Enhanced epidermal dose caused by localized electron contamination from lead cutouts used in kilovoltage radiotherapy

Purpose: To investigate and quantify electron contamination from the lead cutouts used in kilovoltage x-ray radiotherapy. Methods: The lead cutouts were modeled with the Monte Carlo EGSnrc user codes DOSXYZnrc and DOSRZnrc for x-ray beams ranging from 50 to 300 kV p . The results from the model were confirmed with Gafchromic film measurements. The model and measurements investigated the dose distribution with and without gladwrap™ shielding under the lead, and dose distributions with round, square, and serrated edge cutouts. Results: Large dose enhancement near the edges of the lead was observed due to electron contamination. At the epidermal/dermal border, there is double the dose at the edge of the lead compared to the central dose due to electron contamination for a 150 kV p beam and three times the dose for a 300 kV p beam. gladwrap™ shielding effectively removes the contaminant dose enhancement using ten and four layers for 300 and 150 kV p beams, respectively. Conclusions: The contaminant dose enhancement is undesirable as it could cause unnecessary erythema and hyperpigmentation at the border of the treated and untreated skin and lead to a poorer cosmetic outcome. The contamination is easily removed by gladwrap™ shielding placed under or around the lead cutout.