Monte Carlo simulation of contrast‐enhanced whole brain radiotherapy on a CT scanner

Purpose: To perform a feasibility study of contrast‐enhanced whole brain radiotherapy for treating patients with multiple brain metastasis using a conventional computed tomography (CT) scanner. Methods: The treatment dose was optimized to be applied in a single run using a maximum tube power of 5200 kWs at 140 kV. CT scans of a large and a small head were used as reference. Irradiation geometry, shielding, axial beam collimation, radial beam collimation, gantry tilt, and tube current for beam modulation were optimized using a Monte Carlo simulation and a contrast agent concentration of 5 mg/ml iodine in the tumor. The statistical uncertainty of the Monte Carlo simulation was corrected using back convolution. Results: Using a CT tube with a beam collimation of 28 . 8 mm, a mean tumor dose of 1 . 76   ±   0 . 13 Gy was achieved, while the head bone dose was 2 . 61   ±   0 . 18 Gy with a normal brain dose of 0 . 98   ±   0 . 06 Gy, eye dose of 0 . 19   ±   0 . 05 Gy, and lens dose of 0 . 15   ±   0 . 03 Gy, respectively. Using a CT tube with dose modulation and a beam collimation of 40 . 0 mm, the mean tumor dose was 2 . 00   ±   0 . 11 Gy with a head bone dose of 1 . 96   ±   0 . 14 Gy, normal brain dose of 1 . 13   ±   0 . 08 Gy, eye dose of 0 . 21   ±   0 . 05 Gy, and lens dose of 0 . 16   ±   0 . 02 Gy, respectively. Thus a standard CT scanner enables an effective tumor dose of 37 . 0 Gy to be administered in 13 fractions, while exposing healthy brain to an effective dose of 17 . 2 Gy and head bone to 69 . 3 Gy. Additional radial collimation implemented in the hardware improves the therapeutic tumor dose by 25 . 2 % in relation to the bone dose. Conclusions: Contrast‐enhanced total brain radiotherapy is feasible using a conventional CT tube with optimized dose application.