Chapter 6 External Photon Beams: Physical Aspects - IRSN

676.11 SINGLE FIELD ISODOSE DISTRIBUTIONS IN PATIENTS6.11.2 Missing tissue compensationPhysical wedges are usually available with wedgeangles of 15 o , 30 o , 45 o , and 60 o .Dynamic wedges are available with any arbitrary wedgeangle in the range from 0 o to 60 o .Physical wedge filters may alter the x-ray beam quality,causing• Beam hardening at energies of 6 - 10 MV• Beam softening at energies above 15 MV.IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.11.2 Slide 3 (133/170)6.11 SINGLE FIELD ISODOSE DISTRIBUTIONS IN PATIENTS6.11.2 Missing tissue compensationBolus is tissue equivalent material placed directly ontothe patient’s skin surface:• To even out irregular patient contour.• To provide a flat surface for normal beam incidence.In principle, the use of bolus is straightforward andpractical; however, it suffers a serious drawback: formegavoltage photon beams it results in the loss of theskin sparing effect in the skin covered with the bolus(i.e., skin sparing effect occurs in the bolus rather thanin the patient).IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.11.2 Slide 4 (134/170)