ≤ 4 | 29 | 0.91 (0.48, 1.70) | 0.97 (0.52, 1.79) | 41 | 1.18 (0.60, 2.32) | 1.20 (0.61, 2.35) |
F > 3 | M > 4 | 31 | 0.75 (0.41, 1.39) | 0.76 (0.42, 1.38) | 62 | 1.77 (0.93, 3.37) | 1.71 (0.91, 3.21) |
p-trend | | 0.454 | 0.409 | | 0.073 | 0.076 |
Walking (hrs/wk) | | | | | | |
None | 74 | Reference | Reference | 78 | Reference | Reference |
F < 1 | M < 1.2 | 28 | 1.21 (0.64, 2.26) | 1.41 (0.76, 2.62) | 30 | 1.06 (0.52, 2.20) | 1.40 (0.68, 2.89) |
F < 2.5 | M < 3.5 | 14 | 0.60 (0.29, 1.25) | 0.72 (0.35, 1.48) | 52 | 1.99 (1.02, 3.90) | 1.93 (0.99, 3.76) |
F ≥ 2.5 | M ≥ 3.5 | 33 | 1.06 (0.59, 1.90) | 1.12 (0.63, 2.01) | 59 | 1.99 (1.04, 3.81) | 1.59 (0.85, 2.98) |
p-trend | | 0.945 | 0.921 | | 0.027 | 0.149 |
Moderate activity1 (hrs/wk) |
None | 121 | Reference | Reference | 157 | Reference | Reference |
F < 1.5 | M < 1.7 | 17 | 1.00 (0.50, 2.02) | 0.83 (0.42, 1.65) | 33 | 1.18 (0.59, 2.38) | 1.00 (0.52, 1.92) |
F ≥ 1.5 | M ≥ 1.7 | 11 | 0.66 (0.29, 1.50) | 0.60 (0.27, 1.34) | 29 | 0.95 (0.46, 1.96) | 1.22 (0.59, 2.51) |
p-trend | | 0.325 | 0.192 | | 0.912 | 0.599 |
Vigorous activity2 (hrs/wk) |
None | 144 | Reference | Reference | 211 | Reference | Reference |
F ≤ 8 | M ≤ 12 | 5 | 1.03 (0.32, 3.27) | 0.90 (0.29, 2.85) | 8 | 0.78 (0.29, 2.09) | 0.73 (0.27, 1.96) |
p-value | | 0.961 | 0.861 | | 0.626 | 0.534 |
Recreational activity, total (MET-hr/wk) |
Sedentary | 62 | Reference | Reference | | Reference | Reference |
F ≤ 3.7 | M ≤ 6 | 28 | 0.81 (0.43, 1.51) | 0.91 (0.49 1.68) | 59 | 1.05 (0.55, 2.02) | 1.02 (0.54, 1.94) |
F ≤ 10.7 | M ≤ 18 | 31 | 0.89 (0.48, 1.65) | 0.98 (0.53, 1.82) | 54 | 1.37 (0.72, 2.60) | 1.35 (0.72, 2.53) |
F > 10.7 | M > 18 | 28 | 0.74 (0.40, 1.39) | 0.73 (0.40, 1.33) | 57 | 1.64 (0.83, 3.23) | 1.60 (0.83, 3.09) |
p-trend | | 0.438 | 0.320 | 49 | 0.125 | 0.134 |
Occupational activity | | | | | | |
Sedentary | 35 | Reference | Reference | | Reference | Reference |
Standing | 11 | 0.72 (0.42, 1.23) | 0.53 (0.32, 0.91) | 52 | 0.89 (0.49, 1.61) | 0.92 (0.51, 1.65) |
Manual | 96 | 0.46 (0.22, 1.10) | 0.48 (0.22, 1.07) | 72 | 0.89 (0.51, 1.53) | 0.90 (0.53, 1.53) |
p-value | | 0.155 | 0.027 | 84 | 0.891 | 0.925 |
Among men, the risk estimates for total recreational activity and walking suggested a slightly increased risk in SCC, but they were not statistically significant (except for walking 1.2-3.5 hrs/wk, person-based) and there were no dose-response trends. Occupational activity was not significantly related to SCC in men, but was in women. With increasing work activity, women had a reduced incidence of SCC (P trend = 0.03) in the tumor-based analysis. This association did not reach statistical significance in the person-based analysis.
In additional analyses (data not shown), neither stratification of tumors according to whether they occurred on chronically or occasionally sun-exposed skin sites, nor mutual adjustment of recreational and occupational activity materially affected the risk estimates in the respective physical activity models, with one exception. In men, the recreational activity-SCC association based on tumor counts was slightly stronger after adjustment for occupational activity (RR (95%CI) low, medium, high vs none: 1.36 (0.74, 2.49), 1.55 (0.81-2.96), 2.01 (1.09-3.69); p
trend
= 0.032). Finally, for completeness, we assessed the use of sunscreen (non-user/irregular user/regular user) and found no significant difference in sunscreen use across levels of recreational or occupational activity in either men or women, though compared with inactive women, women with active recreations tended to be more regular sunscreen users (data not shown).
Discussion
In this large prospective study of Australian adults, recreational physical activity was not significantly associated with development of cutaneous SCC when adjusted for other risk factors including indicators of sun exposure. In men, we observed a suggestive pattern of elevated risk with increasing hours of total activity or walking, but trends did not reach statistical significance. In contrast to recreational activity, higher occupational activity, i.e. standing or manual activity, was significantly associated with reduced risk of SCC among women in tumor-based, but not in person-based analysis.
To our knowledge, this study is the first to investigate the association between physical activity, both recreational and occupational, and cutaneous SCC and therefore, comparisons with other relevant reports are limited. Our observation of a suggestive positive association for recreational activity in men corroborates the finding from a large Danish prospective study [
16] showing a highly significant positive association of leisure activity with keratinocyte cancers in most active men, but not in women. Schnohr and colleagues [
16] speculated that the sex difference found in their study may be due to more outdoor exercise, less clothing, and less use of sunscreen in men than women. In their study on risk of cancer in general, the leisure activity-skin cancer association was not adjusted for outdoor sun exposure or any proxy measure reflecting sun exposure, and thus may be confounded by UV radiation. In contrast, in our SCC-study, we accounted for clinical elastosis, a marker of photoaging due to cumulative sun exposure [
30]. This may explain the lack of an independent effect of any of the recreational activity measures on SCC in our data.
Findings from two other reports examining incidence of melanoma or keratinocyte cancers in women with higher versus lower recreational activity during adult life are equivocal. In a study of female college alumnae from the US, there was no difference in the lifetime occurrence of keratinocyte cancers and melanomas between former college athletes and nonathletes [
31]. A Finnish register-based study of life-long physical activity and cancer risk among female teachers indicated that teachers of physical education had a non-significant 2-fold higher standardized incidence ratio of melanoma compared to language teachers during the follow-up period 1967-91 [
15]. Both investigations lack adjustment for sun exposure or any other potential confounder.
In the present study, physical activity at work was inversely and significantly associated with the development of SCC in women when tumor counts where considered. The person-based analysis suggested a similar protective effect with higher work activity. The lack of an association among men may be related to the overall higher proportion of manual activity in women (48% vs 32%) which was predominantly performed indoors and included home duties, in contrast to men. Thus, some potentially beneficial effects of a higher activity level combined with lower outdoor sun exposure may have contributed to the reduced risk in women. Results from a large hospital-based case-control study of selected cancer sites indicated no effect of work activity assessed as energy expenditure and sitting time during work hours on risk of keratinocyte cancers [
14]. Compatible with our investigation, the assessment of occupational activity level was based on job titles, but the cancer outcome was SCC and BCC combined, and sun exposure was not accounted for. Except for this single study [
14], we found no other relevant evidence in the literature on occupational activity and risk of SCC.
Similar to risk of internal cancers, any reduction in risk of SCC in persons with higher physical activity levels may be due to compensatory mechanisms by the physical activity, i.e. beneficial effects, specifically when sun-exposure is accounted for as in our study. However, this cannot fully explain the opposite tendency, i.e. towards a positive association between recreational activity and SCC in men observed in this or one other study [
16]. Further, it is not fully understood yet how much and what type of physical activity is required to diminish risk of different cancer types [
4,
32]. Many of the metabolic changes induced by physical activity are mediated through reduced adiposity and effects on other body compartments (e.g. skeletal muscle), and therefore it remains to be determined whether physical activity acts independently of weight control or not [
5,
7].
Some limitations warrant consideration when interpreting results of our study. Due to missing information on physical activity and BMI, we excluded 450 (28%) participants in the original study cohort which potentially could lead to selection bias. When we compared other study characteristics between those with and without missing data however, we found that the main differences were among men only: men who were excluded were on average 3 years younger than those who were included, and this was reflected in the lower actinic damage in the former. Given the lack of differences between women and the lack of any major differences among men who were excluded or included, we believe that any selection bias would be minimal.
The assessment of recreational physical activity relied on self-reports and referred to a time period of two weeks prior to the baseline examination which may not reflect physical activity patterns over an extended time period. However, the questionnaire activity items and the reference time period were adapted from the 1989-90 National Health Survey (NHS) [
24] and considered reliable measures. In this study 64% of the participants reported some form of recreational activity which is similar to the proportion (65%) of Queensland adults who reported physical exercise in the 1989-90 NHS [
24]. Among Nambour study participants the proportion of people walking was somewhat higher (51% vs 42%), and lower for moderate (24% vs 36%) or vigorous (12% vs 17%) activity compared to Queensland data from the NHS. While recreational activity patterns and thus intensity may have differed, the overall level of activity or inactivity in this study was compatible with national data.
Regular sunscreen use has been shown to reduce the occurrence of SCC in this study population [
22,
33], and thus we routinely adjusted for treatment allocation in all analyses. The known influence of fair skin on both sunscreen use and skin cancer [
34] was also accounted for in all analyses and should not have affected results. Further, baseline (1992) sunscreen use was not significantly correlated with recreational or occupational physical activity in men or women and did not differ by case status. The tendency of women to be more regular users with higher recreational activity levels compared to inactive ones observed here and elsewhere [
35] would be expected to reinforce an inverse association between physical activity and SCC if it were present, thus lending further weight to the null findings.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
PHL conceived the analysis plan and design, provided technical assistance to statistical analysis, interpreted the data and drafted the manuscript. AR carried out the statistical analysis and participated in the design of the study. ACG conceived of the study, participated in its coordination and helped to draft the manuscript. All authors read and approved the final manuscript.