Tissue markers and uses thereof

A method comprising:

• performing a biopsy at a site in a target tissue, wherein the target tissue is a soft tissue selected from the group consisting of breast tissue, colon tissue, rectal tissue, liver tissue, ovarian tissue, prostate tissue, and lung tissue;
• and implanting a tissue marker without the use of adhesives at a target site at, or within 5 cm of, the site of the target tissue, the tissue marker having a flexible polymer matrix, wherein the flexible polymer matrix comprises a sheet, wherein the sheet when in a substantially planar configuration comprises at least two films with one or more contrast agents located between the surfaces of the at least two films, such that the tissue marker is visible by one or more imaging modalities, wherein the flexible polymer matrix is arranged in a rolled configuration as a hollow cylinder, wherein the flexible polymer matrix is selected so that the densities of the flexible polymer matrix and the target tissue differ by no more than 30%, and wherein upon the implantation at the target site, the flexible polymer matrix transforms from the rolled configuration to at least a partially unrolled configuration or planar shape while maintaining the density for a period of at least 60 seconds and wherein the density and the partially unrolled configuration of the flexible polymer matrix prevents the tissue marker from migrating more than 10 mm from the target site over a period of at least one month.