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chemotherapy drugs used for patients. IntrinsiQ provided data on the proportion of colorectal cancer patients (of all ages) treated with chemotherapy who are treated with each regimen for each month between January 2002 and September 2005.8

We derive market shares for the 1993 to 2001 period from the Surveillance Epidemiology and End Results (SEER) data set, which tracks the health and treatment of cancer patients over the age of 64 in states and cities covering 26 percent of the United States population.9 We calculate the proportion of colorec- tal cancer patients who are treated with each drug regimen in each quarter based on Medicare claims data available in SEER. In October 2003, approximately 48 percent of all colorectal cancer patients treated with chemotherapy were 65 years or older.10

In the 1993 to 2001 period, when there were relatively few treatment options for colorectal cancer, we include all regimens that contain drugs that were explic- itly approved by the Food and Drug Administration (FDA) for colorectal cancer and had a market share greater than two percent. Market shares of all other drugs are combined into an outside option, which in this early period will consist primarily of off-label drugs - drugs approved for conditions other than colorectal cancer that are used on colorectal cancer patients.11 In the 2002 to 2005 period, the outside option includes off-label drugs, regimens with less than one percent

8Because we observe the market shares of regimens among patients with colorectal cancer, we do not need to worry about o -label use. O -label use occurs when a physician treats a colorectal cancer patient with a drug that has not been approved by the FDA to treat colorectal cancer, or when a physician uses a drug approved for colorectal cancer on a patient with a di erent type of cancer. In October 2005, seventy-six percent of patients being treated with the four drugs approved solely for the treatment of colorectal cancer (irinotecan, oxaliplatin, cetuximab, and bevacizumab) actually had colorectal cancer. That is, o -label use

accounted for approximately 24 percent of the quantities of these drugs. 9SEER contains data on the incidence rate of cancer among the non-elderly, but only has medical claims

available for Medicare patients. Data from IntrinsiQ. O -label use is more likely to occur if a patient’s initial treatment has been unsuccessful. 1 11

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