Chapter 6 External Photon Beams: Physical Aspects - IRSN

516.9 OFF-AXIS RATIOS AND BEAM PROFILESMegavoltage beam profiles consist of three regions:• Central region represents the central portion of the profileextending from the central axis to within 1 cm to 1.5 cm of thegeometric field edges of the beam.• Penumbra is the region close to geometric field edges wherethe dose changes rapidly and depends on field definingcollimators, the finite size of the focal spot (source size) andthe lateral electronic disequilibrium.• Umbra is the region outside of the radiation field, far removedfrom the field edges. The dose in this region is low and resultsfrom radiation transmitted through the collimator and headshielding.IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.9 Slide 4 (101/170)6.9 OFF-AXIS RATIOS AND BEAM PROFILESFor each of the three beam profile regions there arespecific requirements to optimize the clinical photon beam:• The dose profile in the central region should meet flatness andsymmetry specifications.• The dose profile in the penumbral region should have a rapid falloffwith increasing distance from the central axis (narrow penumbra)to optimize beam sharpness at the target edge.• The dose profile in the umbral region should be close to zero doseto minimize the dose delivered to tissues outside the targetvolume.IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.9 Slide 5 (102/170)