Computational analysis of 'Dose/Collision Kerma' relationship and lateral boundary in Stereotatic circular fields using EGSnrc.

Objectives : This work aimed to obtain data from small fields of X-rays that evidence of the hypotheses cited as cause of difficulties for the dosimetry of these. For this purpose, the verification of compatibility between the dosimetric boundary of field and the geometric size of field, was performed. Also was made, the verification of kerma dose according to the expected relationship for conventional fields. Materials and Methods : Computer simulations of smaller fields 5x5 cm² were performed using the Monte Carlo method by egs_chamber application, this derived from EGSnrc radiation transport code. As particulate sources were used phase space files of a Clinac 2100 head model coupled to cones Stereotactic Radiosurgery. Results : The simulations suggested the existence of a plateau in discrepancies between the dose FWHM and the nominal diameter of the field close to 8%. These simulations also indicated a decrease of these values for fields with diameters smaller than 12 mm and larger than 36 mm. Simultaneously, the dose kerma differences in depth reached values higher than 14% in the case where the phenomenon is more significant. Conclusion : The data showed that in fact the behavior of small fields clashes with that expected for conventional fields, and that the traditional dosimetric conventions do not apply to such fields requiring a specialized approach to the techniques that employ them. Furthermore, the existence of the aforementioned plateau of discrepancies, along with the decrease thereof in less than 15 mm diameter fields constitute a remarkable finding.