Dosimetric verification in inhomogeneous phantom geometries for the XiO radiotherapy treatment planning system with 6-MV photon beams.

We have developed a practical dose verification method for radiotherapy treatment planning systems by using only a Farmer ionization chamber in inhomogeneous phantoms. In particular, we compared experimental dose verifications of multi-layer phantom geometries and laterally inhomogeneous phantom geometries for homogeneous and inhomogenous dose calculations by using the fast-Fourier-transform convolution, fast-superposition, and superposition in the XiO radiotherapy treatment-planning system. We applied the dose verification method to three kernel-based algorithms in various phantom geometries with water-, lung- and bone-equivalent media of different field sizes. These calculations were then compared with experimental measurements by use of the Farmer ionization chamber. The fast-Fourier-transform convolution algorithm overestimated the dose by about 8% in the lung phantom geometry. The superposition algorithm and the fast-superposition algorithm were both accurate to better than 2% when compared to the measurements even for complex geometries. Our dose verification method was able to clarify the differences and equivalences of the three kernel-based algorithms and measurements with use only of commonly available apparatus. This will be generally useful in commissioning of inhomogeneity-correction algorithms in the clinical practice of treatment planning.