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Cancer Research Center of Hawaii, University of Hawaii, Honolulu, Hawaii
Requests for reprints: Marc T. Goodman, Etiology Program, Cancer Research Center of Hawaii, University of Hawaii, 1236 Lauhala Street, Honolulu, HI 96813. Phone: 808-586-2987; Fax: 808-586-2982. E-mail: marc{at}crch.hawaii.edu
| Abstract |
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Methods: Penile cancer among 6,539 men was identified through 29 population-based registries in the United States during the period 1995-2003. These registries were estimated to represent 68% of the U.S. population. Age-adjusted incidence rates were calculated per million population using counts derived from the 2000 U.S. census. A subset of nine registries was used to examine time trends in penile cancer between 1973 and 2003.
Results: Squamous cell carcinomas were the most common histologic type of penile cancer, representing 93% of all malignancies. Hispanic men had the highest age-adjusted incidence rates per million for penile cancer (6.58 per million), followed by Blacks (4.02 per million), Whites (3.90 per million), American Indians (2.81 per million), and Asian-Pacific Islanders (2.40 per million). The highest rates of penile cancer were found among Hispanic men (46.9 per million) and Black men (36.2 per million) of ages
85 years. Penile malignancy was rare among males under age 20 years. Time trend analysis supported a significant decrease in the incidence of penile cancer for Blacks (annual percent change, –1.9%) and Whites (annual percent change, –1.2%). The majority (61%) of penile cancers were diagnosed at a localized stage among all racial and ethnic groups, although Hispanic and Black men tended to be diagnosed at more advanced stages than Whites. No racial or ethnic differences in tumor grade were identified. The incidence of penile cancer was highest in the South (4.42 per million) and lowest in the West (3.28 per million) of the United States. The highest age-adjusted incidence rate was found among Black men in the South (4.77 per million) and the lowest rate among Asian-Pacific Islanders in the West (1.84 per million).
Conclusions: This analysis showed significant racial/ethnic and regional variation in the incidence of penile cancer. The high rate of penile cancer among Hispanic and Southern Black men suggests differences in risk factors for this malignancy, such as circumcision, hygiene, or human papillomavirus exposure. (Cancer Epidemiol Biomarkers Prev 2007;16(9):1833–9)
| Introduction |
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Penile cancer is uncommon, so there have been few etiologic studies conducted of this disease (6-10). Factors associated with the risk of penile cancer include a history of smoking, poor hygiene (especially phimosis in uncircumcised men), multiple sexual partners, and sexually transmitted infections. Recent evidence is compelling for a role of human papillomavirus (HPV) in the etiology of squamous cell carcinoma of the penis, with high-risk HPV-16 detected in 29% of tumors in a recent Dutch study (11).
Descriptive investigations of penile cancer in the United States are presently lacking because of the low incidence of this malignancy. A review of the literature revealed only one recent descriptive analysis of penile cancer using Surveillance Epidemiology End Results (SEER) data, although this report on squamous cell carcinomas focused on survival (12). The investigators found that African American men presented with penile cancer at a younger age and more advanced stage of disease at diagnosis than did European Americans, but these results were significant only among men with advanced, nonmetastatic disease. Based on the limited knowledge of the causes and distribution of penile cancer, we examined racial and ethnic variation in this malignancy by age at diagnosis, stage, histology, geographic region, and calendar year at diagnosis.
| Materials and Methods |
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68% of the U.S. population during the 9-year time period, including 68% of the White population, 59% of the Black population, 48% of the Alaska Native-American Indian population, 83% of the Asian-Pacific Islander population, and 75% of the U.S. Hispanic population. Although there were too few Alaska Native cases to carry out some of the subgroup analyses (N = 20), they were included as part of the "total". All penile cancer cases (N = 6,539 men) meeting the North American Association of Central Cancer Registries standards with International Classification of Diseases for Oncology (15) topography codes C60.0 to C60.9 were included in the analytic file. A total of 98% of the cases were microscopically confirmed. Hispanic identification was available from 5,841 (89%) of the men included in the analysis.
A summary stage variable, developed by the SEER Program, was used to group cases into one of four categories: localized, regional, distant, or unknown (16). Localized tumors were those confined entirely to the organ of origin; regional tumors were those that extended into surrounding organs and tissues (or regional lymph nodes); and distant tumors were those that had spread to remote organs or lymph nodes. Tumor grade was classified as well-differentiated (grade 1), moderately differentiated (grade 2), poorly differentiated and undifferentiated (grades 3 and 4), or unknown grade.
To examine time trends in penile cancer incidence, we used SEER data from public use files including data from nine cancer registries (Atlanta, Connecticut, Detroit, Hawaii, Iowa, New Mexico, San Francisco Bay Area, Seattle-Puget Sound, and Utah) between 1973 and 2003. A total of 2,388 penile cancers were identified through this source, representing
26% of the U.S. population.
Population Counts
Population counts, used as denominators for calculating the cancer rates included in this analysis, were derived from the 2000 U.S. Census (17). The Census Bureau provided estimates of the resident populations of the U.S. counties by 5-year age groups and newborns (0, 1-4, 5-9,..., 80-84, and 85 years), gender (male or female), race (White, Black, American Indian, Asian-Pacific Islander), and Hispanic origin (Hispanic, non-Hispanic).
Statistical Analysis
SEER*Stat (version 6.2.4) was used to calculate both annual and average annual age-adjusted incidence rates expressed per million population (18). All rates were age adjusted by 5-year age groups to compare racial and ethnic groups by demographic, pathologic, and clinical variables of interest. Confidence intervals for the rates were calculated by the method of Fey and Feuer (19). The 2000 U.S. population was used for age standardization. For all analyses, rates were suppressed when the category had fewer than 20 cases although counts were always provided. To control for possible confounding of the difference in the age distribution among specific racial and ethnic groups on the comparison of, for example, stage or histology, a relative percentage of the column (C%) age-adjusted incidence rate was calculated instead of a percentage based on case counts. This quantity was calculated as the age-adjusted incidence rate for a particular category divided by the age-adjusted incidence rate for the total group in the column. For example, the relative percentage of localized cancers for Whites was the age-adjusted localized rate divided by the overall age-adjusted incidence rate for Whites. Additionally, to control for the differences in race distributions among geographic regions on the comparison of stage, the relative column percentages were race standardized using the 2000 U.S. population as the standard. We examined differences in the log age-specific incidence rates separately by race and Hispanic ethnicity using the Poisson model as follows:
To assess a trend in incidence rates, the annual percent change in rates between 1973 and 2003 was calculated. The statistical comparison of trends was based on the t test of the trend regression parameter. Three-year moving averages were used in the trend plots to reduce the effect of random variation ("noise") in the plots.
| Results |
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Geographic Region
The incidence of penile cancer among all races combined was significantly higher in the South (4.42 per million) and significantly lower in the West (3.28 per million) than in the other regions of the United States (Fig. 3
). The high rates of penile cancer in the South and the low rates in the West were consistent for all racial groups (Table 4
). White men had the highest rates of penile cancer in the Northeast and West, in contrast to Black men who had the highest rates in the South and Midwest. The highest rate of penile cancer was found among Black men in the South (4.77 per million) and the lowest rate among Asian-Pacific Islander men in the West (1.84 per million).
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Men were more likely to be diagnosed with penile cancer at a localized stage in the Midwest and more likely to be diagnosed at a regional or distant stage in the West than in other regions of the United States.
Time Trend
An analysis of time trends in the risk of penile cancer showed a significant (P = 0.0002) decrease of –1.2% (95% CI, –0.6% to –1.8%) in the average annual incidence rate between 1973 and 2003 (Fig. 4
). The decline in rates was more dramatic for Black men than White men: the average percent change in incidence was –1.9% (95% CI, –0.7% to –3.2%) for Black men (P = 0.004) and –1.1% (95% CI, –0.5% to –1.7%) for Whites (P = 0.0009) during the 31-year time period. In 1973, the penile cancer rate was 8.2 (95% CI, 3.7-21.4) for Black men and 4.8 (95% CI, 3.6-6.4) for White men compared with rates of 1.9 (95% CI, 0.6-4.9) for Black men and 3.2 (95% CI, 2.5-4.1) for White men in 2003.
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| Discussion |
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Contrary to reports from other countries (20), penile cancer was extremely rare in children—only two cases were reported among 15- to 19-year-olds. A comparison of age-specific rates of penile cancer suggests that the high rate of penile cancer among Hispanic men and the low rate among Asian-Pacific Islander men were not limited to a single age group: the risk profile for this malignancy seems to begin in early adulthood for all racial and ethnic groups. We are unable to explain the sharp increase in the incidence of penile cancer among very old (
85 years) Hispanic and Black men. The possibility of cohort effect–associated reduced rates of circumcision or increasingly poor genital hygiene among older men cannot be excluded (21, 22).
Squamous cell carcinomas represented the predominant histologic type of penile cancer with verrucous ("warty") carcinoma accounting for 6% of all incident cancers. Kaposi sarcoma, a rare histologic type strongly associated with HIV infection (23), was the only penile cancer subtype that was more common among Black men than among White men. Up to 18% of men with AIDS-related Kaposi sarcoma are reported to have penile involvement (24).
SEER data show that the rate of penile cancer in the United States has been declining among White and Black men during the past three decades and rates for these two groups seem to have converged. Etiologic studies of penile cancer have identified several risk factors including HPV infection, lack of circumcision or phimosis (the inability to retract the foreskin over the glans), and cigarette smoking (2, 3, 5, 6, 9, 10, 25).
Presently, no population-based data are available about the prevalence of HPV infection of the penis among men in the United States. In a recent systematic review of the literature, Dunne et al. (26) found a wide range (1-73%) of genitourinary HPV prevalence among men worldwide, with most studies reporting a prevalence of
20%, which is similar to the HPV prevalence found among women (
27%; ref. 27). We note that the differences in the rates of penile cancer by racial/ethnic group parallel those of cervical cancer, another HPV-related malignancy, with higher rates among Hispanic women and lower rates among Asian-Pacific Islander women (1). Indeed, a correlation between penile and cervical cancer incidence rates has been noted by several investigators (28-30). The sharp peak in penile cancer incidence among very old Hispanic and Black men may indicate differences in age at initial infection and/or viral persistence compared with other racial/ethnic groups. The seropositivity to HPV-16, HPV-18, or HPV-45, the most common oncogenic types of HPV, was 46% among penile cancer cases and 12% among controls (odds ratio, 5.0; 95% CI, 1.4-17.2) in a case-control study in Uganda (25). Positive HPV-16 serology was found among 24% of cases and 12% of controls in a North American case-control study (odds ratio, 1.9; 95% CI, 1.2-3.2), and 80% of penile cancer tissue specimens were positive for HPV-DNA (9). Variability in the prevalence of HPV in penile cancer tissue from published reports (21-83%) may be explained by geographic variation in the distribution of HPV genotypes and differences in HPV detection methods (31).
The lower rate of penile cancer among Asian-Pacific Islanders compared with other racial and ethnic groups suggests that oncogenic HPV infection may be less prevalent among Asian-Pacific Islander men because screening variation is not relevant to penile cancer (32). Because genital HPV infection is spread through sexual contact, behavioral differences between racial and ethnic groups may be a partial explanation for the variation in rates. Indeed, Asian-Pacific Islander high school students are significantly less likely than White, Black, or Hispanic students to have ever engaged in sexual intercourse (33).
Data from the National Health and Social Life Survey showed substantial variation in the prevalence of circumcision by birth cohort, racial and ethnic group, and geographic region of the United States that parallels some of the variability in penile cancer rates (21, 22). Laumann et al. (22) observed a steady increase in the prevalence of circumcision from a low of 31% in 1932, peaking in 1965 at 85%, and declining to 77% in 1973, the last year of the survey. The prevalence of circumcision among racial and ethnic groups noted in this survey was inversely associated with rates of penile cancer found in our analysis: the level of circumcision among Whites (81%) was much higher than the prevalence among Blacks (65%) or Hispanics (54%). The convergence of circumcision rates among younger men of all ethnic and racial groups in the United States may explain the steeper decline in penile cancer incidence observed in the SEER data between 1973 and 1999 and the much higher rates of penile cancer among older Black men. Circumcision rates among boys born in the 1940s and early 1950s in the South were much lower than in other parts of the United States (21), paralleling the higher rate of penile cancer found in this geographic region.
The majority of penile cancers were diagnosed at a localized stage, in accord with other observations (34), but stage at diagnosis varied by race and ethnicity. Black and Hispanic men were diagnosed with a higher relative percentage of advanced-stage and unstaged penile cancer than were White and non-Hispanic men, suggesting ethnic and racial disparities in health care. This observation may reflect interregional variation in diagnostic patterns because few Black or Hispanic men with penile cancer resided in the Midwest, the region with the highest percentage of early-stage cancer. Alternatively, socioeconomic status may influence stage at diagnosis, and at least one study reported an inverse gradient between penile cancer and social class (35).
Social class and racial/ethnic differences in the prevalence of tobacco smoking (36) may explain reports of an association of smoking with the risk for penile cancer in some studies (9, 10), but not others (6).
In summary, this analysis revealed substantial racial/ethnic and geographic variation in the incidence of penile cancer in the United States. The high rate of penile cancer among Hispanic and Southern Black men suggests differences in risk factors for this malignancy that may have changed during the past three decades or by birth cohort. This descriptive study of penile cancer may reasonably contribute to the development of novel etiologic hypotheses about this malignancy and provide the basis for future collaborative population-based case-control studies.
| Footnotes |
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 3/26/07; revised 6/ 4/07; accepted 6/13/07.
| References |
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distribution. Stat Med 1997;16:791–801.[CrossRef][Medline]This article has been cited by other articles:
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C. Theodore, I. Skoneczna, I. Bodrogi, M. Leahy, J. M. Kerst, L. Collette, K. Ven, S. Marreaud, R. D. T. Oliver, and for the EORTC Genito-Urinary Tract Cancer Group A phase II multicentre study of irinotecan (CPT 11) in combination with cisplatin (CDDP) in metastatic or locally advanced penile carcinoma (EORTC PROTOCOL 30992) Ann. Onc., July 1, 2008; 19(7): 1304 - 1307. [Abstract] [Full Text] [PDF] |
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