  Most virtual source models (VSM) use beam modeling, with the exception of the
patient-dependent secondary collimator (jaw). Unlike other components of the
treatment head, the jaw absorbs many photons generated by the bremsstrahlung,
which decreases the efficiency of the simulation. In the present study, a new
method of beam modeling using a virtual jaw was applied to improve the
calculation efficiency of VSM. The results for the percentage depth dose and
profile of the virtual jaw VSM calculated in a homogeneous water phantom agreed
with the measurement results for the CC13 cylinder type ion chamber within an
error rate of 2%, and the 80 to 20% penumbra width agreed with the measurement
results within an error of 0.6 mm. Compared with the existing VSM, in which a
great number of photons are absorbed, the calculation efficiency of the VSM
using the virtual jaw was expected to increase by approximately 67%.
