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 Pinnacle(3) 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.