Given the lack of human medical data available to calibrate and tie the sophisticated model calculations of various space radiation exposure metrics, such as “dose equivalent,” to the actual biological harm expected from long-term exposure to galactic cosmic ray (GCR) radiation in the region of cislunar space [nominally at 1 astronomical unit (AU) from the sun], a new spacecraft shielding evaluation metric is proposed that empirically links to the available medical data collected from International Space Station (ISS) operations, and thereby improves shielding material assessment. Adequate protection from radiation is one of the most important unknowns regarding the contemplated long-term spaceflights beyond the Earth’s protective zone afforded by its magnetosphere. This new radiation shielding evaluation metric relies on a comparison of the relative shielding effectiveness of a particular material or shielding design in the cislunar space radiation environment to the verified radiation exposure environment inside the ISS, where hundreds of astronauts have flown and spent considerable time, and for which there is voluminous medical data and continuing medical observation. Thus, the new metric is a measure of how well a proposed shielding protocol operating in the 1 AU cislunar radiation environment replicates the ISS radiation exposure levels. And, because it is calculated on a strict comparable basis, it mitigates the unavoidable errors introduced in the shielding evaluation process inherent in using different GCR models and radiation transport codes, as well as the various versions of radiation quality or weighting factors.