Build-up modification of commercial diodes for entrance dose measurements in ‘higher energy’ photon beams

摘要: Abstract Background and Purpose : Several commercially available p-type diodes do not provide sufficient build-up for in-vivo dosimetry in ‘higher' energy photon beams, only limited information could be found the literature describing correction factor variation and/or achievable accuracy methods this range. The first aim of study is to assess analyze diode factors entrance dose measurements at higher energies. In a second step total build up thickness has been modified order minimize variation. Materials Methods Diode accounting non-reference conditions (field size, source surface distance, tray, wedge, block) are determined 18–25 MV beams provided by different treatment units Scanditronix recommended beams: old type new EDP-20, EDP-30 diodes. Hemispherical caps materials (copper, iron, lead) used increase thickness. Perturbation effects with without additional assessed three types. Results For unmodified field size (C FS ) vary between 1.7% 6%, dependent on unit. example, an EDP-20 C 18 much GE linac (5%) as compared Philips machine (1.7%). Depending type, can reduced 1–2% when adding build-up. distance almost independent By influence trays blocks eliminated. Conclusions reflects electron contamination geometry. A 3O mm ‘best compromise' types investigated measuring doses range 25 MV.