Dosimetry for quantitative analysis of low dose ionizing radiation effects on humans in areas of low dose during therapeutic irradiation

In patients that undergo therapy using ionizing radiation (IR), tissues outside of the primary treatment beam that still receive radiation dose may be sampled to assess the biological effects of low dose IR in the area of 1-10 cGy. Accurate quantification of the dose-response curve in the low dose region requires the prospective identification of the area of tissue that will receive a pre-specified low IR dose. This study assesses the feasibility of using Monte Carlo simulations of the radiation transport to identify areas of the skin that receive low dose IR delivered in a single fraction. The PEREGRINE Monte Carlo calculation engine for radiation therapy planning is extended to the low dose areas of such procedures outside the treatment area (0.5-5% of prescribed dose). Following pre-studies in a simple slab phantom, verification of the entire dosimetry procedure was tested on a custom anthropomorphic phantom that could be set to variable thicknesses to mimic patient set-up and variation in patient size. Measurements were performed with calibrated TLDs and a Markus ionization chamber for three different phantom thicknesses. Single PA beam and five beam setups with and without MLC shaping were investigated. Results of the measurements were compared to Monte Carlo simulations. For single PA irradiation of the anthropomorphic phantom the agreement was within 11%, for all thicknesses. For the 5-beam cases, PEREGRINE consistently under-predicts the dose by approximately 15% (3-21%), again independent of phantom thickness. Since the deviation is systematic, it is possible to correct for it with a single factor and allow the biological experiments to proceed. These studies have demonstrated that Monte Carlo simulation is capable of identifying an area receiving IR dose in the range of 1 to 10 cGy during radiation treatment (prescribed dose of 2 Gy) with an uncertainty of less than 10% (after correction factor) across variable phantom / patient thicknesses