Chapter 6 External Photon Beams: Physical Aspects - IRSN

626.11 SINGLE FIELD ISODOSE DISTRIBUTIONS IN PATIENTSClinical situations are usually more complex:• The patient’s surface may be curved or of irregular shape,requiring corrections for contour irregularities.• The beam may be obliquely incident on patient’s surfacerequiring corrections for oblique beam incidence.• Some tissues such as lung and bone have densities that differsignificantly from that of water, requiring corrections for tissueheterogeneities (also called inhomogeneities).IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.11 Slide 2 (123/170)6.11 SINGLE FIELD ISODOSE DISTRIBUTIONS IN PATIENTSIsodose distributions in patients are determined by oneof two radically different approaches:• Correction-based algorithms use depth dose data measured inwater phantoms with a flat surface and normal incidence inconjunction with various methods to correct for irregular patientcontours, oblique beam incidence, and different tissuedensities.• Model-based algorithms obviate the correction problem bymodeling the dose distributions from first principles andaccounting for all geometrical and physical characteristics ofthe particular patient and treatment.IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.11 Slide 3 (124/170)