WE-B-AUD C-01: Dose Calculation Algorithms in 3DCRT and IMRT

Abstract

The majority of photon dose calculation approaches today are model based dose calculation algorithms, in which the dose distribution is predicted from first principles. At the basic level there are the pencil beam convolution models, with further sophistication introduced by the collapsed cone convolution models. While the basic element of both is a convolution, which is performed between the energy released in each voxel and a dose spread kernel, different approximations have been introduced to deal with limitations in computer speed and incomplete physics. As it is important that the clinical physicist understands the limitations of these dose calculation approaches, this teaching lecture aims to describe these models and discuss the advantages and disadvantages of them. When possible, the individual implementation of the algorithms in commercial treatment planning systems will be discussed. The models covered firstly include pencil beam models, which are still the basis for most dose calculations in 3DCRT and IMRT optimizations today. Secondly, more recent pencil beam implementation that include modelling of changes in electron transport, particularly in the lateral direction, will be discussed. Finally the highly accurate collapsed cone models will be covered. Some comparisons with Monte Carlo generated data will be presented to highlight the limitations of the algorithms in focus. Educational Objectives: 1. To provide an educational review of the physics and techniques behind convolution algorithms. 2. To review the methods used to improve the simulation efficiency i.e. pencil beam and collapsed cone convolutions. 3. To briefly review the vendor codes currently used for clinical treatment planning. 4. To discuss the issues associated with experimental verification of dose calculation algorithms. 5. To briefly review the potential clinical implications of accurate calculated dose distributions.