Dosimetric Study On Dose Perturbation In Inhomogeneities For Photon Beams With Small Field Sizes(Beamlets)

Small and very small field size photo beams (beamlets) have been used widely in modern radiotherapy in recent years. Usually, the radiation treatment planning systems depend on measured beam data to construct the beam models which are utilized for dose calculations. However, it is very difficult to measure the dose profile accurately for small field size beams. On the other hand, the tabular parameters for the treatment planning system to calculate absorbed doses, such as percentage depth dose distribution and cross-profile dose distribution, which are practicable for ordinary field sizes, will be not suitable for small beamlets, especially in inhomogeneity medium.Dosimetric study on dose perturbation in inhomogeneities for small beamlets has been a very intractable question in the field of tumor radiation physics at all times. The measured uncertainty and the complexity of the media, such as bone, lung, cavity and their boundaries, lead to the difficulty of this kind of problems. It is inherently hard to measure the dosimetric profile for small radiation field because the ordinary size of standard ion chambers are too big in comparison to some small field sizes to get enough spatial resolution for narrow central regions and steep dose gradients of penumbra. That is why the dosimetric study for small field sizes in inhomogeneities has been dependent on the theoretic calculation more frequently than other problems. Monte Carlo simulation is one of effective ways to get this kind of dosimetric parameters.In this paper, the absorbed doses at the central axis in different phantoms with inhomogeneity media and finite sizes for small field size beams (beamlets) were simulated with Monte Carlo code system EGSnrc, respectively. The dose enhancement effects and decreasing effects caused by material inhomogeneity were studied quantitatively with a parameter named Dose Perturbation Factor (DPF), which was the ratio of the dose at a point in the heterogeneous phantom to the dose of the same spatial location point in the homogeneous phantom. It was showed that the degrees of the dose enhancement or decreasing depended on the energy of radiation, field sizes of the beamlets and compositions of the media. The dose perturbation for high energy X rays is greater than that for low energy X rays. At the same energy condition, the dose perturbation decreased when the field size increased. And the relationship between them was not linear.