Chapter 6 External Photon Beams: Physical Aspects - IRSN

726.11 SINGLE FIELD ISODOSE DISTRIBUTIONS IN PATIENTS6.11.4 Model based algorithmsModel based algorithms for computation of dose distributionin a patient are divided into three categories:• Primary dose plus first order Compton scatter method is arudimentary method assuming a parallel beam of monoenergeticphotons and ignoring inhomogeneities and all scattering abovethe first order.• Convolution-superposition method accounts for indirect nature ofdose deposition from photon interactions, separating the primaryinteractions from the transport of scattered photons and chargedparticles produced through primary photon interactions.• Monte Carlo method uses well established probability distributionsgoverning individual interactions of photons and secondarycharged particles and their transport through the patient.IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.11.4 Slide 1 (143/170)6.11 SINGLE FIELD ISODOSE DISTRIBUTIONS IN PATIENTS6.11.4 Model based algorithmsMonte Carlo simulation can be used directly to computephoton dose distributions for a given patient and treatmentgeometry.The current limitation of direct Monte Carlo calculationsis the time required to calculate the large number ofhistories needed to reduce stochastic or random uncertaintiesto acceptable levels.IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.11.4 Slide 2 (144/170)