INTRODUCTION
Breast cancer is a leading cause of death among women. As of today little is known about the prevention of breast cancer. Therefore, much rests on early detection, when the cancer can be cured completely.
An ideal screening test should detect cancer or the pre-cancerous condition at a stage when treatment can affect outcome. The rate of false positive and negative results should be low. The test should not cause morbidity, should be reproducible, inexpensive, and suitable for large sections of the population. The best test would be the one that detects abnormal cells before they turn malignant.
Mammography has long been used as a screening test for breast cancer. It involves taking an X-ray of the breast. It is widely accepted that screening mammography leads to early detection of breast cancer. Recently, however, questions have been raised over whether such early detection actually translates into greater survival.

LIMITATIONS OF MAMMOGRAPHY
Since mammography cannot separate normal gland tissue from tumours, it is more effective when gland tissue diminishes with age. Many women retain glandular tissue even as they mature and this camouflages tumours until they are large. Young women have more glandular tissue, which interferes with detection of small cancers.

BENEFITS OF MAMMOGRAPHY
Apart from early detection of breast cancer, there are other potential benefits of mammography screening. Although the amount of dense parenchymal tissue can affect the validity of cancer screening, it is also a marker of breast cancer risk. It has been found in a case control study that women with extremely dense breast tissue are at an elevated risk of developing breast cancer than those with extremely fatty breast tissue. The knowledge that breast density is a marker of breast cancer risk can be used to minimise the risk.

EFFECT ON INCIDENCE OF SURGERY
Incidence of surgery - tumorectomy and mastectomy - has increased dramatically since the beginning of clinical use of mammography. Screening identifies some slow growing tumours that are not likely to grow bigger in the woman's remaining lifetime. These can be followed by mammography or treated aggressively, thus increasing the incidence of surgery.
Improvement in mammography has resulted in the test's ability to detect a higher number of Ductal Carcinoma in Situ (DCIS). Currently, most of the DCIS are diagnosed by mammography, since there is usually no lump that can be detected by touch. These may not always develop into invasive cancer, but since these lesions are often diffuse, women are treated with bilateral mastectomy.

STUDIES TO DEMONSTRATE EFFECT ON SURVIVAL
Critics of the test say that mammography is an imperfect screening test, as it does not detect lesions before they turn cancerous.
The Canadian National Breast Cancer Screening Study found that annual mammograms combined with physical breast examination did not reduce breast cancer deaths compared to physical examination alone in women aged between 50 and 59. The results reflected a median 13 years of follow up in nearly 40,000 women. Study authors stated that annual physical examination by an expert was a valid option for breast cancer screening, especially where access to mammography was limited.
Peter Gotzche and Ole Olsen reviewed eight mammography trials in the US, Canada, Scotland and Sweden. They reported that screening for breast cancer with mammography was unjustified. They found the quality of most trials poor. The best trials, they claimed, did not provide evidence of a reduction in either total or breast cancer mortality. Data showed that for every thousand women screened biennially throughout 12 years, one breast cancer death was avoided whereas the total number of deaths increased by six.

NEW FINDINGS
However, the latest review of the Swedish trials has found reliable evidence of substantial reduction in fatality. According to this review, Olsen and Gotzche did not address the case fatality benefit of screening-associated early intervention. (This, if it exists, becomes apparent only after a delay of several years.) Screening in elder women seemed to have led to a 55 percent reduction in case-fatality rate and thereby, after requisite delay, in cause-specific mortality.
Recently, Nystrom and colleagues presented data for over 15 years from four randomised Swedish trials of screening mammography. Despite the reduction in breast cancer mortality, overall mortality showed a relative risk of just 0.98 between the invited and control groups.
Recently, WHO's International Agency for Research on Cancer (IARC) convened a working group. The group concluded that in women aged between 50 and 69, mammography reduced the chances of dying from breast cancer by about 35 percent. In younger women there was only a slight benefit.

RECENT RECOMMENDATIONS
The U.S. Preventive Services Task Force (USPSTF) has released new clinical guidelines in the wake of growing controversy over value of mammography screening.
These recommendations, which support mammography, are based on a meta-analysis of eight randomised control trials with 11 to 20 years of follow-up, published since 1996. Four of these studies evaluated mammography alone and four evaluated the efficacy of mammography with clinical breast examination in increasing breast cancer survival. The USPSTF now recommends mammography for all women aged 40 and older. The task force acknowledges that strongest evidence of benefit and reduced mortality from breast cancer is among women aged 50 to 69. It found that screening mammography reduced a woman's risk of dying of breast cancer by about 20 percent.
The National Cancer Institute USA, has also reaffirmed its support for mammography every one or two years after age 40, or earlier if there was a family history of breast cancer at a young age.

CONCLUSION
While we should be vigilant in reviewing data on mammography, we must keep in mind that no large screening trial can be perfect.
Until an improved breast cancer screening test is developed, mammography, as an annual screening test will continue to be relevant. After all, this is all that we have at present to detect breast cancer at its early stages.

FURTHER READING
1. Mandelson MT, et al. Breast density as a predictor of mammographic detection: comparison of interval-and screen detected cancers. J Natl Cancer Inst. 2000; 92: 1081-1087
2. Olsen O, Gotzshe PC. Cochrane review on screening for breast cancer with mammography. Lancet. 2001; 358:1340-1342
3. The mammography screening debate: Time to move on. Lancet. 2002; 359:904-905