Notation and Formulae for Dose Specification and Calculations in Radiotherapy

The ideas, parameters, and dose calculations relating to treatment planning for radiotherapy are basically simple. However, the advent of megavoltage radiation therapy and new treatment techniques have acted to expand and complicate early methods and often each institution has adopted its own conventions and symbols. At present, therefore, communication is hindered by the lack of an agreed notation; a significant difficulty exists in the retrospective interpretation of patient treatment records and there is increased need for rapid, accurate dose calculations where the introduction of high-output megavoltage sources has rapidly expanded patient loads. In an effort to simplify records and dose calculations, the notation and formulae presented here were adopted after an examination of our existing system, a retrospective study of our treatment records and a survey of the radiotherapy literature. The system described evolved from a simpler version which was used for a number of years.The system shown in Table I consists of a set of symbols (capital letters with one exception) to define quantities useful in radiotherapy, and an accompanying set of subscripts (lower-case letters) which may be added to restrict or further define the quantities represented by the symbols. The choice of a particular letter has been guided by present usage, a desire to include a mnemonic aid, and by the cohesiveness, clarity and lack of conflict which ensues when the symbols and subscripts are combined and used. An effort has been made in the definitions to achieve maximum generality while retaining sufficient rigour to avoid ambiguity.