Chapter 6 External Photon Beams: Physical Aspects - IRSN

656.11 SINGLE FIELD ISODOSE DISTRIBUTIONS IN PATIENTS6.11.1 Corrections for irregular contours and beam obliquityTAR method or TMR method• PDD corr is given as:TzA ( ,Q, h)PDDcorr= PDD ( z+h, A, f, h) Tz ( + hA , , h )• A Q is the field size at point S at adistance (f + h + z) from the source.• T stands for either TAR or TMR,and an assumption is made thatTARs and TMRs do not depend on SSD.• PDD’’ represents the PDD at depth (h + z) for a standard flatphantom with the surface at C”C”.• Parameter h is missing (positive h) or excessive (negative h)tissue.QIAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.11.1 Slide 5 (129/170)6.11 SINGLE FIELD ISODOSE DISTRIBUTIONS IN PATIENTS6.11.1 Corrections for irregular contours and beam obliquityIsodose shift method The value of the dose at point Sis shifted on a line parallel to thebeam central axis by (h x k).• Parameter h is the thickness ofmissing (+) or excess (-) tissue.• For missing tissue (h > 0) theisodose is shifted away from thesource; for excess tissue (h < 0) theisodose is shifted toward thesource.• Parameter k depends on beamenergy and is smaller than 1.Beam quality kCo-60 to 5 MV 0.75 MV to 15 MV 0.615 MV to 30 MV 0.5IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.11.1 Slide 6 (130/170)