The convergent X-ray lens is a new approach in radiotherapy, where instead of high-energy beams with an energy of a few MeV, we use low-energy beams with an energy of a few tens of keV, which are directed with a lens of an appropriate composition to a focal point tens of centimeters or more deep in the tissue. The dose distribution in the tumour can be calculated by a Monte Carlo simulation, which is accurate but time-consuming, so the calculation is simplified with approximations. In this thesis, we have tested the applicability of the CCC method to calculate the dose of a convergent X-ray beam. The method contains a free parameter, called the beam model, which determines the TERMA distribution. A convolution of the TERMA and the dose point kernel yields the dose. We have calibrated the beam model to fit the calculated dose to the Monte Carlo simulated dose. The calculated dose was validated using a gamma function with parameters $\Delta D = 1\%$ and $\Delta d = 1$ mm. The CCC calculation is reliable and 200-times faster than the Monte Carlo simulation. The robustness of the dose to positional errors has also been verified. Since the focused dose is elongated along the depth, errors along the beam direction are less severe than those in the direction perpendicular to the beam.
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