Detector response and dose calculation lateral to material interfaces for 6 MV photon beam

Dose calculation around inhomogeneities is challenging for many algorithms. A validation of dose distributions in these conditions is not straightforward, as detector response close to material interfaces is affected by the non-equilibrium situation, the changing energy spectrum and the volume effect in a steep dose gradient.

Detector response lateral to an inhomogeneity of high density was studied, mimicking the situation of bone surrounded by soft tissue.

Profiles obtained with different detectors (diodes, ion chamber, synthetic diamond) in water in the vicinity of an aluminum cylinder were compared. Dose deposition was also calculated with different commercial treatment planning systems and algorithms as well as using Monte Carlo simulations within and lateral to the cylinder and compared to measurements.

Dose deposition in the vicinity of an inserted aluminum cylinder changes and is registered by the detectors to a different degree. The ion chamber shows the largest change irradiated with a 10×10 cm² at 3 mm distance from the cylinder surface (2.5%), followed by the synthetic diamond (1.7%), then the unshielded (1.4%) and finally the shielded diode (1.0%). Dose calculated by different commercial dose engines differed up to 3% at that point from the detector values (Collapsed Cone).

Dose calculation near inhomogeneities depends on the used algorithm, dose measurements in the same region differ depending on the detector type used. We recommend verification of dose calculation with second type of algorithm and measurements with at least two detector types.