RESUMO
Irregular fields for boron neutron capture therapy (BNCT) have been already proposed to spare normal tissue in the treatment of superficial tumors. This added dependence would require custom measurements and/or to have a secondary calculation system. As a first step, we implemented the sector-integration method for irregular field calculation in a homogeneous medium and on the central beam axis. The dosimetric responses (fast neutron and photon dose and thermal neutron flux), are calculated by sector integrating the measured responses of circular fields over the field boundary. The measurements were carried out at our BNCT facility, the RA-6 reactor (Argentina). The input data were dosimetric responses for circular fields measured at different depths in a water phantom using ionisation and activation techniques. Circular fields were formed by shielding the beam with two plates: borated polyethilene plus lead. As a test, the dosimetric responses of a 7x4 cm(2) rectangular field, were measured and compared to calculations, yielding differences less than 3% in equivalent dose at any depth indicating that the tool is suitable for redundant calculations.
Assuntos
Terapia por Captura de Nêutron de Boro/estatística & dados numéricos , Planejamento da Radioterapia Assistida por Computador/estatística & dados numéricos , Argentina , Terapia por Captura de Nêutron de Boro/instrumentação , Humanos , Neoplasias/radioterapia , Imagens de Fantasmas , Eficiência Biológica RelativaRESUMO
A method is proposed for calculation of irregular field factors on the central beam axis and homogeneous medium for x-ray beams. The irregular field factor is introduced as the ratio of the output of a field with and without blocks on the central beam axis. The algorithm is based on the sector-integration method and the circular field quantities are calculated from in-phantom measurements. These circular field quantities are the output per beam monitor unit for circular fields defined by a hypothetical secondary collimator and reduced to a circular field by blocking. A derivation of the sector-integration equation is given from first principles. As it is shown, the circular field quantities are evaluated from data measured for rectangular, block shaped fields. Such quantities contain all beam components, including photons scattered from the blocks, the block tray, and photons scattered in the phantom. Consequently, the so called primary and secondary beam components are readily incorporated in this approach. Once the circular field quantities have been determined from rectangular field data, the irregular field factors for other geometry can be calculated. Irregular field factors for square, rectangular and circular block-shaped fields were calculated for 6 MV photon beams and compared with measured values. The results agree within 0.7%, even for heavy blocked field cases, i.e., a 40 x 40 cm2 collimator field blocked to a 5 x 5 cm2 field. The method was tested for a particular source to surface distance, depth, phantom composition, and source to block distance. Calculation of irregular field factors in another set up conditions requires the measurement of the appropriate input data.
Assuntos
Fótons , Radiometria/métodos , Radioterapia Conformacional/métodos , Algoritmos , Fenômenos Biofísicos , Biofísica , Modelos Teóricos , Imagens de Fantasmas , Água , Raios XRESUMO
The equivalence between rectangular photon fields and square fields is currently used to simplify tabulation and handling of data as well as to reduce measurement time. Widely used for routine calculation in the case of rectangular fields of moderate elongation, the so-called "area over perimeter rule" (A/P rule) has a remarkable accuracy. Several approaches have been developed to determine the physical and mathematical grounds of this rule, yet the statement that it is independent of depth and energy was not fully clarified. By means of the Clarkson sector-integration equation and Taylor expansion, this work demonstrates that the A/P rule is a first-order approximation on the elongation variable for all cases, i.e., for moderate rectangular field elongation presents a quadratic deviation. To appreciate the degree of approximation of this rule, a model scatter air ratio for Co60 y-rays at 10.0 cm depth was used to compute rectangular radiation fields and the results were compared to those given by the A/P rule. The model scatter air ratio was the proposed by Day and Aird [Br. J. Radiol. 25, 138-151 (1996)].
Assuntos
Fótons/uso terapêutico , Planejamento da Radioterapia Assistida por Computador/estatística & dados numéricos , Ar , Radioisótopos de Cobalto/uso terapêutico , Humanos , Modelos Teóricos , Espalhamento de RadiaçãoRESUMO
A method to predict rectangular field output factors (OFs) of photon open beams for the Saturne 41 linear accelerator has been developed. The procedure is similar to the sector-integration method but the radiotherapy quantities corresponding to circular fields (circular functions) are calculated from one-dimensional OFs. In this case the one-dimensional OFs are defined as rectangular field OFs, where one side remains constant and equal to the maximum field size. The circular quantities are numerically obtained by inversion of the sector-integration equation which relates both the one-dimensional OFs and the circular function. Two one-dimensional OFs were used to take into account the asymmetry between the x and y collimator systems (collimator exchange effect). The resulting pair of circular functions corresponds to the x and y collimator systems, respectively. They contain all the information relative to head, air, and medium (phantom) scatter and, consequently, there is no need to account for the geometry of the head or fitting parameters. Using the sector-integration method, the OFs for any rectangular field can be calculated by integrating the obtained circular functions. To improve results, a procedure is given to account for corner collimators overlapping. Results agree with data to within approximately 0.4% at 6-15 MV photon beams. The proposed method is thus clinically acceptable for routine calculation. Furthermore, the circular function calculation algorithm could be extended to other radiotherapy quantities.