Effects of treatment distance and field size on build-up characteristics of Monte Carlo calculated absorbed dose for electron irradiation

Abstract

Surface, build-up and depth dose characteristics of a monoenergetic electron point source simulated by Monte Carlo code MCNP4c for varying field size and SSD are extensively studied in this paper. MCNP4c (Monte Carlo N-Particle Transport Code System) has been extensively used in clinical dose simulation for its versatility and powerful geometrical coding tool. A sharp increase in PDD is seen with the Monte Carlo Modelling immediately at the surface within the first 0.2 mm. This effect cannot be easily measured by experimental instruments for electron contamination, and may lead to a clinical underdosing of the basal cell layer, which is one of the most radiation sensitive layers and the main target for skin carcinogenesis. A high percentage build-up dose for electron irradiation was shown. No significant effects in surface PDDs were modelled with different SSD values from 95 cm to 125 cm. Three depths were studied in detail, these being 0.05 mm, the lower depth of the basal cell layer; 0.95 mm, the lower depth of the dermal layer; and 0.95 cm, a position within the subcutaneous tissue. Results showed only small surface PDD differences were modelled for SSD variations from 95 cm to 125 cm and field sizes variation from the values between 5 cm and 10 cm squares to 25 cm. When the field side length is smaller than this, the surface dose shows an increasing trend by about 7% at 5 x 5 cm2.