Cervical cancer incidence, mortality, and survival among Asian subgroups in California, 1990–2004

Abstract

BACKGROUND.

Aggregated cancer statistics for Asians mask important differences in cancer burden among Asian subgroups. The purpose of this study was to describe the relative patterns of cervical cancer incidence, mortality, and survival among Chinese, Filipino, Japanese, Korean, South Asian, and Vietnamese women in California, using data from the California Cancer Registry.

METHODS.

All cervical cancer cases diagnosed among the 6 subgroups and non-Hispanic whites (NHW) from 1990 to 2004 were identified and used to calculate incidence and mortality rates and trends. The Kaplan-Meier method was used to calculate 5- and 10-year survival probabilities by subgroup, and Cox proportional hazards methodology was used to calculate survival differences adjusted for race and ethnicity, age, stage at diagnosis, socioeconomic status, and treatment factors.

RESULTS.

Vietnamese and Korean women experienced greater cervical cancer incidence and mortality than NHW women, whereas rates among Chinese, Japanese, and South Asians were comparable or lower. Five-year unadjusted survival probabilities were greatest for South Asians (86%) and Koreans (86%), followed by Vietnamese (82%), Chinese (79%), and Filipinos (79%), as compared with NHW (78%) and Japanese (72%). The adjusted risk of cervical cancer death was significantly lower for South Asians, Koreans, Vietnamese, and Filipinos than for NHW women, but not for Chinese and Japanese.

CONCLUSIONS.

Cervical cancer incidence rates vary substantially across the major Asian subgroups. Despite higher incidence and mortality rates compared with NHW women, Vietnamese, Koreans, and Filipinos have better survival outcomes. Further studies are needed to examine the factors behind these survival differences. Cancer 2008;113(10 suppl):2955–63. Published 2008 by the American Cancer Society.

Asians in the United States experience lower overall cancer rates than other racial and ethnic groups in the country.1, 2 However, Asians in the United States represent a diverse mix of subgroups, with wide variability in country of origin, immigration history, age distribution, culture, language, and socioeconomic status (SES), leading to distinct differences in cancer risk and burden. Cancer data on Asian subgroups are not available at the national level, and this has been recognized as a gap in national cancer surveillance.3, 4 Cancer data on Asian subgroups are available in California, where 3.7 million Asians comprise 12% of the state's population, making it the state with the largest Asian population in the country.5 Data from the California Cancer Registry (CCR) have recently been used to describe distinctive patterns of cancer risk, incidence, and mortality, as well as differential patterns of survival for specific cancer sites among the state's Asian subgroups.6–15

Previous reports from California have described differences in the relative burden of cervical cancer among the various Asian subgroups. These reports have described either incidence and mortality trends or survival outcomes among some, but not all, of the 6 major subgroups that comprise nearly 90% of the Asian population in California: Chinese, Filipino, Japanese, Korean, South Asian, and Vietnamese. The purpose of this study was to examine patterns and disparities in cervical cancer incidence, mortality, and survival among these 6 Asian subgroups in California, using data from the CCR. An understanding of the variability in the burden of cervical cancer and disparities among these groups is critical for targeting public health policies and practices to cervical cancer screening and prevention among Asians in the United States.

MATERIALS AND METHODS

Study Population

Cervical cancer cases were identified from the CCR database. The CCR is the world's largest geographically contiguous population-based cancer registry, collecting approximately 140,000 new cancer case reports annually. Legally mandated, cancer reporting in California was fully implemented in 1988, with standardized data collection and quality-control procedures.16–18 The CCR routinely links with the National Death Index to ascertain deaths among California residents who have died out of state. The analytic file included all invasive, microscopically confirmed cervical cancer cases diagnosed among California residents identified as Asian or non-Hispanic white (NHW) from 1990 to 2004 and reported to the CCR as of April 2007. Cervical cancer cases with International Classification of Diseases for Oncology-3 histology codes 8010-8671 and 8940-8941 were included in the analysis.19 Squamous cell carcinoma was defined as histology codes 8050-8084 and 8120-8131; adenocarcinoma, as codes 8140-8149, 8160-8162, 8190-8221, 8260-8337, 8350-8551, 8570-8576, and 8940-8941. All other tumors were classified as “other.” Race, age, and sex-specific population estimates for California from the 1990 and 2000 United States Census were obtained from the National Center for Health Statistics (NCHS) and used for the trend analyses.20 Pre-1990 California population estimates were not available from NCHS in the same race- and ethnic-specific categories as those obtained from 1990 forward, a difference primarily affecting the classification of NHW cases. Therefore, cases diagnosed in 1988 and 1989 were not included in the analyses of incidence and mortality trends. However, these 1988 and 1989 cases were included in the survival analyses described below, because population data are not required for these analyses.

By using data obtained from the medical record, the CCR classifies Asian race into 12 mutually exclusive Asian subgroups. A Vietnamese surname list is used to enhance the identification of Vietnamese cases, but such lists are not available for other Asian subgroups.21–22 When multiple races were coded for a case classified as mixed race (multiple race data have been collected only for cases diagnosed from 2000 forward), the primary race code was used. The 6 largest subgroups—Chinese, Filipino, Vietnamese, Korean, South Asian (includes Asian Indian and Pakistani) and Japanese—constitute approximately 90% of all Asians in the CCR database. All cases included belong to 1 of these 6 subgroups. Non-Hispanic whites were included in the analytic file as a comparison population. Hispanic ethnicity is determined by use of data obtained from the medical record and supplemented by use of additional information on surname and place of birth.17

Measures

The Surveillance, Epidemiology, and End Results (SEER) summary staging system was used to categorize stage at diagnosis as localized (confined to the cervix), regional (extension to surrounding organs and/or regional lymph nodes), or distant (metastasized to other organs).23 In 2000, an update to the SEER summary staging system resulted in the reclassification of some cervical cancer cases from distant to regional stage. An analysis by the North American Association of Central Cancer Registries found a minimal impact of this change on the classification of cervical cancer stage, affecting approximately 0.2% of cases.24 Treatment information was extracted from the CCR based on information contained in the patient's medical record on the first course of treatment. Treatment categories included were surgery (none, local tumor destruction, hysterectomy, other, unknown), radiation (none, any), or chemotherapy (none, any, unknown).

The measure of SES used in this analysis was a composite measure previously created by using linked CCR and census data.25 Residence at the date of diagnosis was geo-coded to the 1990 census block groups for cases diagnosed from 1988 to 1995, and to the 2000 census block groups for 1996 to 2004 cases. A composite SES score using 1990 census data (applied diagnosis years 1988-1995) and 2000 census data (applied to diagnosis years 1996-2004) was created by conducting a principal components analysis of 7 census variables, including education level, occupation, median household income, and median house value. Quintiles of this SES score were used in the analysis, with a value of 1 representing the lowest SES level and a value of 5 representing the highest SES level.

Statistical Analysis

The distributions of demographic and clinical characteristics, including age, SES, stage at diagnosis, and first course of treatment of each Asian subgroup, were compared with the distributions of NHWs by use of chi-square tests. Age-adjusted incidence and mortality rates were calculated for each race and ethnic group for 5 3-year time periods: 1990 to 1992, 1993 to 1995, 1996 to 1998, 1999 to 2001, and 2002 to 2004. Rates were calculated by use of SEER *Stat software (National Cancer Institute, Bethesda, Md) and were standardized to the 2000 United States population.26 The annual percentage change (APC) was calculated and considered statistically significant at P < .05.

The Kaplan-Meier method was used to calculate 5-year and 10-year survival probabilities for invasive cancers by race and ethnic group.27 The outcome of interest in the survival analysis was death from invasive cervical cancer. Deaths from other causes were censored at the time of death. Cause of death was categorized according to the International Classification of Diseases (ICD) system.19 Cases with ICD-9 cause of death codes 180.0 or 180.9 and those with ICD-10 cause of death codes in the range C52.0 to C53.9 were designated as deaths from cervical cancer. Cox proportional hazards modeling was used for multivariate adjustment of survival to permit evaluation of differences in survival among the Asian subgroups, adjusted for stage, age, SES, and treatment factors. Log-log plots were used to test the proportionality assumption of the model. No violations of this assumption were found upon examination of these plots. Covariates were introduced into the model in a stepwise fashion to assess the effect of introducing different variables into the model on the risk of death because of race or ethnicity. Hazards ratios and their 95% confidence intervals were used to assess the magnitude of risk and the precision of estimates.

RESULTS

The demographic and clinical characteristics of the study population are presented in Table 1. A total of 13,190 cases of invasive, histologically confirmed cervical cancer cases diagnosed among California residents from 1990 to 2004 were included in the analysis: 2311 cases among the 6 Asian subgroups and 10,879 among NHW women. Larger proportions of Chinese and Korean women were diagnosed at 65 years of age or older compared with women in the other racial and ethnic groups. A larger proportion of NHW women (29.9%) were younger than 40 years of age at the time of diagnosis compared with women in the Asian subgroups. The case distribution in relation to SES level was mixed, with higher SES levels among Chinese and Japanese relative to NHW, and lower levels among Vietnamese and Korean cases.

Table 1. Demographic and Clinical Characteristics of Invasive Cervical Cancer Cases Among Asian Subgroups and Non-Hispanic Whites, California, 1990–2004 (N = 13,190)

	Race and Ethnic Group, No. (%)
NHW indicates non-Hispanic white; SES, socioeconomic status. * Surveillance, Epidemiology, and End Results (SEER) Summary Stage at Diagnosis. For 1990 to 1999, SEER Summary Stage 1977 was used; for 2000 to 2004, SEER Summary Stage 2000 was used. Data for the 2 staging systems are combined, because the differences observed in comparative analyses were minimal.24, 37
Variable	Chinese (n = 485)	Filipino (n = 747)	Japanese (n = 198)	Korean (n = 309)	South Asian (n = 88)	Vietnamese (n = 484)	NHW (n = 10,879)
Age group, y
<40	83 (17.1)	136 (18.2)	49 (24.8)	46 (14.9)	16 (18.2)	33 (6.8)	3250 (29.9)
40-64	230 (47.4)	433 (58.0)	102 (51.5)	176 (57.0)	50 (56.8)	329 (68.0)	5266 (48.4)
≥65	172 (35.5)	178 (23.8)	47 (23.7)	87 (28.2)	22 (25.0)	122 (25.2)	2363 (21.7)
P for comparison with NHW	<.001	<.001	.291	<.001	.057	<.001
SES quintile
1 (low)	56 (11.6)	127 (17.0)	16 (8.1)	60 (19.4)	15 (17.1)	97 (20.0)	1329 (12.2)
2	77 (15.9)	153 (20.5)	28 (14.1)	58 (18.8)	18 (20.5)	127 (26.2)	2290 (21.1)
3	84 (17.3)	185 (24.8)	52 (26.3)	64 (20.7)	17 (19.3)	100 (20.7)	2458 (22.6)
4	123 (25.4)	185 (24.8)	52 (26.3)	61 (19.7)	19 (21.6)	94 (19.4)	2499 (23.0)
5 (high)	145 (29.9)	97 (13.0)	50 (25.3)	66 (21.4)	19 (21.6)	66 (13.6)	2303 (21.2)
P for comparison with NHW	<.001	<.001	.029	.004	.704	<.001
Stage*
Localized	233 (48.0)	343 (45.9)	101 (51.0)	142 (46.0)	39 (44.3)	266 (55.0)	5844 (53.7)
Regional	182 (37.5)	298 (39.9)	73 (36.9)	119 (38.5)	29 (33.0)	182 (37.6)	3405 (31.3)
Distant	33 (6.8)	78 (10.4)	17 (8.6)	30 (9.7)	14 (15.9)	22 (4.6)	1183 (10.9)
Unstaged	37 (7.6)	28 (3.8)	7 (3.5)	18 (5.8)	6 (6.8)	14 (2.9)	447 (4.1)
P for comparison with NHW	<.001	<.001	.350	.013	.165	<.001
Tumor histology
Squamous cell	345 (71.1)	492 (65.9)	127 (64.1)	236 (76.4)	69 (78.4)	348 (71.9)	7329 (67.4)
Adenocarcinoma	88 (18.1)	173 (23.2)	49 (24.8)	42 (13.6)	12 (13.6)	80 (16.5)	2338 (21.5)
Other	52 (10.7)	82 (11.0)	22 (11.1)	31 (10.0)	7 (8.0)	56 (11.6)	1212 (11.1)
P for comparison with NHW	.175	.562	.533	.002	.087	.033
Surgery performed
None	175 (36.1)	254 (34.0)	57 (28.8)	110 (35.6)	31 (35.2)	143 (29.6)	3280 (30.2)
Local tumor destruction	49 (10.1)	75 (10.0)	21 (10.6)	23 (7.4)	12 (13.6)	38 (7.9)	1296 (11.9)
Hysterectomy	251 (51.8)	385 (51.5)	111 (56.1)	161 (52.1)	41 (46.6)	281 (58.1)	5947 (54.7)
Other Surgery	9 (1.9)	32 (4.3)	9 (4.6)	12 (3.9)	4 (4.6)	22 (4.6)	332 (3.1)
Unknown	1 (0.2)	1 (0.1)	0 (0.0)	3 (1.0)	0 (0.0)	0 (0.0)	24 (0.2)
P for comparison with NHW	.048	.034	.678	.003	.586	.018
Radiation
None	247 (50.9)	335 (44.9)	105 (53.0)	152 (49.2)	43 (48.9)	249 (51.5)	5866 (53.9)
Any	238 (49.1)	412 (55.2)	93 (47.0)	157 (50.8)	45 (51.1)	235 (48.6)	5012 (46.1)
P for comparison with NHW	.195	<.001	.802	.100	.343	.284
Chemotherapy
None	385 (79.4)	544 (72.8)	149 (75.3)	219 (70.9)	57 (64.8)	377 (77.9)	8441 (77.6)
Any	96 (19.8)	197 (26.4)	46 (23.2)	85 (27.5)	28 (31.8)	101 (20.9)	2319 (21.3)
Unknown	4 (0.8)	6 (0.8)	3 (1.5)	5 (1.6)	3 (3.4)	6 (1.2)	119 (1.1)
P for comparison with NHW	.604	.004	.675	.020	.005	.933

The majority of cervical cancers were diagnosed at localized stage among Japanese, Vietnamese, and NHW women (51.0%, 55.0%, and 53.7%, respectively), but not among the other subgroups. The greatest proportion of distant-stage diagnoses was found among South Asian women (15.9%) and the lowest proportion among Vietnamese women (4.6%). Chinese, Korean, South Asian, and Vietnamese women had a greater proportion of squamous cell carcinoma (SCC) tumors and a lower proportion of adenocarcinoma (AC) tumors than NHW women, whereas Japanese and Filipino women had a lower proportion of SCC and a greater proportion of AC tumors than NHW women.

Hysterectomy was the most common surgical treatment identified among all groups. Hysterectomies were performed among a greater proportion of Japanese and Vietnamese women than among NHW women (56.1% and 58.1% vs 54.7%). South Asian women had the lowest proportion of surgical treatment by hysterectomy of any group (46.6%). The proportion of cases treated with any radiation was less than 50% among Chinese, Japanese, Vietnamese, and NHW women (49.1%, 47.0%, 48.6% and 46.1%, respectively), and greater than 50% among Filipino, Korean, and South Asian women (55.2%, 50.8%, and 51.1%). The proportion of cases with no chemotherapy reported ranged from 64.8% (South Asians) to 79.4% (Chinese), compared with 77.6% among NHW women.

Incidence rates declined for all Asian subgroups and for NHW women over the 15-year period 1990 to 2004 (Fig. 1). Vietnamese women had substantially greater incidence rates than any other group throughout the period, but they also experienced the greatest average APC in rates between 1990 and 2004 (−8.7% per year). Statistically significant declines in rates also occurred among Koreans (−5.1% per year), Filipinos (−4.6% per year), Chinese (−5.4% per year), and NHW women (−2.3% per year). Incidence rates also declined somewhat among Japanese (−2.6% per year) and South Asian women (−1.0% per year), but the trends were based upon small numbers and were therefore not statistically significant. Trends in mortality rates followed a similar pattern during this timeperiod, with the greatest decline found among Vietnamese women (−7.9% per year) (Fig. 2). Declines in mortality rates were statistically significant for Chinese (−4.3% per year) and NHW women (−3.0% per year), whereas declines among Filipino (−1.1% per year) and Korean women (−3.2% per year) were based upon small numbers and not statistically significant. There were too few cervical cancer deaths reported among South Asian women to permit an assessment of mortality trends in this subgroup.

Figure 1. Age-adjusted incidence rates for invasive cervical cancer by race and ethnicity are shown for California, 1990 to 2004. *Annual percentage change (APC) is significantly different from 0 (P < .05).

Figure 2. Age-adjusted mortality rates for invasive cervical cancer by race and ethnicity are shown for California, 1990 to 2004. *Annual percentage change (APC) is significantly different from 0 (P < .05). †Statistic could not be calculated because of small numbers.

A total of 13,864 cases of histologically confirmed invasive cervical cancers diagnosed from 1988 to 2004 were included in the analyses of the vital status of cervical cancer cases, cause-specific survival, and the adjusted risk of cervical cancer death. Table 2 describes the proportion of women who died from cervical cancer of all those diagnosed with it for each subgroup. Japanese women diagnosed with cervical cancer had the highest proportion of deaths from it (26.3%), whereas South Asian women had the lowest (12.4%). Table 3 shows the results of the unadjusted cause-specific 5- and 10-year cervical cancer survival probabilities by subgroup, using the Kaplan-Meier method. South Asian women had the highest survival rates at both 5 and 10 years (85.8% for both), followed by Koreans (85.7% and 82.5%), and Vietnamese (82.1% and 79.7%). Only Japanese women had lower unadjusted cause-specific survival at 5 and 10 years (72.3% and 69.5%, respectively) than NHW women (77.5% and 75.4%).

Table 2. Vital Status of Cervical Cancer Cases,* California, 1988–2004 (N = 13,864)†

Variable	Race and Ethnic Group, No. (%)
	Chinese (n = 502)	Filipino (n = 805)	Japanese (n = 198)	Korean (n = 328)	South Asian (n = 89)	Vietnamese (n = 499)	Non-Hispanic White (n = 11,443)
* Last date of follow-up was December 31, 2005. † Cases include all first primary, histologically confirmed, invasive cervical cancer cases during the study period.
Vital status
Alive	317 (63.1)	520 (64.6)	116 (58.5)	236 (71.9)	69 (77.5)	336 (67.4)	6975 (60.9)
Died of cervical cancer	102 (20.3)	171 (21.2)	52 (26.3)	46 (14.0)	11 (12.4)	90 (18.0)	2506 (21.9)
Died of other causes	61 (12.2)	50 (6.2)	18 (9.1)	33 (10.1)	6 (6.7)	46 (9.2)	1496 (13.1)
Unknown cause of death	22 (4.4)	64 (8.0)	12 (6.1)	13 (4.0)	3 (3.4)	27 (5.4)	466 (4.1)

Table 3. Unadjusted Cause-specific 5- and 10-Year Cervical Cancer Survival by Race and Ethnicity, California, 1988–2004*

	Survival Probability, % (95% CI)
	Chinese (n = 502)	Filipino (n = 805)	Japanese (n = 198)	Korean (n = 328)	South Asian (n = 98)	Vietnamese (n = 499)	Non-Hispanic White (n = 11,443)
CI indicates confidence interval. * Cases include all first primary, histologically confirmed, invasive cervical cancer cases during the study period.
5-Year	78.6 (74.4, 82.2)	79.0 (75.9, 81.9)	72.3 (64.9, 78.4)	85.7 (80.9, 89.6)	85.8 (75.1, 92.2)	82.1 (78.2, 85.4)	77.5 (76.7, 78.3)
10-Year	77.2 (72.9, 81.0)	74.8 (71.2, 78.0)	69.5 (61.7, 76.1)	82.5 (77.1, 86.7)	85.8 (75.1, 92.2)	79.7 (75.4, 83.3)	75.4 (74.6, 76.3)

Table 4 summarizes the results of the multivariate analysis in which variables were added in a stepwise fashion to assess their effects on differences among the Asian subgroups. With the inclusion of only race in the model, only Korean women had a statistically significant difference in the risk of death from cervical cancer compared with NHW women (39% lower). After adjusting for stage, Korean, South Asian, and Filipino women had a significantly lower risk of cervical cancer death (−51%, −63%, and −16%, respectively) than NHW women. After adjusting for race, stage, and age, the risk of death from cervical cancer was also statistically lower among Vietnamese women than for NHW women (−21%). With the inclusion of all variables in the model—race, stage, age, treatment, and SES—a statistically significant lower risk of cervical cancer death was found among South Asian (−69%), Korean (−58%), Vietnamese (−23%), and Filipino (−22%) women compared with NHW women. The risk of cervical cancer death was higher among Japanese women than among NHW women in each model, but the risk did not reach statistical significance.

Table 4. Risk of Death Due to Cervical Cancer by Race and Ethnicity, Adjusted for Demographic and Treatment Factors, California, 1988–2004*†

Variables in Model	Hazard Ratio (95% CI)
	Chinese (n = 502)	Filipino (n = 805)	Japanese (n = 198)	Korean (n = 328)	South Asian (n = 98)	Vietnamese (n = 499)
SES indicates socioeconomic status. * Cases include all first primary, histologically confirmed, invasive cervical cancer cases during the study period. † The reference group is non-Hispanic whites. ‡ Treatment includes radiation, chemotherapy, and surgery.
Race only	0.94 (0.78, 1.15)	0.97 (0.83, 1.13)	1.28 (0.97, 1.69)	0.61 (0.45, 0.81)	0.57 (0.32, 1.04)	0.79 (0.64, 1.00)
Race, stage	0.97 (0.79, 1.18)	0.84 (0.72, 0.98)	1.19 (0.90, 1.56)	0.49 (0.37, 0.66)	0.37 (0.20, 0.66)	0.83 (0.67, 1.03)
Race, stage, age	0.89 (0.72, 1.09)	0.82 (0.70, 0.95)	1.15 (0.87, 1.52)	0.47 (0.35, 0.62)	0.34 (0.19, 0.62)	0.79 (0.64, 0.98)
Race, stage, age, treatment,‡ SES	0.83 (0.68, 1.02)	0.78 (0.67, 0.91)	1.13 (0.86, 1.50)	0.42 (0.31, 0.57)	0.31 (0.17, 0.56)	0.77 (0.62, 0.96)

DISCUSSION

This study demonstrates that the burden of cervical cancer varies substantially among Asian subgroups in California. In particular, Vietnamese and Korean women experience markedly greater cervical cancer incidence than non-Hispanic whites, whereas rates are lower among Chinese, Japanese, and South Asian women. Despite higher cervical cancer incidence and mortality rates, however, Korean, Vietnamese, and Filipino women were found to have better cervical cancer survival outcomes than non-Hispanic white women in California, as were South Asian women. Well-established risk factors influencing survival—age, stage, SES, and treatment factors—did not explain this apparent survival advantage.

The patterns of cervical cancer incidence and mortality seen in California among these 6 Asian subgroups are broadly consistent with the findings from international cervical cancer surveillance reports. The higher cervical cancer incidence rates found among Vietnamese, Korean, and Filipino women in California mirror the relatively higher rates found in Vietnam, Korea, and the Philippines relative to China, Japan, and the United States.28 South Asians are an exception to this pattern, as India (country of origin for the majority of South Asians in California) reports a higher cervical cancer incidence rate than any of the other Asian countries of origin, whereas incidence among South Asians in California was lower than for any other group. This discrepancy may be attributable to a high level of education among South Asians in California relative to other Asian subgroups, with nearly a third of Asian Indians reporting advanced degrees.29

Variations in cervical cancer incidence among different populations have been largely attributed to differences in access to care and cervical cancer screening, so that populations with low utilization of the Papanicolaou test would be expected to have higher cervical cancer incidence rates.10 However, among the Asian subgroups, there are no consistent relationships between the cervical cancer incidence findings in our study and the prevalence of cervical cancer screening reported by the 2003 California Health Interview Survey (CHIS).30 The 2 Asian subgroups with the highest cervical cancer incidence rates—Vietnamese and Korean—do have low screening utilization as reported by CHIS (72% and 67%, respectively) compared with NHW (89%). However, Chinese women in our study had lower cervical cancer incidence rates than NHW women, despite much lower screening utilization (68%) according to CHIS, whereas Filipino women had higher cervical cancer incidence rates than NHW women despite having similar screening utilization (88%) reported to CHIS. These variations in cervical cancer screening among Asian subgroups described in California by CHIS are consistent with results from national surveys.3 Infection with human papillomavirus (HPV) is a necessary cause of cervical cancer, and variation in cervical cancer incidence rates may be partially attributable to differences in prevalence of HPV among these subgroups. However, because HPV prevalence data for Asian subgroups are not available in California, the prevalence of HPV could not be evaluated. Incidence rates may also vary by other reported risk factors that influence the natural history of progression from HPV infection to carcinogenesis, such as smoking, parity, use of oral contraceptives, coinfections, and diet.31, 32 It has also been suggested that differences in follow-up after cervical cancer screening could account for some of these findings.33 Further examination of these factors may inform future cancer prevention efforts aimed at these subgroups.

Mortality rates from cervical cancer mirrored incidence rates, with the highest mortality found among Vietnamese, Korean, and Filipino women (higher than NHW), and lower among Japanese, Chinese, and South Asian women. However, after adjusting for differences in age and stage distribution, SES, and treatment factors, Vietnamese, Korean, Filipino, and South Asian women had a lower risk of cervical cancer death than non-Hispanic white women. Few studies have examined differences in cervical cancer survival among any Asian subgroups. An examination of cervical cancer survival among Chinese, Japanese, and Filipino women showed that Filipino women had higher unadjusted cause-specific 5-year survival (81.4%) than the other 2 groups.13 To our knowledge, ours is the first study comparing cervical cancer survival outcomes among Vietnamese, Korean, and South Asian women, in addition to these other 3 groups. The survival advantage of South Asian women for cervical cancer that we found contrasts with survival analyses of other cancer sites in this group. For example, South Asian women were found to have poorer breast cancer survival outcomes than Chinese, Japanese, and non-Hispanic white women in California.14

Differences in survival between groups may occur because of differences in the clinical characteristics of cervical cancers. A recent report suggested that survival outcomes are poorer for cervical adenocarcinomas than for squamous cell carcinomas.34 In our study, although Japanese women had the highest proportion of adenocarcinomas as well as the poorest survival outcomes, there was no consistent pattern seen among the other subgroups. The addition of histology as a variable in the multivariate analysis of cervical cancer risk of death did not have any effect on the results and so was not retained in the final model (data not shown). Other possible explanations for these differences in survival outcomes include the prevalence of comorbidities. Such information is not available in the CCR database and therefore could not be evaluated.

This study has some limitations that should be considered. First, although California has the largest Asian population of any state, the number of cervical cancer cases among some of the subgroups was still relatively limited. The findings, in particular for South Asians, are based on few cases, and they should be replicated with additional studies. Second, because data on race and ethnicity are obtained from the medical record and not from patient interview, it is possible that some cases in the CCR database are misclassified with respect to race or ethnicity.35 Data on surname and birthplace are used to enhance the identification of some race and ethnic groups (Vietnamese and Hispanics), but such a methodology does not provide complete ascertainment for these groups. Race-specific surname lists are not available for all groups, and birthplace information is frequently not available in the medical record.36 Third, information on lifestyle or other individual factors such as screening access and utilization, smoking, obesity, diet, comorbidities, and immigration history that may influence cervical cancer outcomes was not available. Similarly, a previously published, area-based measure of socioeconomic status was used because individual-level information on socioeconomic status is not available in the CCR database. Finally, there may be unknown differences in the characteristics or risk factors among Asian subgroups in California compared with other states, and such differences could affect the generalizability of these findings to the experience of these groups outside of California.

In conclusion, this study shows evidence of distinct variations in the burden of cervical cancer among Asian subgroups, and the study supports the importance of reporting cancer data for these groups separately. Although progress has been made, as shown by the decline in cervical cancer incidence among all 6 Asian subgroups, disparities in cervical cancer still exist, with a high incidence and mortality among Vietnamese and Korean women that is particularly noteworthy. Efforts to expand access to and utilization of cervical cancer screening as well as the HPV vaccine in these populations should be a continued focus of public health programs and policies. Further study is needed to identify the possible lifestyle, cultural, and clinical factors that might account for variations in cervical cancer burden and the apparent survival advantage of some subgroups.