Yes.

The research was performed and published by the Center for Impact Research, which is no longer in operation. (Empty web-site), and titled Sisters Speak Out: The Lives and Needs of Prostituted Women in Chicago: A Research Study, by Jody Raphael and Deborah L. Shapiro.

Note: It was published in August 2002, not 2001 as reported above.

On page 18, in Table 1. Percentage of Women Experiencing Frequent Violence in Prostitution, they document the results of a survey. A total of 28 female escorts were surveyed, and 21.4% of them reported having experienced "Forced Sex" more than 10 times.

Yes.

The research was performed and published by the Center for Impact Research, which is no longer in operation (Their web-site is empty web-site).

The report was titled Sisters Speak Out: The Lives and Needs of Prostituted Women in Chicago: A Research Study, by Jody Raphael and Deborah L. Shapiro.

On page 18, in Table 1. Percentage of Women Experiencing Frequent Violence in Prostitution, they document the results of a survey. A total of 28 female escorts were surveyed, and 21.4% (i.e. 6 of 28) reported having experienced "Forced Sex" more than 10 times.

Note: It was published in August 2002, not 2001 as reported above.

It does not appear to have appeared in a peer-reviewed journal. Surveys are of limited accuracy. Small surveys even more so, and reporting the figure to three significant figures, without error bars, may give the wrong impression about the accuracy of the result.

The authors published a similar sounding article in 2004 in Violence Against Women, called Violence in Indoor and Outdoor Prostitution Venues. I haven't been able to confirm if they repeat the claim in that work.