Noncoplanar verification: a feasibility study using Philips’ Pinnacle3 treatment planning system

Noncoplanar fields are normally used to improve the dose conformity of the target
while sparing organs at risk. One of the methods to verify the dose distribution
from the noncoplanar fields is by comparing their planar dose distributions from
the treatment planning system (TPS) and the measured ones; for example, using
film or electronic portal imaging devices (EPID). The planar dose distributions of
the measurement tools, that are normally perpendicular to the central axis of the
beam, can be calculated by creating special structures to mimic them in the TPS.
With TPS commercially available today, however, it is not easy to create these
special structures, especially in the noncoplanar configuration. For this work, we
have written in-house scripts in the Pinnacle3
TPS that can create the structures
and define them as virtual planes. These virtual planes can be generated for any
arbitrary gantry and couch angles, as well as source to planar distance, so that
the planar dose maps at these planes can be computed. Two independent quality
assurance (QA) tools were used to validate the planar dose distributions computed
using the scripts for three open fields and one IMRT field at several different couch
angles. The absolute planar dose patterns measured by the QA tools for all fields
at all couch angles were found to be in good agreement, more than 95% (gamma
criteria 3% delta dose and 3 mm distance to agreement), with the calculated ones
by TPS. The results of this feasibility study can be valuable either for pretreatment
dose verification or for in vivo dosimetry that directly implements the planar dose
distributions from the TPS, particularly for the noncoplanar fields.