A method of forming a soft tissue biopsy cavity marker may include steps of providing a radio-opaque element and a bio-compatible and bio-degradable polymer such as alginate and delivering the provided radio-opaque element and the provided polymer to a biopsy site, such as a breast biopsy site. A gelling initiator that includes divalent cations such as NaCl.sub.2 may then be provided and delivered to the biopsy site, causing the previously delivered polymer to gel and to form a soft tissue biopsy marker in situ (within the biopsy site).