Chapter 6 External Photon Beams: Physical Aspects - IRSN

696.11 SINGLE FIELD ISODOSE DISTRIBUTIONS IN PATIENTS6.11.3 Corrections for tissue inhomogeneitiesRadiation beams used in patient treatment traversevarious tissues that may differ from water in density andatomic number.This may result in isodose distributions that differsignificantly from those obtained with water phantoms.The effects of inhomogeneities (also referred to asheterogeneities) on the dose distributions depend upon:• Amount, density and atomic number of the inhomogeneity.• Quality of the radiation beam.IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.11.3 Slide 1 (137/170)6.11 SINGLE FIELD ISODOSE DISTRIBUTIONS IN PATIENTS6.11.3 Corrections for tissue inhomogeneitiesThe effects of inhomogeneities on dose distributions fallinto two distinct categories:• Those that increase or decrease the attenuation of the primarybeam and this affects the distribution of the scattered radiation.• Those that increase or decrease the secondary electron fluence.Three separate regions are considered with regard toinhomogeneities:• Region (1): the point of interest is in front of the inhomogeneity.• Region (2): the point of interest P is inside the inhomogeneity.• Region (3): Point of interest P is beyond the inhomogeneity.IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.11.3 Slide 2 (138/170)