Quantification of skin dose increase and photon beam attenuation for a commercial couch top and breast board using dosimetric and Monte Carlo methods

Background: to study attenuation and increased skin dose for the ibeam standardcouchtop, and attenuation of the breaststep board, for an elekta compact 6 mvaccelerator. materials and methods: couchtop attenuation were measured forthe range of gantry angles 125°-180° and field sizes 5×5-20×8 cm2. h&n extensionand the breaststep attenuations measured in an 8×8 cm2 field. the couchtop effecton percentage depth-dose (pdd) measured by an efd diode for field sizes 5×5-20×20 cm2 and compared with that produced by a co-60 beam passing through a‘tennis-racket’ couch insert. a monte carlo (mc) model of the couchtop producedto provide more superficial pdds. (pdds that are more superficial) results:maximum couchtop attenuation (7.6%) measured for the 135° gantry and 5×5cm2 field. couch extension attenuation was 1.5% lower. adding breaststepincreased attenuation by 2.4%. mc attenuation results agreed withmeasurements to within 0.2%. the couchtop removed the dose buildup effectalmost completely and increased the pdd at 0.4 mm depth by 60.6%-74.6%.mc-calculated pdds at the depth range of skin basal cell layer (0.1-0.4 mm)increased by 55.3%-63.2%. the couch insert in the co-60 beam increased thedose at 0.4 mm depth by 18.1%. for the same dose prescription at 10 cmdepth, the insert in the co-60 beam produced a skin dose 49.7% lower thanthe couchtop at 6 mv. conclusion: these results provide useful practical data onattenuation and skin dose increase applicable to many centres. the acceleratorcouchtopcombination creates a greater skin dose increase than a tennis-racketinsert on a co-60 unit.