In space radiation shielding-related literature, equivalent BFO doses are sometimes provided in terms of sphere thickness (g/cm^2). For example in Fig. 1 of [Ref 1] (the figure is attached below), the radiation dose is calculated in the annual blood forming organs (BFO) with the radiation data from 1977 and 1990. I find them expressed as shield thickness (g/cm^2) for the equivalent BFO doses in many other studies (for example in this book by Wilson et al., page 490 [Ref 2]).
It seems to be a bit confusing for me to find the difference between these two parameters. For shield thickness, we can divide it by density and get the thickness in cm unit. Then we can also measure the volume of the added protective layer by considering its shape (e. g. a cylinder) for a certain thickness. From the unit's perspective, I can do the same with sphere thickness too. Will it be correct? How these two kinds of thicknesses (shield and sphere) are different?