  Carbon-ion radiotherapy (CIRT) is generally evaluated with the dose weighted
by relative biological effectiveness (RBE), while the radiation quality varying
in the body of each patient is ignored for lack of such distribution. In this
study, we attempted to develop a method to estimate linear energy transfer
(LET) for a treatment planning system that only handled physical and
RBE-weighted doses. The LET taken from a database of clinical broad beams was
related to the RBE per energy with two polyline fitting functions for
spread-out Bragg peak (SOBP) and for entrance depths, which would be selected
by RBE threshold per energy per modulation. The LET estimation was consistent
with the original calculation typically within a few keV/{\mu}m except for the
overkill at the distal end of SOBP. The CIRT treatments can thus be related to
the knowledge obtained in radiobiology experiments that used LET to represent
radiation quality.
