Quality assurance in radiation therapy: clinical and physical aspects. Teletherapy equipment and simulators.

A comprehensive Quality Assurance program in a radiation therapy center is desirable regardless of its size, and should cover among others the following areas: physical parameters of the therapy machines, dosimetric standards, preventive maintenance of radiation emitting sources and measuring instruments. In a radiation therapy center, regardless of its size and patient load, it is advisable to have a quality assurance program covering all the treatment planning steps. The following areas should be taken into consideration: physical parameters of the machines; dosimetric standards; radiation safety procedures and preventive maintenance of irradiators and radiation measuring instruments. The minimum instrumentation required and the critical parameters to be observed to establish a quality assurance program are discussed; the suggestions are applicable to the various sizes of radiation therapy centers. It is essential that all the procedures and results obtained are well documented and a critical evaluation of the program be performed periodically. The procedures and frequency suggested are applicable to low, medium and high energy treatment machines and simulators. The fluctuation on physical parameters currently observed in clinical physics practice strongly supports the efforts and costs of a quality assurance program.

A simple device for testing the equilibrium of ion chambers in ambient conditions.

A simple device for testing the response of ion chambers when ambient pressure is varied was built from an inexpensive device normally used to maintain tennis balls under pressure. The results show that one may use this system in routine as well as for teaching purposes.

Effect of the accelerator design on the position of the effective electron source.

An analysis of the effective source-surface distance was made taking into account the influence of energy, field size, equivalent square field size, and the accelerator design. Measurements made in three accelerators showed the influence of parameters like beam flattening and collimation systems on the effective source-surface distance. A brief review of the theory is made leading to a mathematical expression for the effective source-surface distance, showing a good agreement with the experimental results. An alternative method for correcting the dose rate, maintaining the nominal source-surface distance, and not using the inverse square law, but a greater power function, is suggested.

Estudos dosimetricos para utilizacao de grandes campos de irradicao em terapia com feixe de cobalto 60. / Dosimetric studies for utilization of very large irradiation field in therapy with bundle of cobalt 60

Foi feito um estudo de campos grandes aumentando-se a distancia de tratamento e verificando-se parametros tais como a razao tecido-ar, fator abertura do colimador e posicao efetiva da fonte. Os valores de TAR encontrados para as distancias de 130 cm e 180 cm foram cerca de 3% maiores que os relativos a distancia de 80 cm. Foi observado que o fator abertura do colimador independe da distancia e que o rendimento pode ser corrigido pela lei do inverso do quadrado da distancia

Contaminacao de eletrons na regiao de "build-up" para diferentes unidades de telecobaltoterapia. / Electrons contamination in the build-up region for several telecobalt therapy units

A qualidade e intensidade da radiacao secundaria produzida em uma unidade de telecobaltoterapia e proveniente da fonte, material que a envolve e sistema de colimacao, o que enfatiza a importancia de se medir tal influencia para cada tipo de unidade. Foi feito um estudo da distribuicao de dose na regiao de "build-up" em funcao do tamanho de campo e distancia fonte superficie (DFS) em sete unidades de telecobaltoterapia que apresentam caracteristicas distintas de construcao. Para campos pequenos, onde a contaminacao e pouco significativa, ha uma concordancia entre os resultados obtidos para as diferentes unidades. Com o aumento do tamanho de campo, o ponto de dose maxima deslocase na direcao da superficie criando "picos de contaminacao" cuja intensidade e funcao do sistema de colimacao e da distancia entre a extremidade do colimador e a superficie. O aumento da DFS acarreta uma diminuicao da dose na regiao de "build-up" justificando-se medir a distribuicao de dose para diversas distancias, uma vez que e comum utilizar-se distancias maiores a fim de se obter campos maiores. A utilizacao de mesas de tampo solido tambem altera as caracteristicas de "build-up" do feixe