Page 53 - Fisica in Medicina n° 4 - 2017

DC converts EPID acquisitions (or acquisitions with other 2D detector arrays) of the intensity delivered from a radiation beam to a map of in-air fluences, from which the 3D dose distribution is calculated on the patient CT. This reconstructed dose is then compared to the intended dose from the TPS in order to ensure the quality of the delivery of the treatment. How it works relative monitor units The first step to convert a raw EPID images to in-air fluence involves the conversion of the gray level image to what is called a Relative Monitor Units (RMU) map. Each element of the RMU map contains the number of monitor units that would produce its corresponding gray level at the center of a 10 X 10 cm2 field. In a raw EPID image the gray level increases linearly with the number of MU delivered. A 10 X 10 cm2 open field with a know number of MU characterizes this relation giving the so called calibration curve. With this calibration field the gray levels can be transformed into RMU values [2, 8]. However, EPID images contain internal scatter and dose deposition in the scintillator screen as well as optical photon spreading from the scintillator to the photodiodes [10]. This effect can be represented by the point spread function of the EPID: k(r), and the raw EPID image (IEPID) can be represented by the convolution [2]: IEPID = Iinc  k(r) (1) where Iinc is the incident EPID fluence image without the scatter effects above mentioned and k(r) can be written as the sum of five exponentials [2, 11].