Lung correction and mid plane irradiation dose factors in anatomic phantoms of proportional sizes

In vivo dosimetry in combined anterior-posterior, posterior-anterior, total body irradiation (AP-PA TBI) with Co-60 units makes it possible to determine the absorbed dose (D) in the patient's mid plane by means of an entrance dose reading (R) multiplied by a Mid Plane Dose Factor (MDF) and a Lung Correction Factor (CF). A theoretical model of an anatomic phantom, based on a number of cylindrical ellipsoide which could be set in any order following a real therapeutical position, was established. The dimensions of the constituent parts were variable and subject to change. In this work, calculations in the reference phantom with volume VO were compared with measurements in two experimental anatomic phantoms of the same volume and shape; 1. a water phantom from the former investigations and 2. a polystyrene phantom based on real patients' CTs with the lungs' inhomogeneities. A generalisation to proportional phantom sizes with volume V was set up. It was found that in this approach the ratio V/Vo had a significant role in the determination of MDF and CF, when real scattering conditions, e.g. lack of scatter due to the finite phantom width, were included.