SCATTERED RADIATION FROM LINEAR-ACCELERATOR AND CO-60 COLLIMATOR JAWS

Purpose: Solid state diodes and/or thermoluminescent dosimeters (TLDs) are often used to measure scattered radiation doses to critical organ s immediately adjacent to radiation field sites. The energy-dependent response of these commonly used in vivo dosimeters sometimes makes the interpretation of measured values uncertain. This study investigates scattered radiation arising from the collimator jaws of linear acceler ators and the treatment head of a cobalt-60 teletherapy unit. Methods and Materials: A thin window Markus-type parallel-plate ionization cha mber placed in a polystyrene phantom was employed to document the magn itude, energy composition, and sources of scattered radiation at surfa ces near radiation fields. Measurements were taken both with and witho ut additional phantom material covering the ionization chamber, as wel l as with various distances between the ionization chamber and edges o f the radiation fields tested. Results: Data was collected, analyzed a nd compared for treatment units produced by different manufacturers. I t was found that the magnitude of scattered radiation to surfaces imme diately adjacent to radiation fields ranged from 1% to 15% of the maxi mum dose along the beam central axis. These values showed a strong dep endence upon distance from the edge of the radiation field, beam energ y, collimator setting (field size), and the presence of externally mou nted accessories. Teletherapy unit differences due to manufacturing fi rm origins were found to only slightly affect scattered radiation magn itude, while the orientation of upper and lower collimator jaws had ab solutely no effect. Conclusions: Percent depth dose curves of scattere d radiation were obtained and analyzed. The shapes of these depth dose curves suggest the presence of complex energy spectra from secondary electrons and scattered x-rays. Because of the presence of these compl ex energy spectra in areas immediately adjacent to radiation fields, c aution should be observed when interpreting patient doses near radiati on fields, if dose values have been measured in vivo using thermolumin escent dosimeters (TLDs) or solid state diodes. Many of these on-patie nt dosimetry devices are strongly energy dependent and may demonstrate large over- or under-responses in areas dominated by scattered radiat ion. The results of this study, thus, suggest that ionization chambers are preferred for determination of scattered radiation doses in such regions.