背景：過去研究顯示癌症死亡率出現顯著地區不平等現象。但是死亡率的組成包括發生率與致死率，所以較難釐清造成地區癌症死亡率不平等主要來自發生率不平等或是致死率不平等。因此許多研究開始使用癌症觀察存活率來探討醫療照護之差異。但是觀察存活率也無法排除地區醫療照護以外因素對於存活率的影響，因此有學者提出相對存活率比的計算，就是以該地區的死亡經驗來計算癌症預期存活人數，然後再與實際存活人數相比。再者，Victora提出逆轉不公平假說：有效介入反而會加大不平等趨勢，之後才會縮小。

目標：探討台灣五種癌症(口腔癌、乳癌、肝癌、結直腸癌與子宮頸癌)三種存活指標不同不平等指標之年代趨勢，是否出現介入後先增後減之現象。

方法：資料來源是癌症登記資料1987年至2006年與死因統計資料1987年至2011年，透過衛生福利部統計處健康資料加值協作中心比對資料並計算三種存活指標(標準化死亡率、觀察存活率與相對存活率比)。絕對地區不平等指標是人口加權的組間變異和最高與最低地區的差距，相對地區不平等指標是最高與最低地區的比值。以線性回歸檢定不平等指標趨勢是否顯著。

結果：以比值來看標準化死亡率之差異的年代趨勢，在所有的五種癌症當中，都沒有發現Victora所主張不平等差異有先擴大再縮小的現象。但是，若以組間變異來看標準化死亡率之差異的年代趨勢，則可以在乳癌、結直腸癌與子宮頸癌發現縣市不平等的差異有先擴大再縮小的現象，而且差異開始縮小都是發生在2002-2006這個年代。另一方面，若是以觀察存活率或是相對存活率比而言，只有以差距來看口腔癌縣市不平等之差異的年代趨勢，可以發現有先擴大再縮小的現象，差異開始縮小是發生在1997-2001這個年代。

結論：本研究顯示不同癌症存活指標出現不同的不平等指標趨勢改變，可能因為不同介入的執行程度不同造成，未來研究還要進一步釐清。

SUMMARY

We sought to examine trends in regional inequality in cancer survival in Taiwan and to test the “inverse equity hypothesis”. The data used in this study consists of Taiwan Cancer Registration System and National Register of Deaths. According to the ICD-O-3, patients diagnosed with primary cancer of oral (C00-C06), breast (C50), liver (C22), colorectal (C18-C20), or cervix (C53) between 1987 and 2006 were selected and followed up to December 31, 2011. Geographical region was defined according to the original jurisdiction before 2010; offshore islands were excluded. We used Ederer II method to calculate relative survival. Results showed improved relative survival during 1987-2006 in all 5 cancers in all 22 jurisdictions. When we used ratio to examine the trends in regional inequality of mortality rate, it is not compatible to the “inverse equity hypothesis” in all 5 cancers. However, when we used the indicator of between-group variance, it did show the sign compatible to the “inverse equity hypothesis” in breast, colorectal and cervix cancer. The decreased inequality begins in 2002-2006. On the other hand, as to observed or relative survival, it revealed the sign of initial increased then decreased inequality only when we used difference to examine the trends of oral cancer; the inequality began to decrease in 1997-2001.

INTRODUCTION

Previous studies revealed significant regional inequality in cancer mortality. However, mortality rate consists of incidence rate and fatality rate. Therefore, it is difficult to differentiate the origin of regional inequality in cancer mortality whether is from inequality in incidence or is from inequality in fatality. So, there were many studies started to use observed survival to investigate the effect of medical care in cancer. But observed survival still cannot exclude influence from the effect outside of regional medical care.
Berkson proposed the concept of relative survival, it is to calculate the expected survival of cancer patient from regional background mortality and then to divide the observed survival of cancer patient. In the past, there was no study which investigated the trends and compared regional inequality in mortality and survival with one country. Furthermore, Victora suggested the “inverse equity hypothesis”, that is, effective intervention will increase the inequality initially then decrease later. We intend to apply Taiwan’s data to prove Victora’s hypothesis.

MATERIALS AND METHODS

Our data is from Taiwan Cancer Registration System and National Register of Deaths. Patients diagnosed with primary cancer of oral (C00-C06), breast (C50), liver (C22), colorectal (C18-C20), or cervix (C53) between 1987 and 2006 were selected and followed up to December 31, 2011. Geographical region was defined according to the original jurisdiction before 2010; offshore islands including Penghu county、Kinmen county, and Lienchiang county were excluded due to the population sizes were low. We used Ederer II method to calculate relative survival; mortality rate and observed survival were calculated also. The absolute regional inequality indexes include population-weighted between-group variance and difference. The relative regional inequality index is the ratio. Difference and ration is calculated from the highest and lowest values. We used linear regression to examine the significance of the trends of inequality indexes.

RESULTS AND DISCUSSION

In mortality, the highest of oral cancer was 30.7/105 of Taitung county and the lowest was 8.0/105 of Hsinchu city; the highest of breast cancer was 31.5/105 of Kaohsiung city and the lowest was 12.8/105 of Taitung county; the highest of liver cancer was 248/105 of Chiayi city and the lowest was 94/105 of Taipei city; the highest of colorectal cancer was 108.5/105 of Tainan city and the lowest was 73.8/105 of Taitung county; the highest of cervix cancer was 36.7/105 of Pingtung county and the lowest was 20.4/105 of Keelung city.
In observed survival, the highest of oral cancer was 61% of Chiayi city and the lowest was 48% of Pingtung county; the highest of breast cancer was 86% of Taipei city and Hsinchu city and the lowest was 79% of Pingtung county; the highest of liver cancer was 26% of Yilan county and the lowest was 13% of Taitung county; the highest of colorectal cancer was 55% of Keelung city and Taipei city and the lowest was 42% of Taitung county; the highest of cervix cancer was 90% of Keelung city and the lowest was 80% of Taitung county.
In relative survival, the highest of oral cancer was 0.65 of Chiayi city and the lowest was 0.51 of Pingtung county; the highest of breast cancer was 0.89 of Taipei city and Hsinchu city and the lowest was 0.81 of Pingtung county; the highest of liver cancer was 0.29 of Yilan county and the lowest was 0.15 of Taitung county; the highest of colorectal cancer was 0.64 of Taipei city and the lowest was 0.49 of Taitung county; the highest of cervix cancer was 0.94 of Keelung city and the lowest was 0.85 of Taitung county.
From the viewpoint of mortality, the regional inequality of oral and liver cancers increased over time; but it remained the same in breast、colorectal and cervix cancers. From the viewpoint of observed and relative survival, there was no change of trends in the regional inequality of oral、liver and cervix cancers; but there was significant decrease in the regional inequality of breast and colorectal cancers.
When ration is used to examine the trends in regional inequality in mortality, we do not find the phenomenon of increased inequality initially then decreased later suggest by Victora in all five cancers. However, if between-group variance is used to examine the trends in regional inequality in mortality, we do find the phenomenon in breast、colorectal and cervix cancers; the inequality began to decrease in the period of 2002-2006. On the other hand, as to observed or relative survival, it revealed the sign of initially increased then decreased inequality only when we used difference to examine the trends of oral cancer; the decreased inequality began in the period of 1997-2001.

CONCLUSION

Contrast to the suggestion of Victora, our study demonstrated that absolute indexes (between-group variance and difference) rather than relative index (ratio) prove the “inverse equity hypothesis” in both mortality and survival. This study also demonstrated that different cancer survival indicators showed different changes in trends of inequality indexes, it may be caused by different performance of different interventions. Further study is required to investigate these two findings.

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