The effects of thermoplastic masks used for immobilization of patients on surface and build-up for megavoltage radiation

METHODS

Three pieces of Aquaplast masks were used. One of these
masks was used intact as a standard. The hole diameters of
the other masks were stretched 1.25 and 2-fold. The masks
were numbered as 1, 2, 3. Doses were measured with Markus
parallel-plate (pp) ionization chamber, EBT2 film and MOSFET
dosimetry.

RESULTS

The 1, 2 and 3 numbered masks were increased the surface
dose from 24.2% to %73.3, 55.6% and %40.6, respectively,
for 10x10 cm² at Co-60. For 6MV, the surface dose of 14.4%
increased to 50.1%, 34.7% and 23.7%, respectively. Markus
pp results were compatible with the EBT2 film results. MOSFET
measured higher surface doses.

CONCLUSION

The surface dose increases in the presence of mask. Mask
material thickness and hole diameter significantly affects the
surface dose. The diameter of the holes should be enlarged or
cut the relevant region.

Keywords: Surface dose; Markus parallel plane ionization chamber; EBT2 film; MOSFET; thermoplastic mask