Journal of Medical Physics
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Year : 1998  |  Volume : 23  |  Issue : 2  |  Page : 46-50

Build-up Characteristics Of 60Co, 6 And 10MV Photon Beams

Correspondence Address:
H K Malhotra

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Source of Support: None, Conflict of Interest: None

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Precise knowledge of photon beam characteristics in the build-u region is essential for accurate dose delivery in a clinical set up. In the current study, build-up characteristics of 60Co, 6 and 10 MV photon beams have been determined, using a PTW-Markus chamber in a polystyrene phantom. The variation of dmax with increase in field-size is also investigated. . For 60Co, the decrease in dmax with field-size was maximum (60%decrease in dmax when the field-size increases from 5 x 5 to 35 x 35 cm), which may compromise skin sparing at larger field-sizes. All the three photon beams were found to have comparable build-up curves when plotted against d / dmax (%). Errors exceeding 5% were observed in the dose ionisation build-up curve, particularly at shallow depths for 6 and 10 MV photons. Beyond 2 mm depth for 60Co and 5 mm for 6 and 10 MV photon beams, the over-response of the Markus chamber against extrapolation chamber reduceds to less than 2.5%.

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