Chapter 6 External Photon Beams: Physical Aspects - IRSN

686.11 SINGLE FIELD ISODOSE DISTRIBUTIONS IN PATIENTS6.11.2 Missing tissue compensationCompensators are used to produce the same effect asthe bolus yet preserve the skin sparing effect of megavoltagephoton beams.Compensator is a custom-made device that mimics theshape of the bolus but is placed in the radiation beamat some 15 cm - 20 cm from the skin surface topreserve the skin sparing properties of the radiationbeam.IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.11.2 Slide 5 (135/170)6.11 SINGLE FIELD ISODOSE DISTRIBUTIONS IN PATIENTS6.11.2 Missing tissue compensationTypical compensator materials are:• Lead• Special low melting point alloys such as cerrobend (Lipowitz’smetal).• Water equivalent materials such as wax.Since compensators are placed at some distance fromthe skin surface, their shape must be adjusted for:• Beam divergence• Linear attenuation coefficient of the compensator material.• Reduction in scatter at various depths in patient.IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.11.2 Slide 6 (136/170)