Dosimetric measurements for-small beams in X-ray stereotatic irradiation / 中华放射肿瘤学杂志

ABSTRACT

Purpose:To analyse the dosimetric parameters measured by the silicon diode, and to describe the features of the dose distributions for small fields.Materials and Methods:P-type silicon diode with small volune and high sensitivity was chosen to measure the PDD,OAR and Sc,p at isocenter for circular fields of 5mm ～50mm in diameter used for stereotactic irradiation .The dosimetric data measured are evaluated by comparising with ones by other methods, such as ionization chamber and films,and ones in literature.Results:The values PDD of 10 and 30 fields at depth from 5cm～20cm are almost the same as those in reference 7 within ?0.6; The effective linear attenuation coefficient extrapolated to zero fiels size is 0.0510cm -1 for 6MV-X rays. The Sc,p value measured by diode and chamber(NE2571) are agread each other within variations of ?0.4% for large field size, but a great difference appears when the field size is 2 times smaller than the diameter of chamber. The Sc,p values by diode for field size of 12.5mm～27.5mm are agreed well with the results by Monte Carlo calculations. The OARs measured by diode are ageed well within 1mm with ones by films developed by ourself. The penumbras(90%～10%) measured by diode are coincidence with the results published.Conclusion:Due to the high dose gradients at the beam edge and the absence of lateral electron equilibrium for the smaller fields, the detector size and positioning are the most impostant factors influenced the measured results. Our practice shows that the silicon diode with diameter of 2mm or smaller is the best choice for the dosimetric date collection for beams used in X-ray stereotactic irradiation.