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 Standard couchtop, and attenuation of the BreastSTEP board, for an Elekta Compact 6 MV accelerator. Materials and Methods: Couchtop attenuation were measured for the range of gantry angles 125°-180° and field sizes 5×5-20×8 cm2. H&N extension and the BreastSTEP attenuations measured in an 8×8 cm2 field. The couchtop effect on 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 produced to provide more superficial PDDs. (PDDs that are more superficial) Results: Maximum couchtop attenuation (7.6%) measured for the 135° gantry and 5×5 cm2 field. Couch extension attenuation was 1.5% lower. Adding BreastSTEP increased attenuation by 2.4%. MC attenuation results agreed with measurements to within 0.2%. The couchtop removed the dose buildup effect almost 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 the dose at 0.4 mm depth by 18.1%. For the same dose prescription at 10 cm depth, the insert in the Co-60 beam produced a skin dose 49.7% lower than the couchtop at 6 MV. Conclusion: These results provide useful practical data on attenuation and skin dose increase applicable to many centres. The accelerator-couchtop combination creates a greater skin dose increase than a tennis-racket insert on a Co-60 unit.