I’ve written more times than I can remember about the phenomenon of overdiagnosis and the phenomenon that is linked at the hip with it, overtreatment. Overdiagnosis is a problem that arises when large populations of asymptomatic, apparently healthy people are screened for a disease or a condition, the idea being that catching the disease at an earlier stage in its progression will allow for more successful treatment. Two prominent examples include—of course—screening for breast cancer with mammography and screening for prostate cancer with prostate-specific antigen (PSA) testing, and I’ve written about the problem of overdiagnosis with each of them on many occasions. Basically, overdiagnosis occurs when the screening test picks up what we call “preclinical” disease (i.e., disease that hasn’t become symptomatic) that, if left untreated, would never become symptomatic or endanger the health or life of the patient). Although intuitively, it seems to the lay public (and, truth be told, most doctors) that detecting cancer earlier must be inherently better, it turns out that it’s way more complicated than you think. There is a price to be paid for early diagnosis in the form of overtreatment of disease that doesn’t need treatment and for disease that is destined to threaten the life of the patient earlier treatment doesn’t always result in better outcomes. Also, whenever you screen for a condition in asymptomatic people, you will always—always—find much more of it, and the significance of those added diagnoses is not always clear, as a new study in JAMA Oncology shows.
DCIS and mammography: Some background
Before I get to the meat of the study, from my perspective, nowhere is the problem of overdiagnosis and overtreatment in cancer screening as pronounced than in the condition known as ductal carcinoma in situ (DCIS). DCIS is commonly referred to as “stage 0” breast cancer and is characterized by milk duct cells that appear malignant but remain confined to the milk ducts. In other words, they haven’t invaded the tissue surrounding the ducts. In general, DCIS is treated similarly to breast cancer, with surgical excision, either by mastectomy or breast-conserving surgery, followed by radiation therapy if breast conserving surgery is used. Then, depending on its hormone receptor status, adjuvant treatment consists of blocking estrogen for five years. The rationale for this treatment is the view of DCIS as being a precursor to fully invasive breast cancer and that treating the DCIS will prevent the development of breast cancer. Over the last couple of decades, however, it has become clear that not all DCIS is created equal. Much of it will never progress to breast cancer in the lifetime of the woman (particularly if the woman is older, which means less time for fully malignant transformation to occur). Evidence suggesting this includes studies showing an increase in DCIS incidence by 16-fold since the 1970s, when mammography started to be introduced on a large scale, with little change in the incidence of invasive cancer. Today, 20-25% of mammography-detected breast cancer diagnoses are DCIS; forty years ago, DCIS was an uncommon diagnosis, except associated with an invasive cancer.