Answer:
Neutral triglycerides were considered as an alternative to silicone or saline. Initially peanut oil was used, manufacturer then changed to soybean. Atomic number (5.9) similar to breast parenchyma (6) which makes it radiolucent (saline atomic number 7.6, silicone 10.5); if leaked, soybean oil is rapidly absorbed and metabolized. No longer manufactured.