Page 54 - Fisica in Medicina n° 4 - 2017

The incident EPID fluence is obtained using a deconvolution process. It is straightforward from equation 1 that: Iinc = F-1(F(IEPID)/F(k(r)) (2) where F is the 2D Fast Fourier transform. Finally, each deconvolved RMU value is then multiplied by its in air offaxis ratio to restore the flatness and symmetry of the beam, since the EPID flood field image calibration removes the horn effect that is supposed to be present [2]. In the case of images acquired in vivo, a further correction must be made due to the spectrum change and the scattered radiation of the patient. This correction is taken into account by using a kernel that is a function of both radius and water equivalent thickness of what is in the beam [12, 11]. calculation algorithm Each of the pixels of the RMU map represents a specific weight of the in-air fluence map, i.e. the input for the dose calculation algorithm. DC employs a Pencil Beam (PB) algorithm for dose calculation in the CT images of the patient. Contrary to source model algorithms employed by most of modern commercial TPSs, DC calculates the intensity distribution of the pencils from a measured source model through the RMU map calculation [3]. The PB algorithm relies on a poly-energetic dose kernel calculated with Monte Carlo, using the beam data such as the central axis percent depth dose data. Along with this, off-axis correction factors are generated which accounts for the changes in beam penetration due to the change in beam energy by using a diagonal profiles measured in water [10]. It can be assumed that some dose differences will arise from the PB algorithm used in DC and the more sophisticated algorithms used in TPS.